Piston valves, square or round ports?

Currently building a 5“ Tom Rolt as drawn by Don Young. I’m going to use ptfe or similar for the piston valves rather than DY’s forced fit iron ones. Should I use round ports in the valve liners to minimise risk of damaging the ptfe bobbins or should I stick to square ports? Cheers.

IMHO (and relatively ignorant opinion), I'd always use square ports.Steam admission & exhaust is sharper. Filing round holes square in liners is easy enough, if tedious.
I'd caution against using "straight" PTFE for rings; much better in terms of resilience to wear if you get the graphite -filled PTFE. There was an excellent article produced by "Artisan" some while ago, extract here.......:-
"Virgin PTFE is too soft and has a very high coefficient of thermal expansion. The material I use was recommended to me by a supplier who I was in contact with when trying to obtain a material called Flouroscint, which is a mica loaded form of PTFE and has been used by a number of model engineers for piston rings. The firm concerned said that they could not understand why model engineers persisted in using this material because it is very expensive and graphite loaded PTFE was not only cheaper but, in their opinion, more suitable for the duty! I therefore followed their advice and have used rings of 25% or 35% graphite loaded PTFE ever since. The material still has a fairly high coefficient of expansion, so rather than have a large gap in the rings my design incorporates an overlapping joint."

I had produced a few pages showing his jig method of making overlapping joints in the rings in more detail & with photographs. PM me if you're interested & I'll email a copy.

My concern is PTFE expanding into the hole and then being cut by it's edge(s). I guess a round hole would be more efficient as a cutter, imagining the PTFE expanding into the hole as the piston slides over it until it reaches the half-way point where it then is nipped and sliced by the reducing semi-circle. But maybe a square hole is worse because there's more edge to cut. A long run experiment would show the difference and how bad the effect is. Possibly the answer is to chamfer the edges to stop them cutting as the PTFE expands.

Cast-iron rings don't expand into a hole in the same way - they push evenly all round the cylinder.

Dave

Chap in our club had a Simplex with carbon filled ptfe slide valve. This worked fine against a rectangular port