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

[brucegill]

Been out of the game for a long time now and can’t find any of my old notes on 77k with a 25mm piston. What’s the best starting place for these? 3.6mm springs to mind but I don’t trust my memory lol. .22 if that maters...

[Shed tuner]

Been out of the game for a long time now and can’t find any of my old notes on 77k with a 25mm piston. What’s the best starting place for these? 3.6mm springs to mind but I don’t trust my memory lol. .22 if that maters...

holy revival - bruce is back from the dead - long time mate, how are you ?

Start with 3.4 and test

[brucegill]

holy revival - bruce is back from the dead - long time mate, how are you ?

Start with 3.4 and test

thanks Jon, nice to be back 3.4mm it is.....

[Garry]

I know nothing about this subject as my following question will indicate

I'm assuming the transfer port is directly behind the pellet, so why aren't the ports the same size as the pellet calibre?

[Shed tuner]

I know nothing about this subject as my following question will indicate

I'm assuming the transfer port is directly behind the pellet, so why aren't the ports the same size as the pellet calibre?

'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

[Rich]

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.

[Shed tuner]

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.

yup, I have a taper reamer that goes from 2.5-3.5 perfect range. I also have a couple of 3.0 and 3.2 parallel reamers, as it's amazing how often that's where the sweet spot ends up (even allowing for the variation in port length, calibre, piston weight, preload, piston mass, and the phases of the moon )

[accused]

Being around in the 80's we drilled them...no chronos just the "thats better through the bit of wood" scientific approach.
I think we did see improvements up to a point but I really don't recommend this type of Tinkering.

Leave them alone or try to limit it to 3.5mm unless you can make plugs and revert if required.

I can't deny its addictive to tinker with springers though.