Fitting a tachometer.

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Pharisee
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Fitting a tachometer.

Post by Pharisee »

Has anyone got knowledge of fitting a cable driven tachometer to a heavyweight 350 / 500 engine? Years ago, I fitted one to a Norton ES2 that I turned into a presentable cafe racer. If memory serves correctly, it was a tacho drive gearbox from a Velocette, driven by a slotted nut securing the mag/dyno drive sprocket onto the inlet camshaft. Back then I had a full workshop at my disposal, these days I only have a garden shed but I would still like to fit a tachometer to my 18S. Would anyone know if parts are available or could be adapted from another make?
Sorry about the awful photo but I had this thing about black engines back in the day...
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Mick D
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Re: Fitting a tachometer.

Post by Mick D »

Hi

A, possibly, easier option would be to fit an optical or Hall effect tacho and run the cable via a mechanical gearbox case and cable outer to preserve the look.

Regards Mick
39speedtwin
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Re: Fitting a tachometer.

Post by 39speedtwin »

To fit either a Chronometric or a Magnetic Rev-counter to be driven from the magneto pinion, you will require a revering drive, this is because the magneto turns anti clockwise and the instruments require a clockwise input. A 1:1 drive requires a 2:1 rev-counter and a 2:1 drive a 4:1.
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Pharisee
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Re: Fitting a tachometer.

Post by Pharisee »

39speedtwin wrote:To fit either a Chronometric or a Magnetic Rev-counter to be driven from the magneto pinion, you will require a revering drive, this is because the magneto turns anti clockwise and the instruments require a clockwise input. A 1:1 drive requires a 2:1 rev-counter and a 2:1 drive a 4:1.
Yes, I'd figured that much out. I was thinking along the lines of using the tacho drive from a late 60's Triumph. They are driven from the end of the exhaust camshaft which rotates anti-clockwise. These are a 2:1, reversing gearbox and 4:1 tachometers are relatively easy to get hold of. It shouldn't be too difficult to fit as it would just require a threaded hole in the chain cover... Maybe a threaded boss if the cover is too thin... and a special nut made for the sprocket to drive it.
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Re: Fitting a tachometer.

Post by g5wqian »

the cheap indian chrono type tachos can be obtained in clockwise and anti clockwise types , and also the M12X1 thread type which is same as the smiths instruments used for the cable .

many of the indian tachos are m18 thread but they also produce the m12 ones which will fit the smiths cable nuts, you may need to silver solder a bit on the square end of the cable or may need to grind it down to fit the tacho but thats the easy bit .

at around £12 for one of those indian tachos it can be a good way to make a project work until you take the plunge for a real chronometric tacho .
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the indian types are magnetic tachos and will work to some degree for indication but i dont know just how accurate they are .

i have put one on an ariel arrow via a tacho drive to crank and it works ok .

you need the tacho drive that has a ballrace in to run it from the crank , ie i think its BG1508 type .

one some amc twins there is a different timing cover which as the tacho mounting cast into it but i think you need a shaped spacer to fit the outside as the timing case isnt flat on outside .

you can use a SCITSU electronic tacho , these are used by classic racers , they use iignition pulses and have their own rechargeable battery fitted inside .

there are also electronic chronometric tachos available but are not cheap .

i have one of those cheap £4 chinese counter/tacho on my amc twin , it just connects a wire coiled around a plug lead to it and has its own batery , it works to some degree although they say they dont work on magneto ignitions i find it does work in a manner of speaking .

i cant say it is accurate everywhere but it works and reads something , i think it tends to jump around though but for £4 it was just put on as a tool to try and indicate what revs i am going at a given speed .

f you have one of those gunson ignition dwell type test meters and you have coil ignition you can use one of those to check idle revs etc but perhaps not really much good for what you need .
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