Measuring compression.

[mcostello]

Since troubleshooting small engines seems to come up regularly, I was wondering if a compression gauge would have any meaningful numbers? Such as spinning an engine over with a drill motor. Might help someone find a problem.

 

[XD351]

If you just want to test for leakage maybe a leak down tester ?
Compression gauges are good for comparing compression between cylinders to see if you have a bad one .

 

[GailInNM]

I find that they can be useful, mostly for comparative measurements as in multi-cylinder engines. I have built two compression gauges. Here is a simple one that is useful with engines of at least 5cc displacement.The problem with getting any absolute measurements with small engines is the dead volume of the /measuring gauge and iwith this type of gauge it varies because of the piston movement in the gauge.But it is easy to build and only costs a couple of dollars for the tire pressure gauge from the discount auto supply store.

I have also bluilt a gauge using an electronic tire pressure gauge which uses an electronic sensor and stores the max pressure. It is a bit more complicated to build but still under US$10 to make up. The dead volume is much lower and therefore more ulseful on lower displacement engines. I don't have a p hoto of it at the moment.
Gail in NM

 

[gbritnell]

The biggest problem in trying to measure compression pressure in small or tiny engines is that the check valve needs to be right at the end of the threaded adapter otherwise you're adding volume to the already small combustion chamber. If your spark plug has a 1/4-32 thread then the check valve assembly is going to be quite small. For hit and miss type engines it's a real pain because most of them are started by hand so unless you lock out the governor and make an attachment to spin the crankshaft you won't get a reliable reading.
gbritnell

 

[XD351]

Never thought of using a digital tyre gauge !
If you were to attempt to make a compression gauge for a small engine the digital tyre gauge modification would be well worth the effort .