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MG MGB Technical - Engine teardown after 3000 miles

Heres my story:

I had my engine rebuilt 2 years ago, at which time upon first startup the head was found to have a crack above number 3 spark plug and was leaking coolant. Patched with Marine-Tex (thanks Les Bengston - that really worked well). Oil consumption is roughly 1 pint per 150 miles, and has never improved since the rebuild. Despite this, the engine runs strong, does not smoke. I see a mist of oil being spewed from the joint between the head and the block on the spark plug side. Searched for uncracked 12H2923 head, finally found one after 1.5 yrs. Machinist who rebuilt the engine has prepared this head and it is almost ready to go on to my car. I removed my head, and the attached photo shows what I found. The tops of the pistons are coated with oil (there is actually a puddle on top of the number 2 cylinder). The walls of the cylinders are also coated with a layer of oil. Also, the cross-hatching appears to be horizontal, instead at 45 degrees in both directions (as you can see in the second photo), which I gather is not correct. Also, it seems that the piston skirt has been rubbing the bore in the number 2 and 3 cylinders? (see second photo).
The machinist looked at the block, and has said that the rings are bad, and has re-ordered rings.
My questions: 1. is it bad rings, or bad cross-hatching that has led to this situation?
2. If its the cross-hatching, can it be re-honed to fix it, or must it be bored out 10 more thou, and new pistons be used?
3. Can you hone the block in the car or does it have to be removed?
4. Is there something wrong that has allowed the skirts to scuff the bores?

Erick Vesterback

And here is the second photo:

Erick Vesterback

1. I don't know the answer to your first question. The absence of cross hatching didn't help but how the engine was broken in and the type oil used for break in may have also prevented the rings from seating.

2. You should be able to hone the cylinders and get a good cross hatch assuming there are no deep scratches. That said I would want to examine the pistons after removal before deciding what to do.

3. It's possible to hone cylinders without removing the block. The big problem is keeping grit and swarf out of the main bearings.

4. The lack of cross hatching didn't help. Incorrect piston to cylinder clearance could have caused the scuffing.

Hope someone like Chris Betson will provide some input, he builds engines every day. I only do one every few years.

Clifton

Clifton Gordon

Looking at the second picture it would appear that the cylinders were never honed. The pattern on the walls looks more like the marks that are left by the cutting of the boring machine. If in fact that is what happened the scuffing may be caused be the bore being too tight. Normally you bore just under the desired diameter and then take the last few ten-thousands with the hone. You should be left with a nice crosshatch with no signs of the boring cutter.
John H

Erick. It should be noted that the use of Marine-Tex is a temporary thing, a "get you home fix", not a long term solution to a cracked cylinder head. That being said, however, I do not see the cylinder head as being part of the problem noted in the photos you have posted. Normally, a cracked cylinder head constitutes a cooling system problem, not an oil system problem.

The photos, and your written description, would describe a very large oil control problem. Unless there is catastrophic cracking of the cylinder head, it is not going to be capable of supplying the amount of oil we are seeing and you are describing. Thus, we look at the head gasket.

When the head gasket fails in a catastrophic manner, we see visible effects relating to the cooling system. The cooling system is pressurized far beyond the normal 7-15 psi that most of them are designed to operate at and coolant is blown out of the overflow on the radiator or expansion tank, rapidly depleting the system. When the cylinder head is removed, significant damage to the head gasket is noted and, often, burned areas on the block and cylinder head mating surfaces. I am not seeing that form of characteristic damage in your first photo.

Thus, I agree with John H. that the root cause of the problem is the cylinder bores. The second photo is not as clear as the first. But, I see what John mentions about it looking like the block was not honed to size after it came off the boring machine. I also note the arrow pointing to the shiny spot in the bore. This is a characteristic of the skirt striking the bore. When this happens, the skirt can be damaged and large amounts of oil can enter into the cylinder bores, the rate of oil consumption becomes highly excessive, and any form of PCV system is overwhelmed. This is not a problem with "bad rings", but a problem with bad machine shop work.

I would suggest that the pistons have to come out and be inspected for damage. I suspect you will find the pistons to be damaged and in need of replacement. The pistons and bores need to be measured to see if the bores have been correctly sized to the pistons. The cylinder bores are slightly tapered with there being more clearance specified at the top than at the bottom. (In most cases. But, exact clearances vary by engine. I just looked at the 18G/18GA, the 18GB and the 18GD-GG engine specifications and each are slightly different.)

