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MG MGB Technical - Compression Test

Just did a compression test on my 78 MGB.Results:

#1 125
#2 130
#3 85
#4 125

with oil the #3 came up to 105

Compared to the test I did last January:

#1 122
#2 123
#3 112
#4 128

I drive the car once a week and it doesn't noticably burn any oil. Rings, valve, in need of a total rebuild?

George
G.E. Bulwinkle

Did you adjust the valves before you did the compression test? If so, you may have a weak or broken compression ring on #3 cylinder. Since the compression numbers came up substantially after adding some oil, I don't think it is a valve or a head gasket problem. RAY
rjm RAY

Adding oil only works to seal rings IF the oil gets on the rings. In the old days, from whence springs this technique, pistons were almost invariably flat or domed, so oil ran down over the rings. With dished pistons, the oil lays in the dish and does not get to the rings, but it does reduce the unswept volume, which raises the compression. The oil will then splash up and help seal a marginal valve/seat.
My guess is you have a bad exhaust valve/seat on #3, with the others not far behind. Broken compression rings are not very common unless the engine has sat and been stuck from rust, or has been severely overheated. A pressure test will show whether the leakage is through valves or rings.
What is the mileage and do you know if it ever had a valve job?

FRM
FR Millmore

FRM

The car has 130,000 miles on it, but the head was replaced last winter with all new valves. Reason for replacing the head - overheated and could not be solved by replacing the head gasket. Could be that one of the valves is sticking or burned. I suppose the next step is to take it to a shop that can determine if its a valve or rings.

george
G.E. Bulwinkle

The overheating might well have resulted in a ring breaking. At 130, the rings (and all else) are pretty suspect anyway, if the thing wasn't reconned at some point. Also, at 130 cam wear might be drastic, more than enough to give bad/variable compression readings. Measure lift on all valves. Were the last year readings after the head swap? I see that #3 was a bit low then. Also possible that a valve is sticking, especially if bronze guides were used.

FRM
FR Millmore

Yes the readings taken last year were right after the head swap. I bought the car with 119,000 miles on it (ten years ago). At the time the complression readings were 130,128,132,129. I did replace the cam prior to the head swap- #2 intake was almost flat.

George
G.E. Bulwinkle

I'll change my guess to just plain wore out! Cams are usually bad enough to replace by 50,000, though they may still work OK, and certainly by 100, so I'd say your engine has never been rebuilt. The overheat likely finished it off.

FRM
FR Millmore

Thanks FR. Probably time for a total rebuild. I've rebuilt a number of American and VW engines, but this will be my first MG.

George
G.E. Bulwinkle

A bad valve is generally audible in the exhaust or intake. Those figures would tend to indicate rings/bores, but given the time and apparent mileage since the last good dry test that seems less likely. No big deal to get the head off, then you can put petrol or paraffin/kerosene in the combustion chambers and see what leaks out.
PaulH Solihull

OK I got the head off. All the valves looked good and zero leakage when I put kerosene in the combustion chambers. Time for a total rebuild.

George
G.E. Bulwinkle

Did the leak test again more carefully. #3 Combustion chamber was leaking. Hand lapped the intake and exhaust valve in #3 combustion chamber. Not leaking.

George
G.E. Bulwinkle

Head is back on.Compression readings:

#! 124
#2 129
#3 119
#4 123

Close enough!

George
G.E. Bulwinkle

This thread was discussed between 30/12/2010 and 12/01/2011

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