Cable counsel
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.