You need to measure the size of the pistons, then the size of the bores and compare what you are seeing with the specifications for your specific engine. This will, most probably, require that the engine block and pistons be taken into a machine shop for the measurements to be performed. (Unless you know someone with the proper tools to do it in your garage.) I would suggest having the measurements made by someone other than the machinist who originally rebuilt the engine. He should have crack inspected the cylinder head before machining. (Yes, they can crack when being assembled, but this is very rare.) The statement that the piston rings are bad sets off some alarm bells--if they were properly sized to a properly prepared bore, I have never seen a "bad" set of rings. Add this to the fact that, from what we can see, the cylinder bores were never honed and it builds up to some doubts about the quality of the original machine shop work and assembly process. I would have the block and pistons checked out by at least one other shop, two would be better.

Les
Les Bengtson

Thanks guys, I appreciate your input. I have removed the pistons, and I cannot find any evidence of the skirts being affected in any way by contact with the bores. All I see is quite alot of carbon (I think) caked on the uppermost piston ring.

The pistons that went in were 030 over, and were a matched set with the rings ie the rings were already on the pistons when they arrived. I wondered more about the bores than the rings myself, so questioned in my mind about the statement that the rings are bad, and that new ones will solve this. I'd rather not put it all back together with new rings and have to pull it apart next year.

The second photo is with the piston removed, and the shiny area next to the pointer is below the level of the rings when the piston is at bottom dead centre (I think). That led me to believe piston skirts making contact. But there is as I say no evidence on the pistons that I can see.

Thanks again for your help.

Erick
Erick Vesterback

BTW, the machinist who did the work has done several engines for people in our local MG club, and I have heard no bad things - every one seems completely satisfied with what they got. In fact he was recommended. Not sure why it didn't turn out well in my case.
Erick Vesterback

If this was a first rebuild it is unlikely, but if the motor had been rebuilt before and this time the bore was only taken up by .010 the shiny spot could be caused by this spot in the bore being more then .010 over. In this case the boring machine would not have touched this area and it is shiny because it was not cut and is a spot of the old bore.
John H

Does it strike anyone else odd that the top of the cylinder looks so different from the rest of the bore? I don't think I've ever had the misfortune to have to tear an engine down after only 3000 miles, but it seems odd to me.

The top of the cylinder is the part that would usually be ridge reamed (unless you found some ridge dodger rings) if you were only replacing the rings, and were not boring the block.

It seems to me that this top part of the bore would still show the machining marks after only 3000 miles, be it boring or honing marks.

It makes me wonder if it was bored to an oversize during the last rebuild.

Charley
C R Huff

The top of the bores are covered with a very thin film of carbon I believe resulting from burning oil. I could not feel it when I ran my fingernail up and over it. It did however offer slight resistance as I pushed the pistons up out of the bores.

This is the first rebuild of the engine, and I saw the bores before the pistons went in, and they looked perfect (although its seeming to me that the honing was either left out or improperly done).

I will have a look at the top area of the bores to see if I can see evidence of cross hatching. Its hard to photograph really well.
Erick Vesterback

Erick. None of what you have posted negates the fact that you need to have the pistons and bores measured, then compare the measurements to the specifications listed in the factory workshop manual for your engine block and the pistons you are using. The clearance specifications changed, three times, during chrome bumper production. You need to know what you actually have before any form of analysis can take place.

I had to tear down an engine at the 2K point after a rebuild. At that time, cross hatching was clearly evident on the cylinder bores and the piston rings were not fully seated as indicated by several areas of blackening remaining on the surface of the rings in contact with the cylinder bores.

If your problem is supposed to be "bad rings", how are they bad? What do they look like? What is the ring gap in the cylinder bores? This is the type of information needed to give focused comments. Without it, only suggestions, not information, can be offered.

Les
Les Bengtson

I have not done anything with the pistons and rings yet. The head is completed now, so my machinist is coming over this evening and I will go over it with him. There is what I believe to be carbon on the rings, which I haven't disturbed because I want him to see how it looked when they came out. Thanks to all who replied, I feel that I can talk to him more intelligently about how to proceed. I think the block has to come out and the bores be at least re-honed or maybe more needs to be done. It may turn out that the machinist I used won't agree, and if thats the case, I will get it sorted by another shop. Its this far apart now, I don't want to put it back together until its right.

