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A65 tacho ratio #451395 08/28/12 1:15 pm
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fatmarl Offline OP
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I want to fit a tacho to my 1967 A65T (originally fitted with just a sppedo). What ratio tacho head should I buy? Cheers.

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Re: A65 tacho ratio [Re: fatmarl] #451397 08/28/12 2:18 pm
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There a bit rare, 3:1 clockwise,there was a new one on eBay america .


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Re: A65 tacho ratio [Re: fatmarl] #451479 08/28/12 10:37 pm
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You must be careful, as many folks list such things as, "Triumph, BSA, Norton."
Wak is correct, our tach drives use a 3:1 ratio, Triumph uses 4:1. Communicate with the seller to be sure you're getting the right one. Or zoom in on the picture, the ratio is on the face.


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Re: A65 tacho ratio [Re: DavidP] #451526 08/29/12 8:35 am
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'67 Thunderbolt should have RSM 3003/06 on the bottom of the dial.

According to my Smiths application CD you can also use ones with /02 (8000rpm max) or /04 (9000rpm max) as substitute for the /06 10,000rpm unit. Not sure how common those are though.

Chris

Last edited by ChrisX; 08/29/12 8:35 am.

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Re: A65 tacho ratio [Re: fatmarl] #451527 08/29/12 8:50 am
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wak Offline
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if your not to bothered about originality then why not use a cheap electronic car one,or even fit the workings in to a busted smiths body?


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Re: A65 tacho ratio [Re: wak] #451528 08/29/12 9:01 am
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Gees, for once Wac & I agree.
No one will see it when t is running so get a digital and fit it inside an old stuffed Smiths head.
A lot of vintage riders do this with push bike speedos, works a treat.


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Re: A65 tacho ratio [Re: fatmarl] #451600 08/29/12 7:20 pm
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fatmarl Offline OP
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Thanks for all of the replies, I've now managed to source a 3:1 tacho (needs refurbishing though).........

Re: A65 tacho ratio [Re: fatmarl] #451703 08/30/12 12:27 pm
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Sii Offline
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Hi All,

Would this apply to my 1972 OIF A65n aswell? It's currently got a 4:1 ratio pattern tacho unit on it and it certainly seems to be reading too high.

Many thanks

Re: A65 tacho ratio [Re: Sii] #451706 08/30/12 12:43 pm
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Originally Posted by Sii
Hi All,

Would this apply to my 1972 OIF A65n aswell? It's currently got a 4:1 ratio pattern tacho unit on it and it certainly seems to be reading too high.

Many thanks


Yes, it will read high. For example, at 4,000rpm the instrument will read over 5,000rpom (5,333rpm in theory). Mind you, not all the original tachos were necearily all that accurate!


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Re: A65 tacho ratio [Re: fatmarl] #451755 08/30/12 5:58 pm
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Believe it or not but it is possible to adjust the speedo and tacho to give the correct readings. Details of this can be found in a very good book "Magnetic Speedometer Repair" by Graham Bligh. This explains how to rebuild and adjust the Smiths SSM speedo and tacho.

The tacho uses the same mechanism as the speedo to make the needle move. Both mechanisms rely on a magnetic disc spinning inside a metal cup to move the needle around the dial.

Inside both mechanisms there is an adjuster screw which varies the distance between the spinning magnet and cup. When turned, the adjuster varies the needle movement on the dial.

Apparantly, it is possible to adjust a 4:1 tacho so that it reads like a 3:1 and vise versa, although you would be at the limits of the adjustment to achieve either. You also need something driving the cable at a known RPM to make an accurate adjustment. The book suggests using a cordless drill coupled with a digital cycle speedo and magnetic trigger to obtain a known speed/rpm and use this for calibration of the speedo/tacho.

To make any adjustments, the bezel needs removing to gain access to the adjuster screw and its worthwhile drilling a small hole in bottom of the case so that future adjustments can be made without removal or disassembly. New bezels and glass are available on eBay and can be simply siliconed in place for easy removal in future.

I rebuilt my A65 speedo using the book as a guide. It now works well and IM pleased with the result.

Hope this helps.

Last edited by gunner; 08/30/12 6:11 pm.

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