Walchaerts Valve Gear For Sweet Pea

Initially, it was my locomotive friend John who gave me the idea of fitting a Walchaert’s gear to his Sweet Pea, which is fitted with a Hackworth gear at present and doesn't run particularly well.

It so happens that I have a Walchaerts design which I developed for my 7¼ inch gauge engine derived from Romulus and named Maggie, which appears in the Blackgates catalogue. This design worked well in six of the engines I have built over the years and the design will suit Sweet Pea and also Metre Maid, using existing holes on the frame (photo 1). All that is required is to remove the Hackworth gear.

It is important to know the names of the parts of the gear and fig 1 should help.

The parts fall into two groups: existing parts, to be modified, and new parts.

The existing parts to be modified are the crosshead, the slide bar and the reversing lever and we’ll deal with those first, followed by the new parts we need to make.

Crosshead

There is a change to the gudgeon pin holes in the crosshead (fig 2) and a new gudgeon pin is needed to hold the drop link in place, together with a screw in the 5mm hole tapped in the crosshead on the outside faces.

The slide bar (fig 2) is to be shortened or made new with a 5mm tapped hole tapped in its end. A four jaw chuck in the lathe will help to do this. A 2BA thread can be substituted but I prefer 5mm as I can get 5mm screws from F.W.B. (usual disclaimer), which are plated and made from high tensile material - and no stripping.