3½" 75000 Class Piston Valves


One of our club members collared me at the club on Thursday and asked if I would have a look at his 75000 class loco chassis made to the LBSC design. He'd taken it to an open weekend at Cardiff where it had performed very poorly and had tried running it on air when he got back. The results were a very strong 'blow' from the exhaust which pointed to the piston valves leaking. He had mentioned before that the valves seemed to leak when cold but things improved when the cylinders got hot and the oil had got to the valves.

I brought it home and connected it to the large aquarium pump and indeed there was a continuous flow of air from the exhaust. I disconnected the valve rod from the valve gear so I could move the valve manually and the leak was still there when the valve was in the mid position when both ports should be closed. Inspection of the valves showed them to be a 'rattling good fit' in the valve liners so it's no wonder they leaked. Measuring the valves showed both to be about 0.003" smaller than the bores of the liners. I suspect that they probably weren't that good a fit to begin with as I don't think the loco has done that much running since it was completed. The present owner had bought it part built and someone else had machined the cylinders.

The valve liner bores seemed in good condition so I decided to replace the old valves with new bobbins fitted with PTFE heads. I also decided to replace the old method of valve adjustment (nuts either end of the bobbin - Yuck!) with a similar method fitted to Helen Longish so that valve adjustment could be easily carried out from the front end of the valve liner.

I made the new bobbins from brass as that was all I had in a suitable size but as there is no contact between the bobbin and the bore there won't be any wear to worry about. The bobbins are in three pieces consisting of the central part and two end caps that screw on to secure the PTFE heads which are simple rings with a thickness of 0.125". The end caps were screwed on tightly to secure the PTFE and also Loctited. The ends of the bobbins were then skimmed flat. The PTFE heads were left oversize and will be turned to a push fit in the liners by mounting the bobbins on a mandrel in the lathe.

New valves versus the old


Today I made the adjusters for the valves and finished the PTFE heads to size. I turned a couple of shallow oil grooves in each head but I don't know if that is really necessary.

The adjusters consist of a sleeve with a hexagon head threaded inside to fit the valve spindle and a thin nut that secures the valve bobbin. The bobbin is a loose fit on the sleeve to allow the valve to float slightly. The valve is adjusted by screwing the sleeve up and down the valve rod and then locked with a nut using two custom made box spanners - a large thin walled one to turn the hexagon head of the sleeve and a smaller one inside the large one to tighten the nut.

Valves and adjusters

After refitting the valves to the cylinders and adjusting the timing, the chassis was given a run on air with good results. There is still a very slight blow from the exhaust but I am sure this will disappear under steam when the valve heads expand and seal tightly. The chassis actually ran nicely using the aquarium pump which only gives a few psi of pressure.


I saw Nick, the owner of the loco, on Thursday and he had steamed it the previous Sunday with very good results. The only problem had been that one of the return cranks had shifted, probably due to the initial tightness of the valves putting more load on the valve gear. He's now pinned the return cranks properly. He was a bit worried when he had tried to run it on air afterwards and found the valves to leak badly. This is the problem with the PTFE heads on the valves. After the initial steaming, the valves 'shrink' and won't seal against air pressure. As soon as they see steam they expand and seal perfectly again. He had run it on steam again after the run on air and it had indeed been fine. There was a design in Model Engineer recently where the PTFE heads were fitted on tapered seats and spring loaded to expand them against the liner but I doubt if that would expand them enough to seal against air once they had been exposed to steam?

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