I'll post ring gap info etc when I have it, and have pistons and bores measured.
Erick Vesterback

I have checked the ring end gap with the "bad rings", and there appears to be absolutely no gap in any of the 2 compression rings or the oil control rings. I see no gap, and can't fit the smallest feeler guage I have in there. Is it possible that they have changed as a result of use over the last 3000 miles? Wouldn't the engine seize when up to temperature of there was no gap? As far as the compression rings, they "look" normal, but beyond that there could be flaws that I am not able to detect. The oil rings don't look all that high quality, in that the spring that presses the two thin rings against the bore appear weak and of poor quality, compared to a set of Hastings rings that I have. Maybe that explains why the oil consumption is high. As far as the cross hatching is concerned, I don't think the marks we see are the boring marks, as the marks I see are extremely fine, and the bores feel perfectly smooth with my fingernail. I think the marks are cross hatching marks done very horizontally. I understand how this might make for greater oil consumption, but how much difference could it really make? Are we talking a litre of oil every 2000 miles instead of 2500? If so, I can live with that. But then again, are we talking 300 miles instead? Then I need to have it rectified.
I really think the source of oil loss is from the oil hole to the head not being well sealed (as well as the pushrod holes) by the head gasket, as those areas were oil soaked when I removed the head gasket. I have a Fel-pro/Payen on hand to use when it goes back together.
Erick Vesterback

Check www.totalseal.com, they only do piston rings, lots of techtalk on the site.
Laurens CGT

Erick,

I am curious to know what ring gap you have with your new rings. Zero gap on compression rings seems like it could be caused by using a ring one oversize bigger than the nominal bore. It's not uncommon to do that on a worn engine that is not being bored, but it shouldn't be normal on a fresh bore. It will be interesting to know what your bores measure.

The only time I had much trouble with rings was when I put a set into an air compressor, and put them in upside down. When I fired it up, using a blow gun was like spray painting with oil. I turned them over and everything worked fine. As I recall, I could not see the difference visually (well, except for the word "top" printed on one side).

Charley
C R Huff

Erick, properly hone the cylinders, check the bore clearance, get a set of Hastings rings, check the ring gap and adjust if necessary. Check everything else, if this engine was assembled by the machinest and they skipped the ring gap what else didn't they check. Find new machinest.
There is nothing wrong with using a fine hone but it should still be a cross hatch, it will take longer for the rings to seat with a fine cross hatch.
I find it sad that we see many on this board who have had an engine rebuilt by "professional" and get a botched job for there money. Engine assembly is not that difficult, if you are reasonably good with a wrench. It can work out to ones best interest to tear an engine down, have the machining work done, and assemble it yourself. That way you know what you have and that it is done to spec. In some cases I think todays shops are used to the replacement parts suppliers holding very tight tolerances on modern day cars that allows them to get away with installing things such as rings without checking them. Unfortunately a lot of the parts available for older cars from questionable suppliers are not held to these same tight tolerances and adjustment is required.
John H

Erick-
Nightmares such as yours are the reason that I build my own engines, including doing the machinework myself! Where to start?

First, the marks on your bores are tooling marks that are the typical result of boring. I can see no indication that the bores were ever honed after boring.

Second, Check the end gap of the compression rings by both oiling and inserting each of them into the cylinder into which they will each be installed. This must be done after squaring the rings to the cylinder wall by turning a piston upside down and pushing each of the rings with the top of the piston down into the cylinder approximately one inch. Measure the end gap with a feeler gauge. Check this against the end gap specification chart at the back of the Deves Piston Ring catalog or on the instruction sheet that is included in each set of rings. If these gaps are too small, the ends of the rings need to be filed. On the other hand, if the gaps are too large, you may have the wrong size rings, or your cylinder may have been accidentally bored oversize. The dimensional setting of ring gaps is often a confusing and misunderstood art. There are minimum and maximum ring gap specifications that must be observed for the best performance of a new set of rings. Minimum gap tolerances must be observed in order to prevent the ring ends from butting together while the ring expands as the engine approaches operating temperature. The Society of Automotive Engineers recommends a minimum of .0035 gap per inch of cylinder diameter. For example, the proper minimum ring gap for a set of +.020” oversize pistons would be: [(3.180”) + (.020”) bore clearance] x .0035 = .011” minimum gap. Maximum ring gap is an important part of ring performance in that too much gap results in lost compression, power loss and ultimately poor oil control. Manufacturers rigidly adhere to these tolerances and that the ring gaps are inspected in gauges accurate to .0001” at the cylinder diameter the ring is manufactured for. Any increase in the cylinder diameter beyond the designated size that the ring is designed for results in an increase of approximately .003” in ring gap for each .001” increase in cylinder diameter. For this reason, each piston should be individually matched to its most appropriate diameter cylinder bore and the gap of each ring should be measured only in the particular cylinder bore in which it is going to be used. In order to check ring gap, the rings should be placed at the lowest possible part of the cylinder below the ring travel area, as this is the portion of the cylinder for which the ring is sized. The gap of the piston rings should be precisely set by carefully working with a fine toothed file. It should be noted that if the pressure between the second ring equals or exceeds the pressure above the top ring, it could cause the top ring to lift off the bottom of the piston ring groove and lose contact with the sealing surfaces, resulting in a loss of compression and “blow-by”. It also inhibits the ability of the rings to transfer heat away from the piston. Most amateur engine builders (and some tradesmen) are unaware that the gap of the second ring should be .004” greater than that of the top compression ring in order to vent away gases and thus increase the top compression ring’s ability to seal off the combustion gases, especially at high engine speeds. This larger escape path reduces inter-ring pressure and thus assists in keeping the top ring seated against its groove, preventing combustion pressure from escaping. Without this adequate escape path, the trapped pressure beneath the compression ring will nearly equalize with that above it, thus allowing the compression ring to become unseated from its seal against the bottom of its groove as the piston travels down the bore. These conditions will cause reduced cylinder sealing and ring flutter at high engine speeds. In addition, a fluttering ring cannot transfer heat away from the piston to the cylinder wall, a condition that will be aggravated by the blow-by of combustion gases blasting the heat-conducting film of lubricating oil on the cylinder bore downwards, away from where it is needed at the worst possible moment in the piston’s cycle of travel. These conditions can result in piston overheating, top ring groove “pound-out”, ring side-wear, and scuffing. If the gap is not perfectly both symmetrical and vertical, blow-by and blowback gases may generate thrust, causing the ring to rotate in its bore. Secure the butt end of a small sharp file into a vise. File only one end of the ring in order to allow you to verify that you are keeping the gap straight and parallel. File from outside face toward inside diameter to avoid chipping the coating on its face or leaving burrs on the edges of the Outside Diameter. Remove any burrs created by the gapping process with a fine stone. This will produce a smooth finish that will preclude the formation of stresses in the material that can lead to fracturing.


