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MG MGB Technical - Compression values

Hello all,

I am curious to know if the compression values on my B (1975 ex-USA, converted to SUs) are within the "acceptable" range.

Measured on a cold engine, with the throttle closed, engine reved until pressure reading stable:

Cyl 1 2 3 4
Dry 8.9Bar/128psi 9.2Bar/135psi 9.2Bar 9Bar130psi
Wet 9.9Bar 10Bar 10Bar 11Bar

Wet means with some oil poured in the cylinders.

My conclusion is that the compressions are on the lower acceptable limit, rather balanced between cylinders, but the piston/rings/cylinders are slightly worn (over 10% difference between dry and wet).

What do the pros say?

Thank you all

Eric Feron

The US cars have low compression pistons in them but I can't off the top of my head remember what the reading should be (seems about right though?). But as you say the fact the readings are similar is a good thing.

I thought when you do the compression check you're supposed to turn the engine over on the starter with the throttle wide open?

Simon Jansen

Eric. There is a tech article on the compression check, on the MG section, at should you desire to read it. Simon is correct that a compression check is normally run throttle fully open with a warm engine. That is the closest we can come to simulate the engine as the car is being driven (i.e. the throttle is partly open and the engine is hot). Your figures sound perfectly fine and might be a little higher with the throttle open/engine warm. Well within acceptable limits right now.

As to US engines having "low compression pistons", they were sent over having the 8:1 compression ratio rather than the 8.8:1 that the "high compression" engines had because, at that time, unleaded gasoline was only available in "regular" grade. What any engine my have inside it, after 26+ years, can only be determined by stripping and inspecting it.

If it runs well, enjoy it and worry more about the consistancy of the compression between cylinders rather than the actual readings.

Les Bengtson

Hello guys, this is really helpful. I will redo the compression measures tomorrow for the sake of completion.

I do have this very high oil consumption problem (see other post). So this might not be due to worn cylinders or valve guides... hmmm
Eric Feron


Looking at your compression test data and your statement that you have high oil consumption, the probably answer is that your cylinders and rings are worn and you need to think about a re-bore with new rings. Since this means taking the engine out of the car, you are looking pretty much at a total engine rebuild.

The fact that the compression went up with the injection of some oil into the cylinders is typically an indicator of ring/cylinder wear since the oil is now sealing the gap between the rings and cylinder wall. Had there been no compression change, then the indicator would have been valve guides or valves.

Since you have to pull the engine (most likely) if you are going to repair the situation, the question is whether it is more cost effective for you to have the engine rebuilt or to purchase a replacement engine. If you want total originality, then rebuilding is the way to go; but if you don't care about having the original engine in the car, you might be able to buy a refurbished engine for less cost (and maybe get a bit more HP in the bargain).
Ron Kluwe

And taking the engine out can be an expensive experience since all the 'might as wells' get you. Since it's coming out might as well change the clutch, might as well change the cam, might as well do some head work, might as well have it all balanced! Still you end up with a nice, new engine at the end of it all.

Simon Jansen

Interestingly, others elsewhere tell me that my compressions are within perfectly acceptable limits.I redid the test today on a warm engineand full throttle an dot all values up by about 10%.

Looks like their is some life in the old lump still...

As mentioned in another post, I was burning enormous amounts of oil, until I discovered (yesterday) that disconnecting the crankcase breather from the carbs solves that problem: exhaust fumes clean as a whistle...

Me happy :)

Eric Feron


What have you done with the crankcase breather? When my engine was on the way out disconnecting the engine breather did stop the smoke out the back. However I put the breather into a home made catch tank (a plastic milk bottle!) and found I was collecting a lot of oil. This was due to worn (and some broken) rings that caused a positive crankcase pressure. If you have just left the breather open to air you may find you end up with a very dirty engine bay!

70 BGT
I D Cameron

A number of things can contribute to pumping oil up the breather, in one case at least replacing the front tappet cover was the only thing that cured it. The catch tank is quite a quick and easy way of determining if that is the cause of your smoke. You can check crankcase pressure by putting a sheet of paper over the oil filler hole at idle. If mine is anything to go by whilst the paper flutters when held very close, when it is placed right over the slight suction holds it there without fluttering. North American engines with the vent port on the rear of the rocker cover may need to have this temporarily blocked to get the same result.
Paul Hunt 2

Hello folks, this is all very helpful.

But one crucial piece of info is missing so far: the confirmation that my compressions are correct or poor.

I believe that only this can tell if the rings/bores are worn or not.

Can any experienced pro positively confirm this?

My engine is a 18V ex USA, so compression ratio 8:1

I believe that the values I got (see above) tell that the rings and bores are aged but still ok (or am I fooling myself from a rebuilt engine?)

(Note that NO blue smoke comes out of the exhaust and no pinking is observed when the crankcase breather is not connected from the carbs. The crankcase breather does spit out substantial amounts of oil, causing blue smoke and pinking).

Eric Feron

I'm no expert, but from everything I've read your compression numbers are a tad low but perfectly fine. Most of the compression checks I've heard about on anything other than newly rebuilt MGB engines seem to run 125 to 135 psi per cylinder.

My own un-rebuilt engine routinely shows somewhere in that range. Here's how my mg specialist put it: "the engine is a bit tired, but it should run fine". Indeed, except for having to rebuild the head due to valve seat recession, it has run perfectly fine for my taste. Although if you care about performance, the car might lack a little punch.

In your shoes, I'd leave the engine alone and keep driving the car as-is. As has been noted, removing and rebuilding the engine can be costly and time intensive. You should be able to keep driving the car normally unless the compression numbers dip down to 110 to 100 psi range.


BTW, Les' article about the compression check is very helpful. Here's a relevant excerpt from the article:

"First, cylinders with less than 100 psi compression (dry) are probably not firing efficiently and, if the compression is much below 100 psi, may not be firing at all. Secondly, most MG engines in good condition will read about 150-160 psi maximum. If you have more than this, you probably have modified, high compression pistons (assuming the standard camshaft is still being used) or the cylinder head has been milled significantly. The normal range, on an MG, is between 120 psi and 150 psi for an engine that is operating well. A newly rebuilt engine will be slightly higher than 150 psi after it is broken in and may be somewhat lower than 120 as the rings and bores are mating themselves together. The biggest concern is consistency of the readings. The closer the cylinders are to reading exactly the same the better. This means that, all other factors being equal, each cylinder is operating exactly the same when it fires. This will give the best balanced engine from a smoothness standpoint."
J. Palgon

This thread was discussed between 14/05/2006 and 24/05/2006

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