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MG MGB Technical - Combustion Chamber Volumes are uneven

I have a cylinder head with different combustion chamber volumes.

Cylinder 1 44.4 cc's
Cylinder 2 43.1
Cylinder 3 42.5
Cylinder 4 41.7

This seems like alot of variance, but it is the first head I've cc'd. Is it alot? Will a machine shop be able to skim the head so as to make the volumes all the same? I'm shooting for a 9.7 to 1 compression ratio, so the volume I want will be about 37 ccs.

Thanks,
Jim B
Sacramento, California
J Brown

Jim

Skimming the head will bump up the compression ratio, but it won't balance the combustion chamber size.

To balance the combustion chambers, you have two choices that I am aware of. You can do it by how you grind the valves (grind more off the valve face and you increase the combustion chamber size). Or you can grind iron out of the three smaller combustion chambers until they equal the largest one. After doing one of these two things, you can skim the head to get the CR you want.

I would suggest getting all new valves, decide if you need valve inserts, do a good three angle valve job, and then cc the head and see where you are. If you are close, you can then cut it out of the new valves. If not, you can cut iron out of the head. You can of course do some of both.

I have only done this a few times, so wait and see what others say before you pick up the tools.

Have you magnafluxed or pressure checked your head. It would be a shame to do all the work to a cracked head.

Charley
C R Huff

Jim,

I think you will find that skimming will not solve your problem. If you have access to Peter Burgess's "How to tune the MGB 4 cylinder engines", he explains how this is done.

Each chamber or each intake valve will require material to be removed to balance the chamber to largest one (44.4).

I am sure the experts will chime in, and offer advice. For road going, I think Peter recommends 1 cc variance is acceptable.

Shareef
Shareef Hassan

Jim a couple of things that can put them out that far:
The first and most likely is that the head has been trued up before and a poor job. Too much taken off one end hence the 44-41 slope. This could be fixed by a good machinist. Measure the thickness of the head at either end and compare. When truing a head many shops use a machine where the head is hand held and it takes skill to do a good job, If yours is out it will probably need to be jigged up.
The other thing is the distance the valves are let into the head either through wear and tear or during a valve grind and it may be by chance that the volumes slope. The amount the valves sit in the head is one of your adjustments to get your volumes even. Measuring the head thickness is your first job, head surface to rocker cover machined surface.
DENIS4

Denis4,

Ah yes. I hadn't noticed that they were getting progressively smaller from one end to the other.

Good catch. That may be the cause.

Charley
C R Huff

Jim,

first of all you should mesure the thickness of the carsting. Stock heads should give you a reading of 80 mm.

when byretting, it is easy to post a acrylic glas on the chamber testet. Drill a hole in it and fix it with greas to the head.
With a syringe and ATF, it is a fast, simple and an exact job if you are not common with this.

When going to 37 ccm chamber volume, you aught to check wheter it is a 18V block, otherwise valve pockets have to be cut to the bores.
It is also a point to considre whether flat top pistons are not the way to go. Dished ones of the HC version have a dish volume of 6 ccm. The usual chamber volume of 41.5 to 42.5 ccm will give you a comparable compression but without irreverible skimming of the head and you have a larger choice of roller rockers or high lift cams, that can be fitted to your engine.

Ralph
Ralph

I found when I balanced my CCs there were enough burrs and rough edges there that I could get them close just by removing these. For the rest I just made sure I removed a little bit evenly so I wasn't changing the shape in any way. I used a sanding drum on a Dremmel.

Whatever you do do not accidentally nick a valve seat! It is VERY embarrassing having to take it back to the shop to get them to fix it after you stuff it up.

Don't ask how I know!

"Have you magnafluxed or pressure checked your head. It would be a shame to do all the work to a cracked head."

Yes, I second this fully!

Don't ask how I know that one too!

