CRUNCHY GEARBOX
Drew Wegg has this very nice 1952 500cc BSA, which he inherited from a friend. After a two-year lay-up, he took it to a garage for some cosmetic jobs, but now finds it won’t go any further than first gear and there’s a lot of crunching and clattering. The bike has a fourspring SRM clutch conversion and, after initially suspecting it had stuck, Drew has discovered that this is not the problem. These gearboxes can be a bit iffy on selection – the M21 I built in CB always needed a tap on the pedal after changing to re-centralise the lever – but in this case I suspected that the clutch needed adjusting or the plates had got ‘gummy’ with standing. But then I remembered the old adage: ‘What’s the last thing you did before it went wrong?’ Although the bike only went in for cosmetic attention, was the rear wheel disturbed? On all pre-unit bikes, the gearbox is clamped in the frame separate to the motor, and the torque of the rear wheel can pull the gearbox backwards, slackening the rear chain but stretching the front – and a tight front chain badly affects clutch operation and gearchange. The BSA’S ‘plunger’ suspension moves the wheel vertically rather than in an arc, so it may need more slack than a modern machine. If it’s been set too tight, a big bump could tug the gearbox back. Drew came back to say the job was fitting a new rear mudguard, so that could be it. Whenever you adjust the primary chain, you should overtighten it and then back off to the correct setting, putting adjuster backlash the right way to resist pull from the back wheel. After that, check the rear chain, which is obviously also affected.