by Howard Wakefield

MAKING model locomotives calls for well-manufactured parts along with good fitting skills.  One of the areas where a lot of people, including myself, have difficulties is in getting the piston valve bobbins to seal correctly in their liners.  Even the slightest play in the valve and liner will lead to problems of loss of steam and a poor performing locomotive.

Some modellers use piston rings but this in itself can lead to port timing issues as the rings have to be set inside the outer edge of the bobbin, and this will effect the exact opening of the ports.  Piston rings can also be quite expensive.  With these facts in mind I decided to experiment with PTFE valves.

What I came up with is nothing new to the model engineering fraternity but the actual construction may be of some interest to fellow modellers.  A picture tells a thousand words so I have drawn up my solution to the problem in getting a good seal on these valves. The drawing illustrates actual valves that I manufactured for Geoff Watts’ (Northampton SME) 9F locomotive; of course dimensions will vary for different models so this is for illustration only. 

The only precise dimensions are the outer diameter, the width (A) and the distance between the two PTFE bobbins (B). The rest can be varied as required. If you should chose to go down this route and try these yourself is quite possible to use the existing valves and modify them to incorporate the PTFE tyre. If you want to retain your existing valves then start with a fresh bit of stainless steel bar. 

Looking at the diagram the construction is quite obvious; I made the PTFE tyres about 0.020” oversize on the diameter and a similar amount oversize on the width. Make the PTFE tyre a good press fit on the bobbin and make the groove on the outside such that the end cap compresses the tyre when screwed home.

Once you have the bobbin with both PTFE tyres attached and the end caps firmly screwed up (a bit of thread lock may be worth applying) mount the whole unit in the lathe between centres. With a very sharp parting tool cut back the inside of both PTFE tyres so the distance B is correct for your model. Do this carefully as you can’t add material back on if you get it wrong!

When you are happy you have got this bit right use the same parting off tool and reduce the outside edge of the PTFE tyre to the dimension (A). Again go slowly taking a small cut each time. You should now have a valve with correct porting and all that is left is to turn the outside diameter.  Make sure that the valve is at room temperature before proceeding.

With the valve still between centres carefully reduce the diameter to a push fit into the valve liner. I made mine about ½ thou under size to allow for expansion. It is worth making a gauge for the exact inner dimensions of the liners prior to sizing the valves. Don’t forget that the dimension may be different for each liner.  Refit the valves on their stems, reassemble and retime for the correct port openings. If it all goes wrong just make two new PTFE tyres and start again!

Refit the valves on their stems, re-assemble and re-time for the correct port openings. If you need to run your model on compressed air you may find the valves will leak a little, but once you steam up the PTFE will expand and provide a perfect seal.

The photograph illustrates a valve I made for my current project, an Adams T6 tender engine (pictured right). These are slightly different from the ones I made for Geoff, as they are on a solid spindle, but the actual construction is very similar. The Adams ran perfectly on its first steaming but unfortunately I can no longer peep in the workshop and look at it, as it is now in a hundred pieces waiting for the de-greaser and spray gun!