Third- The Original Equipment specified clearance gap between the cylinder bore and the piston skirt is .0021” to .0033” at the top of the stroke and .0006” to .0012” at the bottom of the stroke (Yes, the cylinder is actually supposed to be tapered in this manner! The purpose of the taper is to allow for the greater expansion of the top of the bore due to both it and the piston crown being exposed directly to the heat of combustion). If your builder didn't hone the cylinders, how much do you want to bet that your cylinders are not properly tapered as well?

Fourth- Your piston rings have almost certainly had their sealing surfaces damaged by forcing them up through all of that carbon at the top of your cylinders. You'll need to replace them. Since they came already assembled on the pistons, I presume that they are from AE.
Steve S.

Has your machinist had a look yet? Here's a pic of cross hatching and ring gap by way of comparison, in this case after about 50k.

Steve Postins

Steve, I have always read and been taught that a cylinder bore should have any taper removed at rebuild because tapered cylinders aren't good for efficient ring sealing. 30 years ago I was taught the basics of cyliner boring in auto mechanics school. The boring machine we had wasn't capable of boring a cylinder with a taper. The writer of this article tells everyone the cylinder walls should not be tapered.
http://www.aa1car.com/library/honing98.htm

His second paragraph reads.
"The main objective when refinishing the cylinders is to make the walls as straight as possible (no taper), the bores as round as possible (minimal distortion, which is especially important with low tension rings), to have the right amount of crosshatch for good oil retention and ring support, and to produce a surface finish that meets the requirements of the rings. This is done by boring and/or honing the cylinders in one or several steps with various types of abrasives (vitrified or diamond).

Another article also recommends no taper
www.spearsenterprises.com/pdf/cylinderBoringFinishing.pdf

How would a machinist bore a cylinder with a controlled taper?

Clifton
Clifton Gordon

Erick. Again, the bores of the cylinders need to be measured at the top and bottoms--front to back and side to side, to see what the bores are. Without this information, nothing can be done correctly.

Piston rings which show no gap either indicate oversized piston rings or improper sized cylinder bores for the pistons and rings. Until the bores are measured, you do not know which.

Steve S. is correct. The factory specifications call for a tapered bore with less clearance at the bottom than at the top. But, I have not found a machine shop, in my area, that has the equipment to bore the cylinder bores in this fashion. All of them perform the process as Clifton was trained in and has described. Has seemed to work fine over the years. Specifications for US engines, over the last 40+ years have been a straight, non-tapered cylinder bore.

Les
Les Bengtson

Thanks to the two Steves. Steve S - I have replaced the rings on the pistons with Hastings rings as recommended by the machinist. Unfortunately, I damaged one of the oil control rings in my attempt to insert it, and have had to order a new set from Hastings! Another week to get them. I had gapped them to .014, Hastings recommendes .004 per inch cylinder diameter (apprx 3.375 in my engine) which comes to .0135ish, so I went to .014. Interesting to have slightly more on the second ring, will pay attention to that. I used a piston with an extra ring in the second groove to square it while checking the gap.

