Leak down is a standard test on an airplane engine because the cylinder is tapered (tightest @ the top). When you perform this test on any engine you need to bring the piston up to but not past TDC as the ring seat changes from bottom land seal to top lan @ TDC. You can get very different test results if you go past TDC. As stated above leak down is an excellent diagnostic tool. One other technique used on aircraft is to stake the valve, meaning a small particle on the face of either valve can show excessive leak. A socket placed on the valve spring and hit with a hammer can clear the partical and improve the test results.

Look at this link. Good poop and a real world example of why leakdown is a necessary part of engine troubleshooting.

http://vmaxoutlaw.com/tech/leakdown_tester.htm

Happened to stumble on this thread and find some of it interesting.

I have comp tested and leakdown testing so many marine engines in my day but sometimes found neither one to be completely 100 % solid at pin pointing internal engine problems that you would think it could or should have.

Anyways thought it was a good thread if anyone wants to add experiences or anything to it.

In the last 6 years many outboard techs leakdown 2 STROKE outboard engines and claim to others, throw out the comp gauge as leakdown testing by far is the way to go. I am not a believer in leakdown 2 stroke outboard engines. Just adding this info on top of.

Not one comment at all ? Speechless up here huh ? Anyways thought someone would have something to say about experiences good, bad or indifferent.

I'll add to it and say again what Ive said in past, when you build a NEW motor after rings are seated after dynoing it a leakdown test at that point is VERY good for future reference!! If you come off dyno and ypu have a cylinder with high numbers would be a REAL good time to tear in and find out WHY! My recent 540 I freshened had 3-4% leakdown after thrashing on dyno both NA and with blower, if it was going back in a boat every season when I was winterizing it I would again do a leakdown, gives you all winter to address issues vs hoping for the best and wondering, fwiw, Smitty

I like that.