[Nasher]

Hey Mr G

I wasn't going to bother with crack testing the piston crown, but it could be a good idea.
I'd use my method of Paraffin and talc or chalk dust.

I think that wherever the compression is escaping from must be quite a hole to measure Zero on my gauge, especially as there is actually a decent puff out of the spark plug hole when the engine is turned over with the plug out.