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MG Midget and Sprite Technical - Compression Figures

I've just done a dry compression test on the frog and got the following figures: 1-165 psi; 2-180 psi; 3-175 psi; 4-165 psi. They don't seem that good to me cos they're uneven, but I can't remember what sort of difference is acceptable??
It's 1330 cc with a 276 cam and HIF 44, if that's any help. Any suggestions , idea's?
Graham.
Graham P 1330 Frogeye

They aren't too far off...about 10% is OK IIRC

Do you have a problem which prompted the compression check?
Dave O'Neill 2

I overheated on the way to the Battlesbridge show on the 10th due to a leaking top hose joint and it has a tendency to run hot since. Thought it might have blown a head gasket, but since then it hasn't lost any water, there's no mayonaise in the sump or rocker cover, so I bought a compression tester to double check, but couldn't remember what the tolerances were!

I suppose it's possible that a load of sediment was stirred up when it boiled so I've bought some flushing liquid and I'll give the frog a good clean out this weekend.
Graham.
Graham P 1330 Frogeye

It might be worth checking the thermostat hasn't suffered any damage.
Dave O'Neill 2

I would re-torque the head, reset the valve clearances and retest. It is unlikely to be gasket problem with those figures. More likely just variations in the tests contributing to the normal accepted 10%. They look acceptable to me. I presume the test was done on a warmed engine with WOT?
Guy Weller

The last time I did a compression test on my midget - basically standard 1275 engine - it returned 180/162/168/170.
Andrew Noakes

Dave, I'll replace stat as I want to replace stat housing anyway.
Guy, warm engine but what is WOT??
Andrew, very similar so I'll breath easier,
Graham.
Graham P 1330 Frogeye

Sorry Graham, assumed it was a common abbreviation as I have seen it on here so often.
Wide Open Throttle :-)

The vulnerable bit on the 1275 gasket is the narrow wall between 2 & 3 so low figures for those two can indicate a failing gasket there. But your figures don't follow that pattern so are probably good.

Guy
Guy Weller

If you have all plugs removed when doing the test, and the gasket is leaking, then you will usually hear hiss from the leakage through the other spark plug hole. (For example, if testing no 1 then you would hear a hiss in no 2 if the gasket was leaking between them).

This is also a useful clue for a leaking exhaust valve in the centre two cyinders.

Although I have never checked for it, with a 276 cam you might also expect the centre two cylinders to be a bit lower due to the robbing effect from the siamesed inlet ports. This arises from the end cylinder at the beginning of its inlet stroke overlapping the beginning of the compression stroke of the next door middle cylinder - just before its inlet valve closes. This causes fuel/air to be sucked back out of the centre cylinder through the siamesed port into the end cylinder. It's the effect that scatter cams seek to minimise, it's effect on BHP is measureable
so presumably it could show up on a compression test.
Paul Walbran

Guy, taa, it was WOT.
Paul, unfortunately it appears to be the opposite to the theory! But at least it doesn't look as if 3 & 4 are mutually leaking.
Graham.
Graham P 1330 Frogeye

This thread was discussed between 23/05/2009 and 24/05/2009

MG Midget and Sprite Technical index

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