Burp oil before leakdown test???

Since I love research I did a test a number of years back. I took 12 aircraft with Rotax engine's.. Did a leak down compression test while the engine was hot and then the next morning when it was cold. Never burped the engine. You know what the difference was with all 12 engines? Anywhere from no difference to 1 psi. Continental's and Lycoming's are lose tolerance engines and should be warm, but Rotax is a nice tight close tolerance engine. Even if you had a 2 psi difference it isn't a big deal because you're looking for a number within a range and not just a single specific number.

Thanks Roger. I always value your replies and experience here. However I'm failing to see the connection between a hot/cold engine and one with oil remaining in the crankcase and oil return line to the tank.

Hot engine = minimum oil in crankcase.

Cold engine = depending on relative height of reservoir/oil tank, may have significant or minimal oil in crankcase

In my experience when doing a leak-down test on a Rotax the pressure in the cylinder will bypass the rings and pressurise the crankcase. This in turn will then force any oil sat in the bottom back to the tank pretty quickly. To affect the leak-down results you would probably have to have a pretty good seal in a number of places to stop the pressure leaking out via other cylinders where the valves are open.

In summary I don't think that you need to worry about it, if you have a problem with a particular cylinder you will hear the air leaking out somewhere.

Why? You’re checking cylinder compression not crankcase.