Simon
Simon Jansen

The cc difference seems a lot for a standard head, usually chamber 2 from the thermostat end is 1 cc more than the others, it is more often than not that I cut the first seat on this chamber to get the depth for all the others. It might well be that the valves and seats are in poor condition. Did you use a pespex plate to cover the chamber to measure the ccs and what did you use to measure?

Peter
peter burgess

Ok with Ralph not to skim off that much the head, but use of flat top pistons is a much better solution
under any aspects.
Thanks to Ralph for the std. tickness. I spent too much time in attempt to get it right.
Renou

Denis, I think you may be correct; I have had work done on the head and the machine shop may have done a poor job. I purchased the head in July of 2000 and had it worked on then. This past summer I had it worked on again.

Peter, I understand your suspicion of the measurement results. It is odd that one of the chambers has so much volume. However, I did use a Plexiglas plate with two glass tubes pressed into holes in the plate. I used a burette given to me by a chemist and was able to reproduce my results, so I’m pretty sure my measurements are good. The head is unusual and I don’t think it came off an MGB. Its casting number is 12H4736. It has an offset oil hole beneath the back rocker shaft post. The casting includes the metal necessary for air injection galleries, but the galleries were not drilled. The shape of each combustion chamber is that of the heads fitted to 18V engines. However, the depth of each chamber is deeper than the 18V head and not as deep as the 18G head. (I have an 18G head and a big-valve 18V head to compare it too.) Also, the back of the head has the surface necessary for a water takeoff, but it was not machined for one.

Peter, when I first had the head worked on, I brought your book to the machinist and he opened up the exhaust ports, triple cut the valves and seats, and bulleted the guides. More recently, I had the head checked for flatness, a small crack repaired, and new valve inserts installed. It looks like there is room to cut the intake inserts deeper. However, I like Denis’s idea to machine the surface to reduce the difference in combustion chamber volumes, if that is possible.

Ralph suggests flat top pistons. Where can I get these?

Everyone, thanks for your help! This is great getting help from all over the world.

Jim
jbrownell@waterboards.ca.gov
J Brown

Jim,

You might ask for them (top flat pistons) by MGBMGA
in USA. They have an E-mail adress:
doug@mgbmga.com
I noticed this info:
"Normally, we supply JE pistons in a flat top configuration with a piston pin height of " at:
http://www.mgbmga.com/tech/mgb11.htm
Regards.

Renou

Jim,

there are different choices of flat top pistons.
If you want to use the engine on a road car that does not have to go long distances at high RPM, there are inexpensive carst County brand pistons, that will work to eventual bursts up to 6K and work fine with a modified head and a fast road cam as the Piper 285/2.

If you are planing a race engine for competition use,
JE, Venolia Pistons, Omega Pistons or Arias Forged Pistons (there are also others, i think), might be a better solution, as this kind of engines do long distances at high RPM, so forged ones have to be used.

Just contact Chris Bedson at Octarine Services or Perer Burgees or Doug Jackson. They are able to deliver the quality you need, i think.

Ralph
Ralph

According to http://www.flowspeed.com/cylinder-ident.htm 12H4736 is essentially a CAM-1106 which was the North American head from about 1974 to the end of production, although they were originally intended for other BL products.

Chris 'Betson', BTW.
Paul Hunt

Jim-
In milling the cylinder head, be advised that the 43cc heart-shaped combustion chambers of the 18G, 18GA, 18BG, 18GD, 18GF, 18GG, 18GH, 18GJ, and 18GK engines have a depth of .425” (10.8mm). This being the case, removing .0099” (.25146mm) of material will result in the reduction of the volume of the combustion chamber by 1cc. On the other hand, the kidney-shaped 39cc combustion chambers of the 18V engines have a depth of .375” (9.5mm), requiring the removal of .0096” (.24384mm) of material in order to result in the reduction of the volume of the combustion chamber by 1cc.
Steve S.

This thread was discussed between 12/11/2008 and 15/11/2008

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