I don't believe a TP smaller than 3.0mm ever works in a 12FP springer....

[TORNADOS7]

Leave them alone unless you can make plugs and revert if required.

The very reason I haven't touched the Xocets transfer port buddy...

[Garry]

'cos that'll be too big
Interestingly the old pre-wat BSA underlevers were made like this, as the loading tap was reamed through. They benefit massively from being sleaved down to 3mm or so - more power, less harsh..

Another practical reason is to stop your pellets falling back into the cylinder through them

Any void space has to be filled with the air that the piston is compressing, so a port larger than necessary will also reduce the maximum potential pressure.

A while back I made a number of screw-in ports for a chap who was experimenting with a Walther rifle. The screw-in ports started out as brass grub screws that would screw into a tapped 'port', and they were all primarily drilled 2.0mm. Then with a taper reamer I opened them out, one by one, from 2.1mm in 0.1mm increments up to 3.0mm which was the largest he wanted. I reamed them back to front, if you follow me, so that the hole in the port diverged marginally, as this ought to give a better flow characteristic than a port that got smaller.

I made the taper reamer from silver steel and the taper was chosen so that I could easily judge the bore size by the depth the reamer had entered.

I think a taper reamer is a better way to go than a twist drill; it would be remarkable if a twist drill was that accurate, as any imperfection in the grind will make it cut oversize.

Thanks both, dimly beginning to understand now.