G'day Dmitr and all
it concerns me that when I compress air in through the head, it leaks past the piston when the piston is at the top.
Yes it's perfectly normal for pressure to leak past the piston
With a pressure test ,the piston should be at bottom dead centre with a pressure test ,you are checking for leaks in the complete engine not just the top end and remembering piston rings have a gap at the ends so by design will always leak ,as we know the bigger the ring gap the more the rings will leak.
I was thinking it may have been the starter O ring as some o rings won't fit in properly when installing unless you give them a coating of rubber grease before installing.
The pressure test would have confirmed a crankcase leak and the starter is the easiest part to check first.
Buy a new O ring and put some rubber grease around it, if it's a good quality O ring and you don't have rubber grease Vaseline can be used to lubricate the O ring ,then fit the O ring in the starter and then install the starter evenly.
I never use silastic anywhere near fuel as it's not compatable ,everytime I put fuel on silastic the fuel just disolves the silastic ,not sure how Norm gets silastic to work with petrol as the silastic always breaks up for me .
There are a lot of different grades of silastic ,it's just the ones I've tried haven't worked with fuel.
Cheers
Max.
