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© March 31, 2002 

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Valve Gear Conversion

by Phil Heath

                             click image to enlarge
I recently undertook to change the valve gear on a small 7-1/4 gauge tank engine, which I had built, from Horizontal Marshal gear to outside Stevensons gear.  I've built or rebuilt three locomotives in my modelling career, all with Stevensons gear, and they have all been very successful.


The first was a 5 inch tank engine of the old British LBSC Railway, a 0-6-0 side tank locomotive of William Stroudley’s design for suburban 19th century London. These locomotives were very small, compact and powerful for their size.  The model proved to be just as fast and powerful, and very economical with fuel and water consumption.  The valve gear fitted was direct inside Stevensons with the valve chest between the cylinders and the valves back to back. They were driven off the centre axle from four eccentrics between the cranks.  The loco was a treat to drive and could be notched right up to mid gear. It would move whichever way it was pushed.


The second was a rebuild of a 260 Mogul tender loco, 7 ½ gauge, which my son bought in very poor condition.  It was reboilered and totally overhauled mechanically. The locomotive was fitted with inside Stevensons gear driving outside slide valves through rockers. It would work in a fashion, but we decided to throw it out and rebuild from scratch. This was done and the locomotive proved to be a good one, capable of a lot of work. Owing to the indirect drive to the valves, and a certain amount of lost motion, it would not notch up to mid gear, but once it had settled down there were no problems and it ran with a very even beat.


The third locomotive is a model in 7-1/2 gauge, which my son bought as a kit, and asked me if I would put it together for him. I liked the look of it so agreed to give it a go. It arrived at my home in several boxes with a set of drawings, and proved fairly easy and interesting to build. The valve gear supplied was a form of marshal gear driven by two eccentrics on the centre axle, there was also a third one driving a water pump. I fitted the gear with great difficulty.  It proved impossible to get the valve events the same in both directions. The motion was transferred from inside to the outside valve spindles by rockers. These proved to be a poor fit with a lot of lost motion in several areas. To have removed this would have meant a complete remake of the supplied parts.  The locomotive was duly finished up to "steam testing level", but when put through its paces, left a lot to be desired. It was heavy on steam and fuel without a great deal of power to show for it. The valves were reset in various positions but with no real improvement. The trouble seemed to be basic to the design as far as I could see, and needed redrawing and rebuilding.  


The drawing and building were decided on but not Marshal gear. I thought about Walchearts and Joy’s but decided on Stevensons if there was room on the axle for the four eccentrics and the water pump drive. By this time the locomotive had been reduced to the chassis again, so measurements were taken. Unfortunately it was found that the pump mountings were in such a position as to foul the left hand eccentric rods. The pump mountings were part of the chassis so a great deal of head scratching took place. I again thought of fitting Walchearts gear and then I remembered that one of the Stanier class five 460’s had been fitted with outside Stevensons gear as an experiment by British rail.


The locomotive in question has been preserved, and runs on the North Yorkshire Moors Railway so I believe, though I have not seen it myself. I made a few enquiries and was told that though it was very successful with many drivers thinking it was the best engine in the fleet of class fives, it remained a "one off" (one of a kind). British Railways, in their wisdom, considered further conversions to be uneconomic. After further inquiries I managed to unearth a photo of the locomotive showing the general layout. This picture suggested to me that outside Stevensons gear would fit the  tank locomotive and look the part. I had no doubt, from previous experience with the gear, that it would be as efficient as if fitted inside.


The drawing out was not very easy as my drafting isn't very skilled, but I could understand it, and that was all that mattered. I found a small file on the web that demonstrated many different gears as working line models, so with that help, the proportions were soon worked out. One of the design puzzles was how to drive the eccentric rods. I could see from the Photo that they were driven by return cranks, but how?  The simple things are usually the most difficult to see, but the penny dropped at last and I realised there were two return cranks at 90o to each other. In the photo the inside one is hidden behind the other.



Work started and soon the parts were ready to assemble. The crankpins were changed for longer ones to accommodate the double return cranks and all fitted together as reasonably as one can expect.  One thing I found was that the length of the return cranks (overall length and length in relation to each other) was critical if the valve events were to be accurate. I ended up making three sets of return cranks until I was satisfied with the valve openings in all positions of the reversing lever.



 The locomotive was rebuilt to steam and tested. I was better than I could have hoped for. It notched up, sounded great and used far less steam. It performed equally well in forward or reverse. Relief all round.  I finished it, painted it etc. and shipped it off to my son in Seattle, after a time he decided it wasn’t big enough to do much heavy passenger hauling, so he bought a large pacific and sent the tank locomotive back to me. Now I have the job of converting it from 71/2 to 71/4 gauge so that I can use it over here in the UK.

by Phil Heath

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