Just in today, the rear sprocket for the S90.
I made a calculated guess what ratio I would need and ordered a 40t aluminium rear sprocket for 415 chain.
Still needs some work to make it fit, but that is no problem.
Will save a lot of weight compared to the original steel sprockets, also the 415 chain is a lot thinner and lighter then 428, will also save weight and look a lot more like a racebike.
On the CB50 I also use 415 chain, but that originally has 420.
Front sprockets can be narrowed from 420 to 415, but not from 428 as that has a different pitch and roller diameters. This is the reason I am using expensive RS125 front sprockets, fits perfectly on the S90 outputshaft and has the right dimensions for 415 chain.
Later I might get more sprockets to get the correct ratio for each circuit, but first I want to see how this works, as the ratio depends on the gearbox, max. RPM and wheeldiameter, which are all different from stock.