JIm,
If you have the contra in my head so it is protruding past the lip you wont have to worry about improper scavenging - you will however have to worry about a damaged contra and/or piston! If its that low there is something wrong, as the combustion chamber volume will be near zero. I know they will hang there due to the twin O rings its just that its not possible in normal operation for the cp to be that low.

AJC