The job-lot of castings purchased for my next (HP compound) locomotive were all gunmetal. The received wisdom was that using cast iron rings for both piston valves and main pistons in gunmetal bores was not a good idea. Therefore, for the main piston rings several different materials were compared against their commercial availability, cost and/or ease of manufacture in a home workshop.

One new polymer (Kalrez©) appeared to possess all the necessary properties for O-rings in the main HP pistons but the price was prohibitive. Rings in stainless steel or phosphor bronze, made to order, also proved too costly as well as their suitability with gunmetal being questionable. Yet subsequently, after prolonged research and then discussion with the metallurgist-producer of cast iron Clupet rings, it was decided to employ these rings for the main pistons with assurances that, providing there was good lubrication, there should be no problem using them in the gunmetal


Several articles in M.E. and EIM over the past ten years have described using either virgin PTFE or mica or glass filled PTFE for the valve ‘rings’ so it was decided to adopt this approach. Additionally, the attractive design of Geo Thomas for easily adjusting the position of piston valve ‘bobbins’ on their spindles became the basis of a design which would include PTFE composite rings.

The complete valve spindle based on the Geo Thomas design and with Fluorosint rings

However, a few of the published articles referred to a problem that machining the rings to be a sliding fit in the valve liners at ambient temperature led to their seizure when hot, despite the data sheets showing little or no thermal expansion of the polymer. Indeed, no lesser builder than Doug Hewson had experienced just this paradox.

To avoid post-build dismantling, the offered solution was that the entire cylinder block, together with the valve spindle and its PTFE composite rings should be repeatedly heated in an oven to something like operating temperature and the rings’ diameter progressively reduced on the lathe until they were free to slide at this elevated temperature. An irksome process, although one writer claimed that there was no expansion and the valves operated successfully with compressed air and steam.

A short length of the very expensive Fluorosint 207 (PTFE with mica) was purchased for use in the high pressure, inner cylinder as piston valve rings for this compound locomotive. After machining, some difficulty was experienced in obtaining a smooth fit at room temperature because the Fluorosint seemed to snag and roughen-up on the steam passageways cut into the gunmetal liners, despite their smoothed edges. A particular difficulty was also encountered in the normally simple task of measuring the diameter of each Fluorosint ring. This should have been straightforward but there was very little repeatability in the measurements using a sensitive micrometer; the measured values seemed to vary by about 0.005”. Perhaps consequently, with this middle HP cylinder, the valve spindle felt rather loose in the valve liner at ambient temperature despite the OD of the ring and ID of the valve guide dimensions being apparently the same. At this point, the inner cylinder was gladly put aside until the two outside LP cylinders had been machined.

Months later, when the two outside cylinder castings had been machined the price of Fluorosint had risen further and given the surface scoring observed with the inner cylinder’s valve, somewhat less expensive and probably better-wearing, glass-filled PTFE was purchased.

The two outside cylinder valve spindles were made to the same design as the inner cylinder and it became obvious that again the rings had to be turned down in situ, on the spindle, once their central (¼”) diameter hole had been drilled on the lathe. This necessitated using a collet and female centre to ensure repeatable concentricity for the ‘skimming’. The diameters of the rings (1” OD) were reduced until a sliding fit in the valve liners was obtained.

One of the published articles recommended making the central hole in the rings larger than the diameter of the spindle “to allow some float”. This was done at first by enlarging the hole diameter by 1/32”. Subsequently it proved to be a mistake as the rings would then not run concentrically when later mounted to reduce their diameters. Therefore, new rings were made so that they were a tight fit on the ¼” spindle. The bore may be enlarged later to allow some degree of float.

The next stage was to find the extent of binding, if any, when the temperature was raised. The inside cylinder was therefore dusted off and together with the two new outside cylinder blocks and piston valves, prepared for heating. Because the valve liners had been fixed in place with dowels and Loctite 301 there was concern that too much heat might break the seal, so the domestic oven was set to only 150° C. This was rather less than the steam tables suggested the temperature of steam would be at 150 psi.

The polymer rings were coated with a light mineral oil and the complete spindles inserted into the three cylinder blocks and allowed to equilibrate in the oven for about 30 minutes. After removal from the oven, still at 150°, the valve spindles were all locked solid in their liners due to the expansion of the PTFE composite. So, after waiting until cool, the spindles were returned to the lathe collet and female centre and 0.001” was removed from the ring diameters. The cylinders and valve spindles were reassembled and returned to the oven for another check. The whole tedious procedure was repeated many times until the spindles could be slid with only slight frictional resistance when at 150°C.

Not surprisingly, when now at ambient temperature, the valve spindles were decidedly loose in the valve liners because the rings had contracted. Clearly, the locomotive could never be run on compressed air to bed in the motion work with valves this loose at room temperature.

An attempt was then made to measure the differences between the known bores of the two outside cylinders’ valve liners and the now much reduced diameter of the rings. This proved as frustrating and inexplicable as before:

The two rings on each valve spindle (offside and nearside) had been machined to the same diameter using the collet and female centre each time yet their measured diameters were different. This was attributed to the inability to get a consistent feel with the micrometer. Whether using the ratchet thimble or not appeared to make no difference to the poor repeatability of measurements.


Compared to using cast iron rings, reinforced PTFE was very problematic and appears to prohibit running-in using compressed air because of its thermal expansion. The whole process of fitting and machining the PTFE rings was tedious and very inexact.

The temperature at which the fitting was done (dry at 150° C) gives no confidence that considerable tightening will not occur when the loco is steamed and if so, will demand substantial dismantling to rectify the problem

The cost of Fluorosint or glass-filled PTFE is considerable and while reports suggest that the service life of such rings is lengthy, the disadvantages seem also considerable.

The product data sheets for Fluorosint quote figures for linear expansion to be extremely low and that it is unchanged up to 150°C, thereafter the linear expansion rises only very slightly. But this does not accord with findings in this application and of other builders where the radial expansion was appreciable. From a sliding fit at ambient temperature to a sliding fit at 150°C some 7 to 10 thousands of an inch had to be removed from the diameter of each ring and moreover, judging a sliding fit was very subjective and, therefore, imprecise with these glass-filled PTFE rings.

I have read no negative comments about using PTFE piston valve rings; indeed the authors have expressed every satisfaction. Perhaps therefore my experiences so far are unusual and due to my ignorance and poor methodology.

Regrettably I have passed the stage in the locomotive’s build where I could contemplate scrapping the gunmetal cylinders and reverting to the time-honoured combination of cast iron rings in cast iron liners. Yet I have little confidence that when the locomotive is finally steamed these glass-filled PTFE valve rings as fitted will prove trouble free.



By Crankpin