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LTM 51

Drive units

First step towards a fully functional chassis including motor and gearbox is making the spacers. I turned them from 6 mm brass rod, to 10.5 mm. Their length was mainly based on what I found in other kits. It needs to be proved correct.

I drilled them all the way through and tapped them M2.0. To prevent them from sticking out of the frame ends I milled 1.0 mm off one side.


My cheapy single fluted Chinese spiral mill delivered unexpected good work. The milling marks were so minute that I could not feel them with a nail test and altogether vanished after a few strokes on 1200 grit emery paper.

On fitting the gearbox I realised I had made an error and cut the spacers too narrow. Well, that happens. Fortunately I am more and more able to fabricate my own parts. For now I fitted a few washers under each side of both spacers to widen the frame while I am still fitting things. Once I know the final size I will make new spacers.

Then I added all three axles before installing the gearbox. I immediately ran into the next problem. Can you spot it?





The two rear wheels overlap.

So I set two work. I had basically two options: turn the wheel to a smaller size or putting the two rear axles further apart. The first option had my preference for three reasons.

  • The Alan Gibson wheels were a tad oversize so turning them down was an attractive option as it would not only solve the problem but I would end up with wheels closer to scale
  • The two rear axles were already a bit too far apart as provided per the etch, so taking them further apart was not to my liking.
  • Filing the holes of the axles eccentric is not a very controllable process whereas turning wheels is very repeatable and predictable.

To turn the wheel I needed a mandrel to clamp the wheels tightly down. As turning is not the subject of this page I will not go into detail how I made it.

Some details on the features of the mandrel:

  • I provided a central spigot to mount the wheel. It is 3.20 mm wide, just a bit more than the standard 1/8" axle (3.18 mm) so the wheel would sit very tight
  • The backplate is about the size of the plastic centre of the wheels, smaller than the metal tyre to allow the RP25 tool to get behind the flange.
  • An M1.2 hole was tapped in the front of the mandrel. The bolt serves to prevent the wheel from slipping. In practise one bolt did not suffice. It broke the adjacent spoke of the wheel centre. So I added a second one to hold the wheel.

The Fohrmann RP25 tool is matched to the wheel after completing the wheel for the sake of the photo.

The black line on the tool indicates the front face of the wheel.

One complication while turning was that doing so the tyre heated up considerably. Chances were that the tyre would come off the plastic centre. So after I found that out the hard way I first gave them a drop of thin cyanoacrylate glue at the back of the wheels which by capillary action drew into the separation between plastic and the metal and permanently fixed them together. Turning was done ever so carefully. Only the lightest of cuts and slowest of speeds and ample time to cool down, all served to keep the stress and temperature in the wheel to the lowest possible level.

Old and new in comparison. In all the diameter of the wheel decreased by 0.7 mm measured over the flanges. As the flanges as supplied where a bit higher than the RP25 norm the flange height was reduced in the process. The wheel diameter was reduced by 0.5 mm bringing it to 10.4 mm. The prototype's wheels were 900 mm so 10.34 mm in H0, so within all reason they are now spot on.

And with about 0.25 mm to spare they now go together well. Tight but clearance is clearance. The right wheel broke a spoke and will be replaced.

At long last I could finally trail fit the gearbox/motor assembly.

And yes it fits well.

Alternative frame setup

The frame plates as supplied have holes to accommodate fixed bearing bushes. This results in a three axle rigid frame setup. This works but it has been a long standing wish of mine to build a loco frame with sprung axles for better road behaviour and better electrical pickup. I wanted to experiment with this without possibly destroying the original so I made two identical frame plates from 0.4 mm (16 thou) brass based on the outline of the original ones..

These were the first objects I filed with my newly acquired Vallorbe 140-K2 needle files (24 July 2020)

I bought these files on encouragement by a fellow builder and by ClickSpring's videos. I bought them from VoF van de Gevel in Tilburg, Netherlands, an excellent seller!

And boy did these files work! These files are so straight and true you can make very dependable and predictable strokes. The two squares I filed in the two plates (then still soldered together to make them identical) measure 7.5 by 6.0 mm and all dimensions are within a 0.1 mm tolerance. The hole for the bush was first drilled undersize and then filed to a tight fit. And indeed the fit is so precise that the bush sits without falling out.

Next is mounting the sprung hornblocks.

The loco is not sprung as per the prototype. The model's weight presses down on the hornblocks until it rests on the small bolts. Only if the loco's weight lifts e.g. in case of the depression in the rails a little spring pushes the hornblock in case down to maintain road contact. So this works exactly opposite as a real locomotive's springing where the locomotive is suspended on the spring and only if it hits a real hard bump the spring may reach its end of travel to come to a stop.

The objective of sprung hornblocks on model locomotives are maintaining electrical contact and preventing derailments on uneven track.


Mounting the hornblocks is not overly difficult. I soldered the Alan Gibson sprung hornblocks in place as per the manual.

The only snag is the mounting of the tiny coil springs that are supposed to go on top of the hornblock and be held in place by the adjustment screw. One disappeared even before trying to mount it and the second escaped very soon after mounting. The spring does not sit very well on the top of the hornblocks. It slips easily and becomes trapped on the top of the hornblock in an awkward position. I found no way to make them sit securely. Their tendency to vanish is one reason why I dislike coil springs. A second reason is that they are not adjustable. So in the end I left them away altogether in favour of a springing of my own.

I drilled a 0.3 mm hole through the frame spacers in which I soldered a phosphor bronze spring which just touched on the middle of the axle when it is on its lowest position. Effectively I now have a four point suspension.

I have yet to clean up the soldering, which I will after I have decided that this is the way to go. For now the hornblock is functional.

When completely depressed the sprung axles should all rest on the same height as the fixed middle axle.

So I put the frame on a glass plate, to ensure 100% flatness of the underground, and set up my height gauge. First I measured the middle axle's height and next I adjusted the set screws of the other axles to achieve the correct settings.

Axle Left Right
Front 13.16 mm 13.18 mm
Middle 13.16 mm

13,18 mm

Rear 13.16 mm 13.13 mm


Well, I guess that is well within tolerance. I may to do some more final adjustments in the right side

Finally I weighted down the frame to determine how much weight should rest on the drive unit to make it rest on its set screws. Initially this was a tad on the high side but after some adjustments of the phosphor bronze wires the total weight it needed was about 40 grams. As the boiler cradle in its current unfinished state already weighs 62 grams the pivot side of the drive unit will be loaded sufficiently to settle on its spring. The other side will need some 20 grams. This will be the combined weight of the motor and extra lead and I think I can achieve that.

So far I was very doubtful about a sprung chassis but now I have built one I am pretty happy with the results.