Cable coun­sel

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

Kees Ma­joor from the Nether­lands reck­ons I owe speedo cable man­u­fac­tur­ers an apol­ogy. In Au­gust Fixes I grum­bled about skimped ma­te­rial mean­ing there wasn’t enough thread on the knurled nut to fit a Smiths mag­netic speedo. Kees points out that it doesn’t fit be­cause it’s the wrong cable – a groove machined in the knurl­ing in­di­cat­ing it’s for a Chrono­met­ric type with a shorter one. Hummph... ah, but hang on – the one with the groove did fit (just), the other one was the prob­lem re­place­ment; so not only have they skimped on ma­te­rial, they’ve dodged ma­chin­ing the line! I rest my case.

But jok­ing apart, Kees, who was at one time the ‘go to’ guy for speedo re­pairs in the Nether­lands AMC club, sug­gests it’s worth men­tion­ing that the main case of Chrono­met­ric fail­ure is mud­dling the cables the other way – fit­ting a Mag­netic cable. Be­cause the in­ner cable is longer, tight­en­ing the nut puts up­ward pres­sure on the speedo’s in­ter­nal drive, which is only held down by a small plate; if this bends, the gears move out of mesh enough to strip off their teeth – thanks to Kees for the pic­ture be­low, il­lus­trat­ing the prob­lems. I think this prob­lem started in the ’80s when the only cables eas­ily avail­able were for later bikes like Com­man­dos and T140s with mag­netic clocks. More re­cently, sup­pli­ers have been care­ful what they sell af­ter com­plaints about (ex­pen­sive) bro­ken in­stru­ments.

There’s an easy an­swer: be­fore fit­ting the nut, in­sert the cable in­ner into the speedo head and make sure the alu­minium sleeve butts up against the clock’s threaded boss on the clock with­out any force.

LEFT: Too much cable means plate dam­age lead­ing to den­tal de­cay

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