Dapol Class 121 - conversion to P4

Jeremy Good
Posts: 149
Joined: Fri Feb 19, 2010 10:36 pm

Dapol Class 121 - conversion to P4

Postby Jeremy Good » Wed Jan 10, 2018 2:01 pm

A new item of motive power arrived for Wheal Elizabeth just prior to Christmas in the form of one of Dapol's newly released Class 121 DMUs in BR's blue/grey livery.

This looks like a significant improvement over the old Lima/Hornby offering but is not without its issues. The main being, in my eyes, the raised floor of the interior which covers the drive train - I have yet to decide how to address this but suspect a coat of paint on the floor and seats may at least disguise the problem in the interim. The other omission is the small grille on the body sides that these units received on refurbishment but again I haven't come up with a solution which doesn't involve a full repaint!

Cl 122 approaches Wheal Elizabeth Halt

The model has a central motor driving all wheels on both axles via cardenshafts. The wheels are fitted on stub axles into the fairly substantial gear muff and pick-up is via the wheel bearings which sit in the brass pick-up strip with runs along the side of the bogie moulding (as shown in the photo of the completed conversion, below).

I decided that I wanted to try and retain the "split axle" pick up arrangements of this model rather than having to fashion new pick-ups. Having spoken to Brian Osborne at Branchlines I obtained a set of their Black Beetle wheels fitted on stub axles as supplied for the Bachmann DMU's, Brian kindly supplied a pack of 8 wheels to P4 standards fitted to their stub axles.

Having dismantled the bogie and pulled apart the gears it became apparent that this wasn't going to be quite as straightforward as I had hoped. The Branchlines wheels are on a 1.95mm axle which is turned down to 1.52 mm at the end that goes into the gear, the Dapol wheels were also 1.95mm but 1.58mm at the turned down end. Although the Branchlines wheels fitted the gears they were too loose to simply be a push fit. It also became apparent that the wheels would be spaced too widely apart for 18.83 gauge due to the turned section on the gears being shorter than the original Dapol wheels.

Dapol/Branchlines wheels

The solution was to drill the gear muff to a depth of about 1.5mm/2mm (not too critical) with a 1.9mm drill which then allowed the wheels to be fitted giving a back-to-back of 17.75mm and providing a slightly more secure fit. However this then left the ends of the axles sitting in the gear being quite close together so I filed about 0.5mm off the end of each axle to maintain the insulation gap. I put a small smear of Araldite into the hole at the end of the gear before refitting the wheels keeping the brass bearings well away from the glue... When set, this gave me 4 straight, free rolling, P4 axles and wheels with gears fitted which could then be refitted to the bogies after the wheel faces had been blackened.


The bogie sides needed a little adjustment as the brake gear is moulded on a plastic sprue. The brakes themselves are all individually glued in place but the moulding runs behind the bogie frames preventing the new wheels being refitted. These were simply cut off and then the bogies refitted.

Test running showed that the conversion hadn't affected the pick ups and the running was as good as before. I now have a new complete train for the passenger service at Wheal Elizabeth.

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Mark Tatlow
Posts: 774
Joined: Tue Dec 02, 2008 11:24 pm

Re: Dapol Class 121 - conversion to P4

Postby Mark Tatlow » Wed Jan 10, 2018 5:17 pm

Jeremy Good wrote:I now have a new complete train for the passenger service at Wheal Elizabeth.

Prototypically without any passengers!!

I suspect some people in the compartment around the cover to the drive shaft would help mask it.
Mark Tatlow

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