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TR2/3/3A TR3 Tacho Repair

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V

vivdownunder

Jedi Warrior
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Hi all,

A colleague's tacho repairer has asked him for the speed the tacho cable rotates at, relative to engine revolutions.

Does it rotate through the various drives in the pedestal below the distributor at half engine speed or what ?.

Thanks,

Viv.
 
Good question. I would think it is geared down but what would I know.
 
Where's Randall???

He has all of that stuff when you need it.
 
The tach cable does rotate at 1/2 crank speed. Turning the cable with a test motor at 500 rpm should indicate 1000 rpm on the tach, etc.
 
An cordless electric drill with a square drive bit in the end of appropriate size works well to test--make sure the thing turns freely by hand and is not frozen first, corded drill probably will spint too fast.
 
Aloha Viv,

I concur with Moseso, the distributor rotates at one half engine speed. The tachometer drive is driven of distributor drive shaft and rotates at the same speed as the distributors. On my tachometer on the instrument face at the bottom it reads:
"R.P.M
x 100
2:1"
That's confirmation.
 
Brosky said:
Where's Randall???
Click to expand...
Apparently, my home ISP and BCF are no longer on speaking terms.

I agree with the others, a TR3 tach head reads twice cable rpm, as indicated on the face. (TR6 has a different ratio as I recall.)

Not likely your drill motor turns too fast (2500 rpm is way too fast for an ordinary drill bit), but finding a suitable adapter might be a problem. Besides, most drill motors are series-wound, so the rated speed is only an estimate at best. You can't rely on the name plate to tell you how fast they turn with no load. Theoretically, the no load speed would be infinite, it's only the drag in the gears and bearings that keeps them down.
 
On later Triumph mechanical tachs the ratio appears somewhere on the face as X.XX:1 I assume from the discussion here that Smiths wasn't doing this when the TR3 was made.

When I work on mechanical tachs and speedos I use an old piece of broken speedo cable in the chuck to drive the instrument. If I'm doing calibration work I use a DC motor hobby lathe with digital speed read out.
 
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