Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch
8. Make the valve from gunmetal or bronze. Chuck a piece of rod in the lathe, face the end and turn down to 12 mm diameter. Very lightly centre, then drill carefully no more than 1 mm depth to a diameter of 5 mm. Using a boring tool, open out the recess to 6 mm diameter and a depth of 1.2 mm maximum. Part off the valve. Make two valves at the same time.
File or mill about 1.3 mm off each side of the valve (equally) so that it slides comfortably in the steam chest. Cut or mill the cross slots in the valve. Make sure that the valve rod will fit in the slot without binding. Make the cross-piece from a scrap of 1.5 mm brass. This should fit the slot in the valve without any shake. Drill and tap the cross-piece 10 BA. Take a piece of 1/16" stainless steel or bronze rod and tap each end 10 BA as per the drawing. Screw one end into the cross-piece and check that they fit into the valve slots without binding. You should be able to screw the rod in and out in order to provide adjustment of the valve travel. Check that the valve, rod and cross-piece will fit in the steam chest and move freely. If necessary trim the cross-piece until a smooth fit is obtained.
9. The knuckle at the end of the valve rod is made from 3 mm square brass. Drill this 10 BA and cut a slot for the eccentric rod. Screw tightly onto the valve rod. Turn a gland nut from 6 mm (1/4") hex brass, threaded 3/16" x 40. Put a short piece of Teflon plumbers tape around the valve rod and fit the gland nut. Allow the valve rod to slide freely through the gland.
10. The crossheads are made from mild steel. A skate of brass can be soldered on top if desired. In practice, this is purely cosmetic. The stages in construction are shown in the diagrams below. The recesses and shoulders can be milled or filed to shape.
11. The slidebars are made from 3 mm (1/8") square mild steel bar. Cut to length, they can be used as a gauge for machining the recess in the crosshead. The hole for the retaining screw should be drilled 1.4 mm diameter at the cylinder end, and this then used as a jig to drill the hole in the rear cylinder cover. The hole in the slidebar can then be opened out to 1.8 mm whilst the hole in the cylinder cover is tapped 10 BA. The hole at the rear end of the slidebar is spotted through from the motion bracket and tapped 10 BA. The slidebars are secured with hexagon headed steel screws.
12. The Motion Brackets can be made from 1.5 mm (16 G) mild steel - a piece of the frame material would be suitable. Remember these need to be bent for either the right or left side of the frames. Clamp the motion brackets in position when the cylinders are in place, spot through and drill and tap the frames to take 10 BA hex head screws.
If you have any questions, or comments or find any errors in these notes please contact me by email. Email Keith Bucklitch.
Copyright © 1998, Keith Bucklitch
Last Edited - October 1998