Revised Piston Valve Bobbins


When making a previous locomotive, I found the approach for setting the valve bobbin on the spindle by using shims to be quite awkward. To add a shim, it was necessary to remove the Bobbin/spindle assembly which was no small task. I eventually set the valve position by the screw thread into the Crosshead which is really not desirable. There has to be a better way. This article describes dimensions for a 5"g locomotive, but the ideas are usable for any scale.


After some thought and input from other Model Engineers, I designed a bobbin which is fixed to the spindle via a 1/4 x 40 TPI thread, thus allowing for fine adjustment. As the bobbin will now be fixed to the spindle it will not be able to float, so I countered this by having 'floating' PTFE 'tyres' fitted to the bobbin.

As can be seen in the design above, the centre hole in the bobbin is tapped at one end (1/4 x 40 TPI). This thread screws on the spindle. The two end caps are designed with a 7.8mm gap in which the PTFE 'tyre' will sit. As the internal bore on the 'tyre' is less that the diameter of the end cap, it will be free to float.

Main Body

As well as incorporating a new design, I also decided to change the bobbin material from Stainless Steel to Phosphor Bronze.

First a piece of phosphor bronze bar was faced off and part drilled through at 6.5mm, with the remaining ~10mm tapped 1/4 x 40 TPI:

First a piece of phosphor bronze bar was faced off and part drilled through at 6.5mm,
with the remaining ~10mm tapped 1/4 x 40 TPI:

After drilling and tapping the bobbin centre hole, a crude mandrel (1/4" bar) was set in the chuck, tapped and centre drilled to that I could support it with a dead centre.

With the bobbin so held, it was turned to just over size:
The end cap mounting shoulders were then turned to size and threaded 3/8 x 40 TPI:
The bobbin was again held in the mandrel and using a circular tipped tool, the main recess in the body was turned:

End Caps

The End Caps were turned by initially drilling and tapping (3/8 x 40 TPI) some phosphor bronze and then parting off, to form two blanks:
Again, I made a crude mandrel by tapping a piece of 3/8" bar and fitting each blank to it, before then facing off the inner face,
turning the shoulder and facing the outer face of each end cap:


Here is a view of the bobbin with both End Caps loosely fitted:
Another, this time with both End Caps fully fitted:
Here a couple of views of the completed bobbin fitted to the spindle and with the PTFE 'tyres' fitted:

To complete, I cut a slot in the outer face of each end cap to allow a specially made tool to be used to tighten it to the main body. I also turned a small, circular, locking nut and again cut a slot in the outer face.

Here is a view of them both fitted:


To tighten the end caps, adjust the bobbin on the spindle and to lock it in place, it was necessary to make a couple of tools using BMS. The inner of the two is for the lock nut and the outer for the end cap and to adjust the position of the bobbin on the spindle.

Adjusting tools:

Turning the PTFE 'tyres' should have been a simple turning exercise, but I found it difficult getting the required width when parting off. Holding the tyres to then face them back was a challenge - so I made a simple split collett. Removing the 'tyres' from the collett was not easy due to them being very slippery, so I also turned a small blank to push them out with.

Ejection blanks:

Author: Mark Smith
This article is sourced from the article in Mark's Model Engineering Pages and is used with permission.