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Compression Test- Do it Warm or cold?

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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.

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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.

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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.

later

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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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