Proto:87 Wheels for a Bachmann GP 40

Keith Norgrove

Some 20 years ago I used a Bachman GP40 chassis converted to Protofour to power a British Rail Class 20 diesel. At the time I purchased a spare with the intention of doing a second conversion. This spare had P4 wheels fitted at the time but the rest of the conversion work has never been done.

Looking for a locomotive for P87 experiments I dug out this loco and carried out another wheel change.The original truck sideframes and the original axles have not survived, but this has little effect on the conversion work which amounts to little more than replacing the wheelsets.

Materials

You will need 4 axles of Alan Gibson 40" disc wheels, his item no. P8703/2. Ask him also for inside bearing 2mm axles, these will save having to grind off the pinpoints.

Dismantling

It may well be possible to do this work without removing the trucks from the loco, but the removal is easy, and handling the trucks is simpler after removal.

Spring the bodyshell off the chassis.
Push out the retaining pins that form the axles for the wormwheels and the truck support pivots, the trucks then drop out.
Unclip the moulding that forms the sideframes and keeperplate of each truck. The axles can then be gently prised out of the gearbox.

Preparation of Wheelsets

As I did not have the original axles, I could not use the simpler method described here. I have given alternative steps where appropriate, steps shown as #A describe what I did and are known to work, steps shown as #B may be easier but need to be checked.

1- Remove original wheels from the axles with punch or a suitable puller, I used a NWSL puller. Also remove the new Alan Gibson wheels from their axles, here a gentle twist with the fingers will be enough.

2A - Remove the axle gears from the original axles, here I tapped the axle through the gear over the vice jaws and finished with the puller.

2B - Measure the original axle diameter with a micrometer, or by trial fitting one of the Gibson wheels. If the original axle is 2mm diameter and fits the Gibson wheel snugly proceed to stage 6.

3A- Mark the centre of the new axles, then using a half round file, file a shallow recessed ring around the centre of the axle. The filing should just cut through the axle finish to bright metal and the recess be just perceptible to the fingernail.

4A - Run a 2mm drill through the hole in one of the axle gears, smear a little solder paint or cream in the file marks on the axle and fit the gear. The gear should be a firm enough fit to retain its position on the axle and should be carefully centered. Application of heat from a soldering iron or micro torch will fix the gear in place.

5A - Clean up any stray solder, and if pin point axles were used, grind off the pin points so that the axle will fit between the sideframes.

6 - Remove any moulding pips from the rear face of the Gibson wheels with a craft knife and polish the tyre tread and the tyre rear face with a fibreglass brush. The dark finish should be removed leaving polished steel to ensure good pickup.

7A - The wheels can now be carefully pushed onto the axles and set to the correct back to back dimensions.

7B - If succesful with stage 2B then the wheels may be fitted to the original axles saving a lot of work refitting the gears. If in any doubt about the security of the push fit, a drop of retaining compound (eg. Loctite 601) can be used in the wheel bore.

Assembly

1 - Try a wheelset in the bogie frame and inspect the pickup wipers. Most likely the wipers will contact the plastic section of the rear face of the wheels. The wipers should be tweaked with tweezers to make a nice spring contact with the polished steel rear face of the tyres.

2 - Repeat for the remaining three wheelsets.

3 - Replace the trucks in the frame and clip the bodyshell back on.

4 - Test run, if you can't do this you are going to have to build some track!