So continuing with the valve gear for the 4MT, the next job in the instructions was connecting the rotating arm with the expansion link. Now the instructions say to use nickel silver rod as pins and solder in place. However I like to be able to dismantle the motion and on the prototype all of the “pinned” joints have a steel collar around the pin. So I got a small piece of nickel silver bar, turned the diameter down a little and drilled No.66 for tapping 14BA. The pivot in the end of the rotating arm was slightly too big for 14BA so it was drilled out slightly and bushed. The weigh shaft arm was given a similar treatment, this has a small cast bearing block to fit to the radius rod. 14BA cheesehead steel bolts providing the fixings.
Next I moved onto the slidebars and crossheads, again another nice set of brass castings are supplied in the kit. They needed very little fettling and filing to get into shape. The slidebars are pinned to the rear cylinder cover with a bit of wire into the cylinder end casting.
The end of the crossheads are drilled out and a short length of steel bar is soldered in.
The fixing of the connecting rods to the crosshead is a nice design. The ends of the conn rods are bushed and slot into the crosshead. A 12BA bolt is used to fix the two together, although the nut needs a little filing and the bolt trimming in this photo. However the rear of the cross head is drilled larger than the screw head so the bolt fixes the bush in place and is virtually flush with the back of the crosshead. The only down side being that the supplied bush was about 5 thou under 1/8″, rather than mess about I reamed the conn rod 1/8″ and turned up some new bushes to be a good fit.
Finally I soldered the drop link to the crosshead and fitted the union link. Once again the union link has a small collar soldered on and tapped 14BA so the two are bolted together with a 14BA steel cheesehead bolt.
Once that was completed I turned back to completing the return crank / radius rod bearing. Once the brass bearing cover was fitted then it was going to be difficult to tighten the 12BA screw into the top hat bush. So I had to make a small modification, I decided to drill a small hole in the rear of the radius rod near the bearing and put a small notch in the top hat bearing. So once the brass bearing cover is fitted then I can push in a small bit of wire to hold the top hat bush whilst I tighten up the 12BA screw.
I then continued fitting the rest of the motion, the valve spindle was pinned and the the slide bars pinned and soldered to the motion bracket, so the final result is.
So next job is to see what I can make of the coupling rods. The etchings supplied are for fluted rods but my chosen loco has solid rods so I need to see if I can swap them around to build the solid version.
Radius Rod Fitting
Questions were raised on the Western Thunder forum as it wasn’t that clear exactly how the radius rod bearing to the return crank was fitted so hopefully these photos and drawings illustrate how the top hat bush is locked with a small length of wire allowing the screw to be tightened up.