I know that my bores are not tapered, because I watched the machinist check it with a gauge. From what I can tell, Steve Postins, my bores look like yours from your photo. Thanks for providing it.

Thanks to all who have contributed, I am learning lots!

Erick
Erick Vesterback

I have never bored a cylinder with a machine that could provide a tapered bore. Pistons are normally smaller diameter at the top then across the skirt. Cylinders do wear into a taper and the "specs" for the measurments of taper in a cylinder is normally provided as a guidline as to when the cylinder has worn too far and needs to be bored.
John H

Cylinders are NOT tapered; Clifton & John H are correct, Steve S and followers are not. Clearances are specified at the top and bottom of the piston SKIRT, which IS tapered. Clearances vary by PISTON design and material, not by engine; piston manufacturer's specs apply. Not all MGB are the same, since they had different designs of piston, and replacement pistons may be different than any OE parts. Skirt clearances are measured at 90 degrees to the piston pin; measurements in other locations may/will be different, again depending on piston design. Measurements in the ring groove area or above will be substantially less than given skirt figures.

Position and orientation of rings is critical, as noted by Charley; upside down rings or rings in the wrong groove make great oil pumps.

Erroneous crosshatch patterns may result in ring damage, oil consumption, and prolonged or infinite wear-in of rings. From your picture, the crosshatch appears to be about as wrong as it gets - namely, there isn't any "cross" to it.

Oil coming out the head joint is a guaranteed sign of a loose cylinder head. Given that this appears to be a resin/fibre gasket, the simple fact that the head came off without totally destroying the gasket is proof that the head was loose and/or installed with contaminated gasket surfaces; if installed correctly you cannot get a head off this type gasket without leaving half the gasket on the head and the other half on the block.

The fact that the loose head did not catastrophically blow the gasket and dump all the coolant is evidence that you have been pussyfooting about whilst supposedly "running in" the engine. That is a sure way to make ring seating take forever, or to never happen at all. That's a final and sufficient reason to have large quantities of oil in the combustion chambers, even if all measurements and assembly were correct.

BTW, you have not yet told us what the piston and bore, and thus clearance, measurements actually ARE; any more speculation is a waste of time.

FRM
FR Millmore

Somebody once told me piston rings are actually a lot more expensive than pistons. Also rings get worse over time (new old stock?).

Didn't know this when I had my engine rebuilt with NOS, so far so good. Total seal was recommended by the top running MGC in the british BCV8 championship. Not cheap. Hastings claim to be the only US ring manufacturer, so total seal probably belongs to them?

Running in is critical as well, I did 500 kms straight at 3000 revs, and 2500 kms further before really starting to "use" the engine, all on cheap mineral 15/40 oil (changed this after 500 kms). It doesn't use oil.
Laurens CGT

The cylinders were bored 030 over, and the pistons (which came with the rings already on them) show 030 on the tops. The Hastings 030 rings out of the package went into the bores with about 010 to 012 gap, which I adjusted to 014 (which required only a minimal amount of filing to acheive). This suggests to me that the bores are the right dimension - if not, the gap would be either way big, or not at all. It is possible that the pistons are not right even though they are marked 030, so I will beg borrow or steal a micrometer and measure those values, but if aftermarket parts designs are so variable, how will I or anyone know whether they are a good match?

I don't know what kind of gasket was used. I would say it looks like aluminum (probably isn't though) or some other metal and it has dimples on it. It has extra material around the cylinder holes, and what looks like copper seals around the coolant passages. It was not stuck to the head or the block very firmly. Permatex Aviation gasket sealer was used when it was installed. ARP head studs were used, and torqued and retorqued x 2 (after cam breakin and at 500 miles) to the appropriate values given by ARP (the ARP lube was used on the studs) can't remember the torque value. I don't believe the head was loose, the oil bubbling out did not stop even for a moment after retorquing.

I did not pussyfoot when breaking the engine in. I drove with a mixture of normal and spirited driving, and competed in autocrosses with 400 miles on the engine. Meaning repeated flooring the pedal and driving the s**t out of it. I also ran the car from 35 mph to 65 mph with the foot on the floor, then engine braked back to 35 mph, repeated 5 times, and did this on 3 or 4 occasions.
I felt alittle guilty because other people would say stuff like "you drive that pretty hard for the engine being so fresh". It was hard not to, as the engine performed so much better than before the rebuild. Hard to help yourself from opening the throttle.

