Compression test-how to?

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Bob McCarty
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Compression test-how to?

Postby Bob McCarty » Wed Dec 29, 2004 1:44 pm

I've never checked compression and could not find a previous post with enough details, here's what I understand, would you make any corrections for doing it properly:

1. warm up engine
2. remove all four spark plugs
3. open throttle wide
4. insert compression guage in one of the holes
5. turn over engine four or five times, or until reading no longer rises
(will hand cranking accomplish the same as using an electric starter?)
6. repeat on the other three
7. repeat as a "wet test" to compare results. I understand this to mean
that it is done with about a tablespoon of 30 wt oil in each cylinder
prior to cranking.
8. results indicate health of engine, ie. rings and valves

Does that sound right? Thanks, Bob

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Postby Cub-Bud » Wed Dec 29, 2004 2:10 pm

Sounds good to me. 8)
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Postby Donny M » Wed Dec 29, 2004 3:10 pm


The hand crank will accomplish the same results but will be harder on you :lol: :lol: 8)

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compression testing

Postby mike mix » Wed Dec 29, 2004 4:09 pm

The steps you list look good. If you are hand cranking the engine the readings will be much lower. Keep in mind you would like the readings to be within about 5% between each cyl. If all readings are about the same say 100psi using a starter motor to crank the engine, it is not necessary to add oil the cyl. because the engine mechanical condition is good. If you have one cyl. that has a low reading say 100 80 95 95 add oil to the lowest cyl. 80 psi and redo the test. Readings comes up possible bad rings, reading stays the same possible bad valves. Two cyl. with a low reading say 100 80 80 100 adding oil does nothing, possible bad head gasket. When you get your compression numbers post them and hopefully someone can lead you in the correct direction.. Mike

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Postby John *.?-!.* cub owner » Wed Dec 29, 2004 7:37 pm

You have it correct, but hand cranking will usually give a lower compression reading than starter.
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