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I am looking at buying a second hand 2-stroke engine. I have seen a lot online about checking compression before buying used engines. I don't have a compression tester, but will buy one if necessary.

My question is, if the engine starts and runs fine, does that automatically mean the compression is okay, or should I still check the compression?

Thanks in advance!

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You cannot assume just because it starts it has good compression. It can have minimal compression and still start and run just fine. That minimal compression is going to be different for every engine, so you'd have to look up the specifications for the exact engine you're looking at. The deal with it is, the more compression the better it will run (make more power). It can still make enough power to run with low compression. Really, checking the compression is the only way to be sure.

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Engines can run with compression below the intended spec. But the compression amount alone is not the whole story.

On multi cylinder engines differences between cylinders can be an indicator of wear or damage.

Damage could mean worn or broken piston rings, damaged pistons or cylinder surface, burnt valves or valve seats, a blown head gasket or a cracked head.

A good reading (within spec and consistent between cylinders) is a good indicator the engine is healthy, and should give you confidence when buying.

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