I'll check the piston and cylinder bore dimensions, and see if that sheds light on the situation. I am at a loss, because there seems to be conflicting info coming from what I perceive to be very informed people. I hope I haven't wasted anyones time with this. I just want an engine that doesn't use a litre of oil every 200 miles.
Erick Vesterback

Erick, for a quick measurment of the piston to cylinder clearance, insert the piston upside down, hold it against the cylinder wall at the bottom of the skirt 90 deg from the pin and insert a feeler gauge between the other side of the skirt and the wall. What ever size gauge gives you a light drag when inserted will be very close to your actual clearance. While a micrometer and a bore gauge will be more accurate you do need to be well practiced with them to be accurate. When ever the shop I worked in bored cylinders we would number each piston, measure it and bore the cylinder to match it. This method assured that each cylinder to piston fit was uniform for all cylinders.
John H

Erick,

First, let me preface my comments by saying that I am not a pro. I consider myself to be an upper-branch shade-tree mechanic, so feel free to take what I say with a grain of salt.

Second, don’t worry about wasting anyone’s time with your postings. Anyone who thought it was a waste of time would not respond, and I’m sure that those who do respond do so because they enjoy the “hunt” for the right diagnosis. In other words, we are enjoying ourselves by playing Monday morning coach at your expense. If we didn’t enjoy it, we wouldn’t respond.

Here is my opinion so far. It looks like you plan to measure both the bore and the pistons after you have begged, borrowed, or stolen the proper equipment; good idea. If you can’t come up with the inside mic, I agree with John H that you can get a pretty good measurement with a feeler gauge. I also agree that the feeler gauge might be more reliable than the inside mic if you haven’t developed the “touch” with the inside mic.

I may have missed it, since this is a pretty well visited posting, but I haven’t seen that you plan to address the surface texture of your cylinder walls. That is, I haven’t seen that you plan to re-dress them with a hone/glaze breaker. In my opinion, if I had to choose, I would take a good surface over a proper dimension any day. Up to a point, or course.

On larger engines, with significant wear to begin with, I have pushed the bore way past spec with a drill-driven hand hone (glaze breaker) to get rid of imperfections in the walls. I mean like maybe 30 to 60 minutes of glaze breaking time. I may have also expanded the pistons with a small punch because I think at that time no machine shops had piston expanders/knurlers any more.

I tried as best I could with hand tools to accomplish cross-hatching at the proper angle as dictated by the manufacturer. This was a 743 cu in 6-clyinder Cummins, so it was a different case than an MGB, but the result was a maiden voyage from Kentucky to California with only one quart of oil usage. If it had been a gallon I would have been delighted. I don’t know what the long-term prognosis would have been because I only put about 100,000 miles on it before I got fed up with trucking and went back to school.

I believe the same is true with brake hydraulic rebuilding. I would rather hone it a lot and have a good surface than worry about how much metal I am taking off. So, if I owned your engine, I would not put it back together with out resurfacing the cylinders.

I have honed cylinders in-frame with the crank in place by plugging the crank journal oil galleys with grease, covering them with oil soaked rags, and then rinsing everything real well afterwards with a solvent suction air gun. I’m not saying I recommend it, but I’ve done it. It was a lot more trouble to pull the engine and crank in my case because of the size of the engine.

I don’t really remember the spec for the angle of the cross hatch for the Cummins, but I think it was something in the neighborhood of 30 to 45 degrees. In some other source I read the explanation of the purpose of the score marks created by the honing, and the reason for the angle. It made sense to me, and it was something like this:

The course texture gives oil a place to hide out so that it can be available. The angle of the crosshatch allows the sharp edges of the new rings to meet the coarseness of the cylinder wall with a scissor action during break in. As the ring comes up, it only shears a bit of the metal at a time. If the cross hatching were flat, like yours, the sharp edges of the new rings would go up and down the bore going “chunk, chunk, chunk” at every scratch mark, and while doing so would tend to peel metal off the bore and rings in a directly vertical fashion, and thus leave a bad surface for the future. Think of what it would be like to use a reel mower if the blades were not spiraled.

As for the Permatex Aviation cement on your head gasket, that may be the reason that it came off so cleanly. It may also be the reason that it leaked. If you had a gummy gasket, the Permatex probably prevented it from adhering to the block and head. I do a final clean of the block and head with xylene, alcohol, brake cleaner, or some high flash solvent, and put them together dry.

