Quote Originally Posted by Oddbob08 View Post
A thread I came across on the collectable thread suggests that BSA fitted grub screws to the tp of some rotary breeched rifles, mine doesn't appear to have one but they have been found on all three calibres used by BSA. Screwing this in or out affects the pressure in the tp and ultimately the power.

Going back to my problem, if I may (please say if I'm 'hijacking a thread'), I've been to the shed and measured my RB2 piston.

Front to rear of the piston itself is 134.9mm, the slot is 100.7mm long and ends 14.8mm before the tail-end of the piston. The rod extends 56.6mm past the end of the skirt and the bevelled cut (which latches with the sear) is 11.3mm from the tip. The piston diameter is 28.8mm at it's widest. I forgot to measure the cylinder of the gun, specifically how far past the cocking slot in the cylinder the cocking slot in the piston goes.

I can't tell if it has a steel top hat fitted or this is just where the cocking rod is secured.
The lump at the bottom of the piston is where the rod is attached to the piston, I had to build up the space around that lump so that the delrin top hat could sit on a flat surface, regarding the adjustable transfer port, I was once told by Tony Wall at SFS that he used to back the screw out so the TP was fully open and then tune the rifle from there, apparently you can then us less spring.

Pete