Quote Originally Posted by exftrplt View Post
Can anyone tell me where I can find how the size of the calibration orifice in the compression tester was determined? Theoretically would the orifice size change for each type of engine?
.040" for your standard Aircraft engine compression differential tester. A .060" special orifice can be used for big bore engines although most don't bother. Most mechanics will compare compression reading against the calibration port on many of the testers. The reality is, the actual compression numbers people seem to hang their hats on (because that's all that gets recorded in the logs), aren't terribly meaningful as they vary significantly with the displacement and wear of the engine. If a cylinder is leaking, what is more important is where it is leaking, and how much, which is where the number comes from. A low compression reading typically warrants a boroscope inspection. If it's a leaky exhaust valve, the color and pattern will tell right away if the valve is burning or just has a piece of crud caught under the face. Leaky rings may just be a stuck ring, but a boroscope inspection will tell if there is significant scoring or sometimes the plating coming off the upper cylinder face.