Compression test

My take on this issue for what it's worth is this:

The leak-down is a great diagnostics test when you have done a compression test and found there to be discrepancies. Then it can be a good tool to to isolate the source of the leakage.

By itself, it is only measuring leakage at one point in the bore, at TDC so it isn't really giving a full picture.

I personally think that a normal pressure test should be carried out at 200 hour intervals which would show a trend of normal wear compression losses over time and if unacceptable differences between cylinders are found, then is the time to do a leak-down test

The spec is two hundred hours. I personally do one every annual because I want to see if there are any trends and changes. A differential.compression test only takes about 15 min.