So made a top hat and a cast iron ring (two actually) and couldn't get them into the groove the o ring dimensions are too big for the cast ring to work,so made a synthetic one from the precious material I have been using in the tx
Built it all up with a tx spring , but needed a lot of pre load, and it made 11.2 with 7.9 and 10.8, with exacts only had 4.53 to hand I think it would do 11 on 4 .52, this is on the standard transfer port. The tx spring is too close in diameter to the bore of the piston and with the large pre load (50mm) I can feel it catching on cocking, so plan b is a 97 spring smaller diameter and a bit longer .

https://photos.app.goo.gl/g8JVFUNDqYmKMRPy9