don young piston valves

Sorry if this is a dumb question but with all this talk about wear on valves and them leaking etc. Why aren't they built in a similar way to a model aero engine cylinder and piston? I've had engines doing 10,000 rpm plus for hours on end without any noticeable drop in performance so they cant have worn that much. They also get a lot hotter than a steam engine cylinder. Or why aren't they made like a car engine with iron bores and piston rings, they seem to last pretty well? Thanks.

Hi, they do put piston rings on iron valves and liners though some seem to go over the top. A design in Henry Greenly's book from way back uses 14 rings on each valve head! I haven't been able to find any other designs, they seem to have been seldom used, especially in smaller gauges. However mine are bronze and the received wisdom is that you can't lap bronze cylinders because the grit tends to imbed in in the metal and continue wearing it. Also they say you can't make rings from bronze but I haven't been able to find out why. The reason for using bronze was that it lends itself to fabrication, I haven't heard of anyone trying to fabricate cast iron cylinders though apparently it can be silver soldered.

Hi John, sorry for the late reply. I've not been on the forum for a bit.

I make the PTFE valves a push fit in the bores. They will sort themselves out once they see steam.

It helps if you can make the PTFE heads as thin as possible as that helps negate the problems with the PTFE expanding. Those on my Helen Long are only 0.0625" thick and they still seal perfectly after all these years.

I've never used anything but the plain PTFE for valves or piston rings , indeed, never found the need to. The plain PTFE works fine for me.

John

Thanks John , that's great to hear from someone who has actually made them and succeded. I will make them as you say, my valves are 15 mm diameter. I will keep the thickness of the PTFE down to 1/16 th. Did you put any grooves in them as some have suggested?

Yes, I did put some narrow grooves in them. I used a 60° threading tool.

John

Thanks John, my fabrication is going ok but it will be a while before I make the valves.

It’s vitally important that the Teflon rings be made according to the tried and true procedure. A friend who thought he knew better, had two failures before he followed the correct procedure the third time and, surprise surprise, he had success. The method is given here:

https://modeleng.proboards.com/thread/13000/piston-valves-rings

Another friend made them too big cold, and they seized on first steaming.