SPROCKET WATCH

Classic Bike (UK) - - WORKSHOP RICK’S FIXES -

Steve France wasn’t happy about a noise com­ing from his pre-unit Tri­umph engine and found that the engine sprocket is slack on the crank splines, caus­ing chat­ter. He asks if it would be worth try­ing some­thing like 660 Loc­tite – and if so, how dif­fi­cult would it be to re­move in fu­ture?

This is a dif­fi­cult one. Loc­tite swells on cur­ing to take up slack around a loose bear­ing, for ex­am­ple. It’s a sim­i­lar prin­ci­ple to things seiz­ing through rust­ing, but with the kind of con­tin­u­ous im­pact a sprocket faces, I’d be sur­prised if a chem­i­cal re­pair would last, and if it was strong enough I’d worry about get­ting it apart. Loc­tite does yield to heat but sprock­ets and cranks are heat treated so you don’t want to go too mad with the blow torch.

But any slack in this area will be a prob­lem that ac­cel­er­ates, so any­thing that can be done to re­duce the move­ment will help pre­vent it get­ting worse. The usual re­pair is to have the crank welded up and re-machined, which some­body like SRM En­gi­neer­ing could un­der­take, but it would mean strip­ping the engine com­pletely.

I re­mem­ber once read­ing an old dodge about bang­ing fine sewing nee­dles into worn splines to pack them out, but the wear would need to be pretty ex­treme be­fore there was that much space; sounds a bit like one of those great ideas no­body’s ac­tu­ally tried. But a new sprocket might be worth a go; be­cause the wear is likely to be spread be­tween both male and fe­male splines, re­new­ing the sprocket may just take up enough wear to cure the trou­ble.

Spline wear could be the crank, sprocket or both

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