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 - Genuine Andover Norton Spare Parts - Andover Norton Motorcycles Parts - Original Andover Norton Parts
Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.andover-norton.co.uk
Number on the dial is 3003/00A or possibly 004.
www.obsoletebikeparts.com
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.
Smiths tachometer gearbox BG1508-05
Condition: NOS Made in England by Smiths Industries Used for checking ratio on bike for one minute only.1-1 RatioWill fit BSA Gold Star (65-9164)