how to test compression

[faredce]

hey guys just starting owt in the world of motorbikes and have just purchased my own aprilia rs125 i see ppl talking about compression test.
how do u do a compression test, equip etc and when do u need to do one, how can u tell when one is neccessary?

thought this would be the perfect place to ask

[surfer]

Basically if your bike is loosing power/not going as fast and you want to know why doing a compression test will indicate if your cylinders are worn and the if the rings need replacing.

[Soul.Trader]

You need a specific tool to do it - they're not overly expensive, but it's not really sensible for the home mechanic to own one, because it wont get used that often. Should take 15 minutes for your local shop to do a compression test.

Basically, all they do is remove a spark plug, put the compression tester in the sparkie hole, unplug the HT lead and turn the engine over a few times to see how much compression your cylinders hold. Then you can put a few dabs of oil into the cylinder and perform the test again - if it holds substantially more pressure, your rings need replacing.