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MG MGB Technical - Compression Test

Hi all.

I have a Gold Seal engine of unknown provenance which I am looking to use to replace the siezed and rusted original for my 69 BGT.

I have just done a cold compression test on it and these are the results:
1 = 50 psi
2 = 50 psi
3 = 140 psi
4 = 75 psi

Number 3 seems to be in the right area. Is it likely to be the head gasket that is causing the low values or should I be looking for more?

It is my intention to strip and clean it as much as I can before taking the head off, but that may be a few months away yet.


Tony Brough

Head gasket normally would fail either on one cylinder, or between two adjacent cylinders.

It looks like you've either suffered both of these, or there is another issue, such as valves not seating properly.

Depending on how long it has been sitting, the piston rings might also be sticking/not sealing.

Have you tried a squirt of oil in the bores to see if it brings the compression up?

Either way, it will need to come off for investigation.
Dave O'Neill 2

Thanks Dave

It has been stood for a number of years. I will try the wet test and see if it makes any difference to the readings.
Tony Brough

I would be prepared to replace the valves. I have pulled the heads on a couple of Bs and the midget and found burn or worn valves. It is easy to pull the head, have the work done and to get the car back into working order.
Glenn Mallory

Is this a runner or a removed engine?

Compression tests need to be done hot, all plugs out, throttle wedged wide open.

Thanks Guys

Paul the engine is out of the car, i am just trying to get an understanding of possible issues before cleaning and stripping.

One good thing, the oil pump is working...
Tony Brough

A cylinder leak test will give you a better indication of what to inspect rather than a compression test, if you don't have a cylinder leak tester or are unable to borrow one, you can always make one, with a spark plug and a hydrolastic suspension valve, and a tyre pressure gauge, and an air supply,
Andy Tilney

I would try squirting some RED_X down the bores and leave soak for a week. Don't forget to eject the surplus by spinning the engine over after the soak. Do this before you remove the head. This should free the rings and lubricate the bore. You could try squiring some down the ports to soften the carbon deposits on the valves. Be prepared for clouds of smoke when you do start it.
Paul Hollingworth

An engine that has been sitting around for years and of unknown history should be treated as just a bundle of parts inconveniently bolted together - only one course of action makes sense, take it all apart, check and rebuild it with the good bits, reconditioned old bits and replaced worn/rusted bits...
Chris at Octarine Services

Given it's uninstalled and the limited compression you can do, and their results, then you can only consider it as a potential donor after stripping and checking as per Chris.

Thanks for all the input. I have a while before I need an engine to put back in the car. It was always the intention to strip, inspect, clean out and replace what is required as I go.

I will crack as per Chris's post and see what see what Pandora has to offer.
Tony Brough

This thread was discussed between 06/07/2017 and 09/07/2017

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