Compression Test- Do it Warm or cold?

My manual says to perform compression test after engine is warm, however some say do it while cold. Go figure. I think since the engine is using its compression while warm it makes sense to test it while it's warm. Any suggestions, maybe do it both ways and compare.

Warm is the correct method.... :cry:

It's really up to the one calling the shots.

Both will give correct readings as long as you do it cold when they say cold, and warm when they say warm.

Thanks, Last night I removed the spark plug after 829 miles of use for an inspection. To my amazement the thing looks like brand new! Not bad for a 1983. Some one must have taken care of it or recently done the top end, but i'm just tickled pink. With a plug in that good of shape I said no need for a compression test. No wonder i'm getting and average of 58 miles to the gallon (+,- 2 miles). Actually i was ready to do the test when i realized i didn't have the correct fitting. I actually drive my xr500r to work, about 26 miles oneway, go about 60-65mph about 2 days a week. After work on the way home their is a track i jump on for a little fun.


I'm not a certified wrench, but I'd think cold because the warm engine (heat expands) would "tighten" up, whereas a cold cylinder would afford you the worst case look @ leak down.

0-5% is the spec for high performance. If you get that while cold, I would think it's only better warm. But if the manual says test while warm... :cry:

Edit: Here's an example of why I think cold...

Before reshimming recently, my bike was very hard to start when cold. Once warm, it started fine. So when the cylinder/piston and ancilarry components were cold, there was less air pressure on the offending valve(s) and therefore not much compression. Once warm, the alloys/metals/whatevers by expanding created a bit more pressure, thus allowing the pressure to build easier and start. That's not to say I didn't have a clearance problem when warm - because I did. The bike would start easy, but not idle. So with a warm engine, I had a lower test reading - great, but why not try and get it as close to 0% as possible by testing cold?

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