gearbox bearings

Dave - I’m pretty sure I sourced one of these for my wideline 88, it was a FAG 6203 TB. They are pricey, but you can get them from 123Bearing.co.uk. About 30 quid. I think the layshaft is better with one of these in than a roller as they positively locate the shaft and stop it floating around. My 88SS had a roller in it and I had to shim 60 thou out of the shaft. 

How does using a ball race stop the layshaft floating about? The layshaft is a sliding fit in this bearing, The end float of the layshaft depends on the thickness of the inner cover gasket, the kickstart shaft assembly and to a greater degree on how well the box was made originally. Using the roller bearing will positively locate the layshaft and has a greater load capacity than any of the 6203 ball races. Using the recommended roller bearing (superblend variety) gives the layshaft an easier life,      

I had the same experience as Russel with excess end float on the kickstart shaft  after fitting a roller race . And i was not convinced that ISSO shims at the KS end of the layshaft was the best solution . I prefer shimming at the roller end . This is not so easy to do as no selection of  shims readily availiable . This is why Hemmings used to recommend the uprarted ball race. And No shims needed .

I think these (or similar) are the type Mick was recommending:
https://simplybearings.co.uk/shop/p5081/SKF-6203ETN9-Open-Deep-Groove-Ball-Bearing-with-Fibre-Cage-17x40x12mm/product_info.html

Some problem with the rivets in the standard metal cages coming apart.

That said, I have never had a problem with standard, metal-cage, bearings from more reputable brtands like SKF and FAG. The roller bearing is better suited to the loadings on the end of the layshaft in the 3 lower gears but, as noted above, you do need to locate the shaft axially (NUP prefix does this).

Some people prefer C3 clearance bearings. In the scheme of bearings, a Norton box is not running particularly fast or hot and standard clearance seems quite suitable.

Bearing issues are much more of a problem on Commandos than earlier twins. Clean, uncontaminated oil is the most important consideration.