I have only used gasket cement on a head gasket twice. The first time I was 16 years old and I slathered it up real good on my TR3. It started leaking again in about two days. The other time was on a beat down Toyota Corona because I was reusing the old head gasket. In that case I was careful not to clean or disturb either surface, I applied it sparingly, and it was still holding 10,000 miles later when the second piston disintegrated, whereupon I drove it to the junkyard because the license plate was expiring and the performance was marginal on only two cylinders.

Charley
C R Huff

Erick, did your machinist check the top of the block for flatness with a straight edge when he came to look at your engine? Are you going to put the engine back together without pulling the block from the car? You talk as if you're preparing to put pistons back in, so I hope you are checking the deck for flatness and doing a proper hone on the cylinder walls.

With the luck you've had so far, I'd pay attention to details like this, or maybe have recurring problems.

Since you had no clearance in some rings, it's possible they lost temper from heat caused by too much tension. This is just theory from me, experts will probably chime in to correct or confirm.

Did you notice if the rings were indeed installed upside down? I'm just trying to condense the thread here / make it easier for you to cover all bases before reassembly.

I know the season is starting and the itch to drive her again is great, but do it right as you can this time.
Tom

Took some measurements of pistons and bores:

Tried inverting a piston and inserting it until the bottom was flush with the deck - could not get my .002 feeler in there - the gap was too small. Amazed me how close a fit it is.

Then using a cheap digital caliper borrowed from a co-worker, I measured the piston crowns at 3.151 - all measured exactly the same. Then measured the skirts at 3.164 to 3.165. The bores using the same instrument measured 3.188 to 3.189. Would seem to me to be much greater than 2 thou clearance of the skirt (why wouldn't the .002 feeler fit?). If 3.160 is the standard bore, I gather I am very close to the correct .030 over bore (lacking by .001 or .002 - could these bores be made to bore of 3.190 by honing? May be further evidence that the honing did not happen? or maybe just very difficult to measure accurately with less-than-ideal tools).

Roundness and lack of taper of bores has been verified. I see no cross hatching, only extremely fine (the walls feel perfectly smooth to fingernail/touch) absolutely horizontal marks, which are likely the boring tool marks. I feel more and more that I have to remove the block and rehone it. But, do the pistons and bores seem right? Or do I need to change them out too?

Tom - I was going to just reassemble the engine, but I have decided against it to make the cylinder finish correct. The itch to drive again is strong, but I want it to be right. Charley - Thanks for all the info, I will be using a Payen gasket dry this time.
Thanks again to all others who have posted who I have not named - I am glad this thread has not died yet.
Erick Vesterback

Erick, not sure why the feeler gauge showed so tight a fit based on your measurments. It is possable to hone out the last .002, in fact that points even more towards the impression that the bores were never honed as you typically bore undersize by .001 to .002 and hone to finish size. If you are going to hone it yourself get your hands on a professional hone such as a Sunnun, not one of the spring loaded hones at the local parts store. If your measurments are correct, the pistons would apear to be .010 oversize and were mismarked by the manufacture as .030, it happens. This would also show that the shop never measured the pistons or they would have caught this. you need to verify your measurments with high quality micrometers and go from there.
John H

Your pistons look like they are +.005" on the assumption that the original bore is 3.160" You can easily hone your bores out another .003" and then replace the pistons with +.030"s. This bore to piston mismatch is a serious error made by the engine rebuilder and they should be held financially responsible for this monumental cock up. Good luck.
Iain MacKintosh

Fitting the feeler gauge with the bottom of the piston flush with the deck - how about down half an inch or so? Is the carbon/oil buildup filling the gap?

You might get by honing with the block in place. I would borrow a good straightedge to check the deck flatness, and if it's out, pull the block and make it flat.

If it's obvious your shop didn't hone properly, they probably should be on the hook here. But I can imagine them saying "Well, you told us to bore it to 30 over, not to finish hone!"
Tom

Erick-
Sorry I am not able to follow this closely due to other pressing issues. You can email or call me direct for more detailed blather.

Meantime:
Glad to know you are not pussyfooting, though your treatment of a new engine is a bit extreme. Not as bad as running it on the interstate at constant heavy load, though!
If the gasket was letting oil out the side and the head was not loose, then it is a very bad gasket, or something is not flat. The use of gasket cement is not necessary or advised, as any gasket I've ever seen has the appropriate sealant applied by the factory. However, like Charley, I have resorted successfully to Permatex #2 at times (reusing metalfaced gaskets). Thin application to clean and dry block, head, and gasket, with careful retorques.
If that is a metal faced gasket with dimples, it's a new one on me, and I figure I've seen most of the gasket types available; that material is normally for manifold gaskets. No telling what people are selling today though.

BTW, Charley gives pretty good real world advice.

