Checking compression

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kittypancake

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Okay...it's been a LONG time since I've checked compression...let's see if I remember correctly...
let the compression gauge needle jump 4x's

I remember something about '20'...is it all cylinders within 20#'s of each other, or 20% of each other, or does '20' not factor in at all?

also...how will the compression differ from a cold engine to a hot engine?
 
It should reach max pressure in ~4 compressions, but I would just keep cranking until the needle stops going up regardless. I go ~10 times in my M-B diesels. Some say a good engine will reach the final pressure in less cranks, but I don't see a reason. It is probably more dependent upon your compression gage. Also, crank w/ the throttle and choke held open.

The manual specs a variation between cylinders. Something like 10 psi. I would be more concerned that all cylinders measure good, which is probably >115 psig for a typical slant. 145 psig would be like new. The values depend a lot on the camshaft installed.

I think the spec is for a warm engine. Even more important is that it has run in the last week so that oil is everywhere. If the engine has sat for years, there may be no oil to seal the rings. If it won't start, you can pour a tablespoon of oil thru each spark plug hole. That is also one way to infer "bad rings", but a bit tricky to call.
 
I think where the "20" comes in is the readings on a dynamic check should not vary more than 20psi across the board, and on a static (differential) check, you should have less than 25% leakdown.
EG...175 on 7 cylinders and 105 on 1 = bad.
On a leakdown check, 175 (minus 25%) = 132 (minimum allowable)
Of course, both of these refer to warm engines that have been run and oiled, as was mentioned.
It's also important to not reject a cylinder based on one compression test reading.....
 
I always spin it over about 6 times. It's not gonna keep pumping up anyway. It will level off and stop.

Ever manual I have read says no more than 10% difference.
 
The right way to do it is to remove all the plugs and prop the throttle wide open' Then screw your tester in and crank it over. It doesn't matter if you give it 4 cranks or 6 as long as you do it the same for all cylinders. What you want to watch is that the first jump of the needle should give you 1/2 of your total compression (approx). The difference between your highest cylinder and your lowest should be LESS than 20% on a good engine.

You can also do a wet test by squirting 3-4 pumps (depending on your squirt can) of oil in the cylinder and following the same procedure.
 
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