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MG MGB Technical - Oil Consumption follow up

I have carried out a compression test on the engine. The readings dont tell me anything, hope someone else can interperate them.
carried out on a cold engine, all plugs out, engine turned over until the readings did not increase.

Dry Wet
Cyl 1- 155 160
" 2- 175 190
" 3- 145 180
" 4- 140 200

Wet readings is after 5-6 squirts of oil from oil can.
I was abit surprised at the differance between the readings.
Trev

Trevor Harvey

Well going back to your original post and your mention of a rebore - you can count on your new pistons but I am not sure about the rest of the work. I hope Peter B might come in here with some professional advice. I really feel for you.
Roger W

Depends on how much oil you put in to judge the increase. But something fishy with #2 being so much higher in the dry test.
Art Pearse

Well, no matter what interpretation you give to the numbers, it doesn't look good. Assuming you did the test correctly (either same number of hits on each cylinder or wait till the compression stops climbing) the dry numbers are spread out pretty far. Not more than about 10% between the lowest and highest is preferred.

Around a 5 or 10 psi jump even on a tight engine is pretty common. Just adding oil reduces the effective combustion chamber size, so this will increase the compression even if the oil is not sealing anything. This is why you try to get the same amount of oil in each.

Your increase on 3&4 is a lot. Unless you really put a lot of oil in those compared to the others, it seems the compression rings are not doing the job. I suppose it is possible for the oil control ring to seat while the compression rings did not, but I don’t think it is too likely. Technically the oil is testing the compression rings rather than the oil control rings.

I suppose you could consider the Bon-Ami approach that I mentioned in your other thread on this subject. And, it can’t hurt to try the additive mentioned in your other post. You might try that with a different oil. See what others say, but I think 30 or 40 wt non-detergent was common for break in. Also, perhaps some hare driving and not holding constant RPMs may help.

Though some may shudder at the thought, I have honed rusty cylinder walls without removing the pistons; once on a Model A John Deere and once on a friend’s Toyota truck. Get two of the pistons all the way down, smear a bit of thick grease between the wall and the top of the piston, and when you are done honing use a lot of motor oil and a clean rag to clean the walls and piston top. Then, move the other two pistons down and do those.

Charley
C R Huff

A leak down test might be more significant to check the rings. The trouble with wet tests is the vol of oil added, say 1 squirt is 1 cc then 6 squirts equals 6cc equivalent of skimming/milling 60 thou off the head, will give a big increase in CR.

Peter
Peter Burgess Tuning

Peter, What is a leak down test?
I Will do another wet test. I will put less oil down and make sure its the same amount in each, didnt think about the increase in CR.
Trevor Harvey

I would offer that I do my pressure tests on a warm engine ie run to operating temp and allowed to cool a little to allow handling of the plugs.
R Taylor

I have carried out a second compression test. taking more care that all are done the same. Less oil was put in for the wet test and given time to settle around the bore. The readings are suprissingly different.

1--135--155
2--140--150
3--140--155
4--155--160

These readings to me look better, can I assume there is nothing wrong with the pistons or the rebore, I understand there could still be a glaze problem.
Trev
Trevor Harvey

They need to be done on a hot engine with the throttle wide open, as well as all plugs out.

If you get similar figures with that, then 4 would look like a rings/bore problem with that big a difference between dry and wet, with 3 not far behind. Whilst not much difference dry to wet with 1 and 2 there is more difference than one would expect between them on both counts, possible valve seating issues.

If the throttle was closed the difference between 1/2 and 3/4 could just be down to the carbs not being adjusted correctly.

P Hunt

Just noticed the customisation had changed for some reason so what I thought were the most recent figures were the original. Hot engine and throttle wide open still applies, and although the later figures are more balanced they still look low, the Workshop Manual quotes 170 ft lb and I'd expect that to be dry.

BTW, it's better to append to the original thread than start a new one for an update as then we have the whole story in one place. We can tell by the date that it is an update.
P Hunt

I have done yet another test as advised, on a hot engine and the throttle open, suprise suprise I got readings of 170/180 on all cylinders.
Would this mean that the heavy oil consumption is more likely to be valve seals/guides.
Trevor Harvey

Trevor - I hope the high consumption relates to your use of synthetic oil. You now know the pistons are good and a reasonable match to the cylinders, the compression rings are doing their job and its a fair assumption the oil control rings were correctly installed. Low compression on a cold engine is not unusual on older engines and burning oil is not necessarily a symptom of something drastically wrong- Some engines just do - I have a Turbo V8 Bentley straight out of the factory in mind - it was using more than yours.

The question about the quality of the honing remains - you question the valve seals. I suggest you drive her a thousand or two miles with a high detergent diesel oil having checked the crank breathing is not blocked - watch consumption carefully and compare any lengthy journey with the day-to-day run around. If consumption goes up you may have some work to do - if there is no change - you know how to deal with it.

It was interesting to read that the US drivers consider bedding-in additives - they know a thing or two but I suspect we are hearing about abrasive agents and I would worry about it getting into the bearings.

Hope this helps.
Roger W

Reasonable compression does not mean the oil control rings are working correctly, it means the compression rings are working. Remember the engine is burning oil and it could be all 4 cyls burning. The oil on the bores will also increase CR. The engine only burnt oil after rebuilding. Some oil control rings are difficult to fit and ensure they are ok. these are usually Grant's rings. The expander can sometimes overide rather than butt up and render the control rings virtually useless. It would be nigh impossible to get that much oil down valve guides. Bad/No honing could also exacerbate the problem.

Peter
Peter Burgess Tuning

This thread was discussed between 26/04/2013 and 28/04/2013

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