All BSA unit twin frame numbers until early in the '66 model year started with A50, and did not match the engine number. Round your measurement off to the nearest even inches, 4', 6', 8'.)Īlso, just because the frame number starts with A50 doesn't mean it didn't start life as a 650. (but they may have slipped a little, or someone may have mounted them a little lower to give a little more spring stiffness. If they're mounted correctly, the distance from the lower edge of the lower tree to the lower edge of the spring collar should give you the amount your forks are extended. Those collars at the top of your springs were what was sold to make an extended outside spring BSA fork work.
