When I rebuild my late diesel chassis I use oilite bushes of the following dimensions, 8mm OD, 4mm ID and 4mm wide.
The Northern chassis sides for where the axle fits are 4mm each, so these will be a perfect starting point. You have probably noticed that the drive axle has two different journal sizes, so once the bushes have been fitted they will then need to be machined to suit the axle.
The larger journal comes out at about 4.56mm, so this hole needs to be drilled to 4.6mm. The other smaller journal is 4.10mm but the problem here is that the knurled bit for the wheel can measure slightly larger at 4.11mm, why Gilbert did this I have know idea. I take this out to 4.15mm but drills of this size can be expensive but it does make for a better fit, so the next common size drill will be 4.2mm.
The next hard bit is to know where to drill the chassis to fit the bushes. I had a look at some of my 336’s and some 6 wheel Pacific/Hudson chassis and the following dimensions are the best I can come up with that might help you and a machine shop. The distance from the bottom of the chassis to the axle hole centre is 12.5mm, this sounds like half an inch but in realty it is just under, I checked several times and I couldn’t get a measurement of 12.7mm which is half an inch.
The other measurement I used was from the end of the chassis/motor end to axle hole centre and this was a bit more difficult but I reckon it is 8.5mm. These two dimensions should be OK for a machine shop.
The other measurement I worked out is the axle hole centres for each one this is 34.35mm and from number one axle to number four axle they are 103.05mm. This could be another measurement to use.
When I drill out the diesel chassis i initially use a 7.7mm drill and then finish with a 5/16” reamer. Then I just press in the bushes. The same principal should apply to the steam chassis. The only thing I would do as a safeguard would be to add a temporary spacer between the chassis sides when pressing in the bushes, just in case it tries to distort the sides inwards.
These measurements are as close as I can get to and you should check them out to make sure they are going to work. I haven’t done one yet myself but I can see a need for a single jig/tool which will cover most of the 4,6 and 8 wheel standard chassis. Another project for retirement.
Hope this helps you or anyone else thinking of giving it a go.
