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Dapol Class 122 conversion to S7 and DCC by Richard Carr

The original source for this article is here: Dapol Class 122 conversion to S7 by Richard Carr

My Dapol Class 122 DMU arrived yesterday after spending several months stuck in a container in the Suez canal. Thankfully they all survived intact.

It looks really good if you ask me.


It comes screwed to a piece of Perspex.


There is plenty of interior detail.



Once you have unscrewed it from the Perspex there are 2 battery boxes that need pushing firmly into place.




There is a bag of detail bits to go on the front - vac pipes, air pipes and speedo.


Then the hard bit - converting it to S7

Both bogies are powered and once you take the retaining plate which is fixed to the side frames you see this.

So, it is going to require very careful removal and replacement of the reprofiled wheels on the axle, I won't be thinning the wheels as there won't be enough axle to push home given they are fairly thin in the first place.

It might be a case that new wheels are needed but hopefully not.


Surely, Richard, it won’t make any difference to the amount of axle the wheel is pressed onto whether you reduce the wheel width or not because the b-t-b measurement is fixed at 31.25 regardless, so take the wheel width reduction off the front face, if that makes sense. If you leave the wheels at Finescale tyre width it might bind on the inside of the bogie frames.

What is the Dapol wheel tyre width? My experience of Dapol wheels is that they are very nearly scale width as they are.

Also, the flange depth is near enough to live with as well.

I just happen to have a Dapol wagon and a vernier by my side so here goes:

  • Tyre width is 3.5 mm, i.e. 6" equivalent compared with 2.9 mm (5"equivalent) for wagons and coaches
  • Flange depth is 0.88 mm (1.5") compared with 0.68 mm (1.125") (data from Website of the ScaleSeven Group).

I've had success before pushing Minerva wheels out to S7 back-to-back so I'll try the same with this Dapol set.

I'm not planning to thin the wheels, they measure 3.5mm thick so only half a mm over scale, 2.5mm should be enough to get a good seating on the axle.

S7 standards for loco/diesel wheels are 3.16 min to 3.25 max so as you say if you leave them as they are you'd only be 0.25mm oversize. But what is more important is flange thickness.

I will be reprofiling the flange; that’s the easy bit.

The Class 122 is now converted to S7 and it isn't too difficult.

This is how I did it.

First remove the 6 screws holding the bogie frames in place.


The wheels and axles will then just lift out.


Next use a wheel puller to remove the wheels from the axle, I wouldn't try pulling them off by hand as you will probably damage the plastic insulation.


4 wheels off their axles, these are then reprofiled in the lathe, I use a couple of cast iron clamps to hold them and a HSS profiling tool you can buy from the S7 group.

Then you need to put the wheels very carefully back onto the axles, I think the best way is to gently push them fully back onto the axle then I used the wheel puller to move the wheels 1mm further apart at each end, checking the back to back with some digital callipers.


You then need to remove the brake support linkage. You can see it in this photo and then in the one below I have cut them out with some pliers.


This now gives plenty of space for the widened wheel sets.


I used some tweezers to help place the wheel sets back in the bogies, and then refitted the bogie frames.

I tested it on my rolling road and it runs very nicely. Now I just need to fit a decoder.


Hypothetical question as I don’t model in S7 and the railcar is out of my era, but didn’t it need some kind of shims/washers/spacers to stop the axles (and the gears) moving out of alignment?

There is a small chance that the wheels might move on the axles but I doubt it will happen. My Class 15 locos use the same axles and they haven't moved at all and have had a reasonable amount of use with decent sized train over the past 3 or 4 years. There is very limited side play in the finescale version and even less once you widen the wheel sets so the gears shouldn't move out of alignment.

I would be interested in knowing what decoder and speaker/s you are proposing to use. I might see myself with a 121 in the near future, somehow, the 122 didn't quite have the same appeal - until the Sandite/route learner era. It will be interesting to see what differences Dapol include on the Loadhaul 122 when they bring it out.

Of course. I've dropped an email to Youchoos to see what they have. The Dapol instructions say it should be an 8-function decoder, so it will be interesting to see what Youchoos comes up with.

I have on order from Digitrains, 2 x Zimo MX644c sound chips, and SP1 speakers, of which Dapol do recommend in the instruction booklet.

Once I have received them, and fitted them I’ll give some feedback, but from what I have read, it seems to be a doddle to fit.

I purchased a Zimo MX644C from YouChoos earlier this week and fitted a single tangbang speaker in the guards van it meausres 66 x 29 x12 mm once you cut the fixing lugs off. I stuck it down with black tack. It all works as per YouChoos instructions.


I have also added the front end detail provided with the DMU (air and vac brakes speedo etc) and put some transfers on it for the head code and route indicator, as mine will be in the north east it is clearly a "special"


This is a much better picture, with lights on.


I did a bit more testing on the decoder this afternoon and found that it will not turn on the main interior lights. It should be F12. F11 turns on the cab light but at the cab at the rear. I have let YouChoos know and I imagine it will just need a few CVs tweaking to fix it.

Youchoos have identified the problem and are doing a guide to to how to fix it.

This is how to fix the cab lights.

The cab lights are the wrong way around because the Dapol leaflet has them listed the wrong way around! This is easily remedied with CV#129=1 and CV#130=2.

They also suggested that changing CV112 to 32 would improve running, I tried it on mine and it didn't make any difference to be honest; mine runs very well.

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