Crosshatching lets the rings slide on a constant bearing surface, made up of the high points in the pattern. As the rings go up, they slide over the oil film, and as they go down they scrape it off and down the low grooves of the pattern. This scraping is the primary function of the second (and third, if the piston has 4 rings above the pin) ring, and the directionality is a function of taper of the ring face, or of notches or chamfers on the inner circumference of that ring, which is why ring orientation is critical.
If the "crosshatching" is at 90 degrees to the bore, as yours appears to be, the ring bounces over the oil in the grooves, leaving it to get up to the combustion chamber. If the crosshatching started out rough - as when the bores are not honed - then it can damage any coating or plating on the ring face. It may also wear the bottom edge/corner of the scraper rings so that they can no longer scrape.

Piston fit:
As I noted above, the basic piston measurement is at 90 degrees to the pin, just below the bottom of the lowermost ring ABOVE the pin (pistons with an oil ring on the skirt below the pin don't count!). This is the "top of skirt measurement" given in the books. Measurements in or above the ring groove area will be considerable smaller; measurements not at 90 deg to the pin likewise. Measurements further down the skirt will be bigger, giving the reduced clearance figures in the books - these are the ones mistakenly taken to indicate tapered bores by some folks.

Per your post of 26 Mar, your rings and bores are about right, working from the Hastings ring end gaps.

Per the 27 Mar post, things get more complicated.
"I measured the piston crowns at 3.151 - all measured exactly the same. Then measured the skirts at 3.164 to 3.165. "
As previous, pistons are NOT measured at the crown; if these are +.030 pistons, then the crowns are -.040 from the "real size", about the usual in my experience. Your skirt measurement is off scale - if you were measuring at the right place and in the right direction, these would be +.005 pistons for a 3.160 std bore, never heard of such. (Pistons for some engines are supplied like this, but not MG, and certainly not marked +.030)

"The bores using the same instrument measured 3.188 to 3.189. Would seem to me to be much greater than 2 thou clearance of the skirt (why wouldn't the .002 feeler fit?). If 3.160 is the standard bore, I gather I am very close to the correct .030 over bore (lacking by .001 or .002 - could these bores be made to bore of 3.190 by honing? May be further evidence that the honing did not happen?"
My conclusion exactly - they were bored to +.030 less allowance for honing, and the honing WAS NOT done.

"or maybe just very difficult to measure accurately with less-than-ideal tools)."
25 or 30 thou is not a small error - very obvious with anything better than a yardstick, IF you measured the skirts correctly.

The key clue is here:
"Tried inverting a piston and inserting it until the bottom was flush with the deck - could not get my .002 feeler in there - the gap was too small. "
Based on your bore measurement of 3.188-3.189, the pistons are in fact +.030 as marked, and the clearances are tight, by about the amount required for honing..
How tight?
Various MGB engines use different clearances, and as I previously said, replacement pistons may be different still.
Corrected from obvious errors/misprints in the Bentley book, and assuming the actual numbers are correct:
18G/GA Top of skirt = .0036/.0045; bottom = .0018/.0024
18GB Top = .0021/.0033; bottom = .0006/.0012
19GD-GG Top = .0021/.0033; bottom = .0006/.0012
18V same

You can see that your .002 feeler would not fit with correct or even slightly loose bores, except on a 3 main engine, and it is rather surprising that the pistons did not scuff the bores IF your bore measurements are accurate, since the clearances would appear to have started out zero to negative.

Those sizes/clearances are typical for an engine of the MGB era and basic design.
For reference, a more modern engine (Mazda)gives .0015/.0020 as the top of skirt new clearance, with up to .006 as a worn service limit - i.e., it's OK to reassemble at this size with expectations that it will work OK for a reasonable time.
If anybody is making MGB pistons with more modern materials and design, then I would expect spec clearances to approach the Mazda numbers.

I suggest that if at all possible you take the engine to a REAL machinist not otherwise involved, and get things measured and fixed as required. Check block and head for flatness, bores and pistons for true size/clearance, rings for correct installation, and put it all together with good gaskets and correct retorqing. I've never had the need to use ARP studs, but be certain that the threads are long enough that there is zero possibility of the nuts bottoming on the threads, and that correct hardened washers are used under the nuts. (I have seen cases where the washers were omitted and the nuts bottomed - guaranteed leaky gasket!)If your current guy has screwed up, then he owes you, and if possible try to get him some education in the bargain. You do not want to go through this again!

Send me a request by email and I will send you my head tightening procedure, which includes a lot more on fasteners and such.

Best of luck,
FRM

FR Millmore

This thread was discussed between 17/03/2008 and 30/03/2008

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