What other ratios were used to drive Smiths tachos? The tach on my atlas seems inaccurate - reads fast - but not by a factor of 2. It's possible I got the meter head off a Triumph or BSA.The camshaft gives a reduction of 2:1 so a 4:1 tachometer needs a 2:1 drive gearbox the same as the early 20M3 Commando (as all Commando tachometers were 4:1).
(The original Atlas tachometer would have been 2:1, therefore, had a 1:1 drive gearbox.)Motorcycle parts and spares for Norton motorbikes - Andover Norton Spare Parts - Andover Norton Motorcycles Parts - Original Andover Norton Parts
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Number on the dial is 3003/00A or possibly 004.
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Thanks, L.A.B. On GM speedometers that I've had apart, there is an adjustment for the spring to calibrate the speed correctly. If there is such an adjustment on a smiths magnetic tacho I could not find it. I have two of these, one I took apart intending to clean the face but never did. How would one check for accuracy? Maybe the one that's apart it better.According to the Smiths list the RSM 3003/00 would be the correct magnetic tacho for an Atlas* (from '64?). The "A" likely indicates a later replacement production run or minor alteration so would still be correct but probably not the original tacho.
*(Also 650, P11, N15, various Matchless, etc.)
Your RSM 3003/00A tacho, therefore, should be marked "RPM, x100, 2-1"...
... the matching drive gearbox would be a 1:1 BG 1508/05.
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