Thumb Compression Check

Bob Bidonde

Bob Bidonde

Well-known member
Somewhere I saw a sketch of using the thumb to do a compression check, but I can't find it. I would appreciate a post of a picture or link to one.
 
I have always considered that if each cylinder has enough compression to force my thumb off the spark plug hole as the starter turns over the engine, that cylinder is healthy enough to drive. My rule of "thumb" (ha, ha, ha!) is that if I can keep my thumb on top of the spark plug hole while the starter rod is depressed, then that cylinder has ring or valve issues to deal with, possibly a blown head gasket near that weak cylinder. The compression is likely below 30psi. This quickie test can be performed by one person standing on either side of the engine, although it's a little easier from the driver's side because you won't have to lean over the engine to push down the starter rod. I leave the other three plugs in place so that there's no "bleed over" between cylinders from an adjacent cylinder.
Marshall
 
quicker to just feel with the hand crank,
pull up against compression, see if compression holds evenly, if you relax the pull how much it springs back.
won’t work on a new engine that the bearings are still too tight, it has to be able to rock on compression at shutdown.
If you take the top distributor cap off you can watch the rotor position to tell which cylinder
 
Here ya go...
 

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Just don't do it THIS way...Turn the thumb OVER to check cylinder compression health!!! Ha, ha, ha! (Akin to "Green side up! Green side up!" Remember that ethnic joke from the 1960's that we're not allowed to find humorous anymore???)
Marshall
 

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Guten Abend,

I also know that an experienced master checks the compression with his thumb and can state whether the compression is reasonably even and good or bad.
 
When I started turning wrenches for a living I smashed a thumb, the old guy that I worked with said “when you get home tonight soak it in cider it will make it feel better “ I was young and naïve it took me awhile to figure that out LOL
 
Yes, the thumb test is quite reliable and simple to interpret to get a pretty good idea of the engine's cylinder health:
* Thumb is pushed off = decent to very good compression; the more force that pushes the thumb off the hole, the stronger the compression; listen for the "POP!" and "WHOOSH!".
* Thumb is NOT pushed off = poor compression, internal problems in cylinder need to be addressed;
A compression gauge will simply put numbers to this simple go-no go test.
Marshall

ADDED: By the way, a warmed-up engine will give slightly stronger compression than a cold engine. Take that into account if a particular cylinder is borderline strong or weak. But if it's that close to failing the test, the cylinder in question is probably weak anyway.
 
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Maybe after using the thumb check for compression one might just hold on to the spark plug connector to check voltage. What would one use to check vacuum?
 
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