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Compression test


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Have you done a search here or elsewhere?  This is such an old chestnut, but everyone has to learn!


1/ A compression gauge is NOT a precision instrument.   Never compare your readings with someone else's, unless done on the same day with the same gauge!

2/ The gauge is most useful to compare cylinders along the block.   Ideally they should all be the same, but +/- 5psi is acceptable.


Procedure (have pen and paper to hand!)

A/ Warm up the engine to operating temp.

B/ Remove ALL the spark plugs

C/ Use wire or some other means to hold the throttle WIDE OPEN.

D/ Install the gauge into No.1.    Tighten it!

E/ Turn the engine on  the starter until you get a constant reading on the gauge.  This may take five or six turns.

F/ Note the reading.

G/ Repeat for the other cylinders

H/ Then  do it again.    This takes practice and at least two runs along the block will be a start.  10,000 hours not required!


If you have any cylinders that are significantly low, then tip a teaspoonful of engine oil into that cylinder, turn the engine over once and repeat the compression test.   If pressure reading does not increase then there is a valve problem, and not bore wear or piston ring failure.

Good luck!


Edited by JohnD
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