I'll add what I can to this discussion. Back in the mid '80s I had modified my PR to be competitive in the AMA's BOTT classes. After breaking two axles and one stock swingarm, I decided to make my own swingarm. The PR has since been restored and is in someone else's private museum, but the swingarm is now in my Norton streamliner. In building the liner, we added the monoshock structure to the swingarm, but you can still see the original twin shock mounts I used when it was on the PR.
It is constructed of 4130 chrome-moly steel, and has opposed pairs of tapered roller bearings on each side. It has a larger rear axle to fit the 18" mag wheels I was using, and is wide enough to clear the 18" slicks of the day. In order to fit the wider tires, I moved the gearbox over to the left by removing the stock top spacer and macining the bottom case lug to match, and using spacers on the right side. It gave me something like .200" or .250" more chain clearance. I don't recall the exact amount now. I had also switched from the 530 to a 520 chain, for a little more clearance. Moving the gearbox was only possible because I was using a belt drive. There's no room to move the countershaft sprocket over with a stock chaincase. I didn't have to space the engine pulley out any. I was using a Bob Newby 40 mm wide drive, and all I had to do was remove the spacer behind the clutch basket to bring it back in line with the engine pulley. I think I machined a small counterbore in the back of the clutch center to capture the stock mainshaft circlip, in place of the stock spacer. I also had to space the rear shock mounts a little further apart, to keep the chain from rubbing against the rear shocks. I'm currently using it on the liner with a 17" Dunlop slick in 160/55 size, and there is still room to go a bit larger. I once fitted it with the same slick in a 155 width on the PR back in the early '90s as an experiment. It fit ok, but didn't give enough ground clearance for racing. I think a 165 would fit, and maybe even a 170.
If I recall correctly, when I was still using the stock swingarm, some of the 18" 120 section tires would just barely fit. The Dunlop 120 of the day was a little wider than the otheres, and I think we had to shave the left side slightly to clear the chain.
For the curious, the gearbox is a heavy duty Quaife 5-speed with a hydraulic clutch.
Ken
