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A J72 for Bethesda Sidings - Part 2

Captain Kernow



So now to the details of the chassis build.


This blog is essentially about putting a replacement etched chassis kit under an existing RTR body, but it's also about me finally realising a desire to get this long-standing loco running properly.


I do like Perseverence chassis kits. They are pretty straightforward and simple to build, they have helpful tabs and slots for the frame spacers (should you choose to use the manufacturers recommended configuration) and are, in my view, entirely adequate for a 'layout loco' that's going to spend more time running on a layout than being closely inspected by the unforgiving macro lens of digital cameras.


I know that High Level do a chassis kit for the J72, and indeed I have one or two of Chris Gibbon's chassis kits under some of my other RTR bodies, and I would imagine that his kit is probably the best and most detailed/comprehensive for a J72 in 4mm scale.


I have used one of High Level's excellent gearboxes, however.


The chassis kit instructions are pretty straightforward, with diagrams and plenty of description. I think that these kits were unavailable for a while, but they are most definitely back on the market now. More information can be found on their website here - http://www.perseverancekits.co.uk/


One of the first things I did, after removing the J72 body from the old Mainline split chassis, was to glue in new chassis fixing points at the front and rear. The front fixing is (as per the kit instructions) one of the frame spacers, with an 8BA nut soldered in place. The rear fixing point is made up of plasticard and has another 8BA nut glued in place and 'trapped' by being closely surrounded by more plasticard, plus a piece on top of the nut. You will need to ensure that the 8BA bolt is cut to exactly the right length for this one.


The rear fixing point also functions as a shelf for the body to sit on the rear of the chassis, so it's important to get the position of this exactly right, relative to the rest of the body:


As I was building this chassis with single beam compensation, the leading and middle axle holes were cut out for the hornblocks:


I then turned to the motor and gearbox. I had a Loadhauler gearbox left over from another project a few years ago, where I decided to change the gearbox. I cleaned this up and attached a Mashima 1220 motor to it and ran it in for a few hours, with the motor sitting on a bit of bluetack and wires directly soldered to it. All seemed to be well.




It was at this juncture that I realised that I ought to check that the motor and gearbox would actually fit in this rather small loco. It didn't, the Loadhauler is too tall for a J72.


I then got out my acetate print-off from the High Level website, that shows the actual size of the gearboxes. I worked out that a Humpshunter was the one I wanted, so I e-mailed Chris at High Level and asked for the price of a 90:1 example. I got a really quick response from Chris, together with a gearbox in the post extremely quickly, so many, many thanks to Chris for his usual excellent level of service.


While I was waiting for the new gearbox, I put the hornblocks together and laid the essential chassis components out:


Although I do have a chassis building jig (the Hobby Holidays one), I didn't really need it to solder the frames together, but instead used these Comet jigs:


Here is the soldered up chassis, albeit with the hornblocks and some other detail still to be added:


Next thing was to get the chassis rolling. I had bought Markits wheels for the J72, and set about fettling the wheels to take the squared off axle ends. I don't know if many of you have had this problem, but 'as bought', it's often difficult to get a Markits wheel to fit the axle end, without a bit of careful fettling to the square hole in the wheel. Fortunately I had a small, square section file that was exactly the right size:


Around this time, the new gearbox arrived, so I made that up and attached a Mashima 1020 motor to it. The smaller motor is necessary, because on the J72 it will need to sit between the frames slightly, where the distance is just under 11mm:


I tested the gearbox and motor combination, having first run the motor in for a while on a bit of bluetack. The slow and smooth motion as the driving wheel revolves so very slowly is just a joy to behold. Chris has refined the construction of his gearboxes in the few years since I last assembled one, and they are just fantastic.


The next task, having soldered the hornblocks to the chassis, using the Hobby Holidays jig and the coupling rods as a guide to the precise correct location, was to get the wheels in the chassis and the coupling rods on. They are temporarily held in place by small bits of plastic wire insulation. Fortunately the chassis rolled really smoothly, as I propelled it in either direction with my Peckett 'Plantagenet':


Similar view but with the J72's body on the chassis:


I hadn't got the rear body fixing point quite right, in that the body was sittling a fraction of a mm too high at the back:


Fortunately there was some metal on the rear of the frames that could be removed to give the correct rear buffer height.


So this is the current state of play:




I did need to add an additional frame spacer just under the motor, so that it wouldn't fall down into the frames and foul the compensation beam:


The next jobs are to test the chassis under power, and once I'm happy with that, fit the brake gear and pick ups. There will then need to be some weight added to the loco body and I can then paint and weather the chassis and give the loco body a little gentle weathering as well.

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I cant with to see the finished model as I have a couple of Mainline loco's I have been waiting to replace the chassis on myself

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  • RMweb Gold

Hi HornbyMM, I'd take the plunge if you want to get your J72s running.


Don't get me wrong, the most detailed chassis kit for a 4mm J72 is probably the High Level one, but there's nothing wrong with the Perseverence chassis if you want a basic chassis that will give you a sweet-running loco. In OO or EM, you certainly don't need to install the compensation. I do it, because I think it does help with current pick-up, even in OO.


I believe Comet also do a chassis similar to the Perseverence one, but I don't think that Comet feature 'tab & slot' in their chassis, which I know Perseverence do. But I believe the Comet range is rather larger than that of Perseverence and their chassis kits are fine, at the end of the day, so you pays your money and takes your choice.


I do think that the High Level range of gearboxes is the best on the market, however, especially following what appear to be their upgrades.


Don't forget that ideally you'll need a smallish motor that's no more than 10.5mm in width, as the J72 works best with the motor partially between the frames.

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