Converting the Murphy-Bachmann 141 Class Diesel to 21mm Gauge

Denis Bates

 

The General Motors 141 Class diesel of CIE was introduced in 1962, and for over 40 years has been among the most successful of the Irish diesels.  So it was not by accident that Murphy Models chose it for their first foray into a completely designed Irish model (the previous Woolwich Mogul and the NCC Jinty were of course repainted versions of the English models).  So, although my main interests are in steam days, and particularly the BCDR, I couldn’t resist purchasing one, with a view to re-wheeling it to 21mm gauge and P4 standards.  The prototype is described and drawn in two of the model magazines: by Tim Cramer in Model Railways for March 1977, and by Shane McQuillan in Practical Model Railways for June 1986. The latter article describes also the building of one, from a kit by the Model Irish Railways group. Comparing the Murphy model with the drawings, I could find nothing amiss – except for the buffer spacing. The buffer centres should be at a spacing of 6’3”; on the model they are at 23mm (5’9”). I presume that is to match the spacing of other Murphy-Bachmann models, at English spacing.

I first determined that a P4 wheelset would fit between the bogie frames, and it does, just. My main mistake was to dismantle the loco as far as possible – this is not necessary, as the bogie frames can be levered off. They are similar to those of the Bachmann Class 20 diesel (described by Keith Norgrove at http://www.norgrove.me.uk/index.htm). A screwdriver is used to prise out the frames, which appear to be identical on the two bogies. The wheelsets can then be prised out of the bogies. Each wheelset has an offset plastic gear wheel, two brass bearings which are a push fit in the sideframes, and brass wheels with insulating sleeves. Measure the distance of the gear from the ends of the axles, before tapping out the wheels and sleeves using a small drift and hammer. The gear wheel can similarly be tapped off the axle.

If the model is to be converted to EM gauge, all that is necessary is to cut 2mm steel rod to the appropriate length, and put the gear and wheels on. Although there is a spline on the original axle for the gear, it seems tight enough on a plain rod (a smear of loctite could be used to anchor it firmly). To keep the axle laterally in place, two brass sleeves, or an appropriate number of washers, should be added between the gear and the bearings (or between the bearings and the insulating sleeves (see figure). For conversion to P4 and 21mm, the original brass wheels can be used, but have to be turned down to receive P4 rims (these are obtainable from Alan Gibson on special order). It is also possible to turn down the existing rims to P4 standards. For those without a lathe, it may be possible to purchase P4 wheels to suit. Before finally inserting the wheel sets, the pickups need to be adjusted so that they bear on the backs of the wheels. On the Bachmann wheelsets, the hub projects further out than the rim, and does not fit easily between the sideframes. However, the insides of the frames can be filed out (about 0.5mm or more) to give clearance. I used a cylindrical dental burr to do this, held in a drill press, and hand held the frames. Once finished, the loco ran just as well as it had done on 00 track. Now to try it on Adavoyle Junction, out of period though it be!

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3 Comments

Filed under Modelling

3 responses to “Converting the Murphy-Bachmann 141 Class Diesel to 21mm Gauge

  1. John McGahern

    Hi Denis,
    great article, thank you. Now, how difficult is it to lay your own 21mm track. Are there gauges available etc…?

    John.

  2. Denis Bates

    Dear John,
    Thanks for comments. The Scalefour Society used to have gauges available, but they don’t seem to be listed now. I have a set of gauges by Joe Brook-Smith, which I got from the sociey. Laying track is no more difficult than for standard gauge with these gauges, which include a triangular gauge for gauge widening for curves.

  3. John McGahern

    Hi again Denis,
    what is the purpose in applying the brass sleeves? I have done the coversion with everything except the sleeves and I am getting a little bit of inconsistant movement along the track….could this be my problem because the wheels are not held laterally in place and could be grabbing the rails from time to time?

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