Can a Smiths Chrono Speedo Be Modded into a Chrono Tacho?

[Peter Gee]

 hear it can be done as the instruments share many the same parts. It surely cant be as simple as swapping the speedo face for a chronometric face, given the ratio off the tach drive matches the speedo?

Anyone know anything or any guides that can be seen or bought? To me the Tach must have the same mechanism, minus the mileometers?

Cheers

Peter G

[Bsareg]

I've changed speedo into a tacho a few times. Quite straight forward, remove odometer and connector and replace dial. Trickiest job  is adjusting  for accuracy. There's a little brass bob weight rocking to and fro with extra steel weights attached.  You add weight to raise the reading or reduce weight to lower. 80mph Speedos have a lighter (thinner) brass bob. 120mph speedos might have problems getting the reading low enough, depends on the ratio. I expect an instument techy will be appalled but the system works fine for my bikes.

[JulianS]

Depends on direction of rotation and gearing being correct for the conversion - some tachs set up for clockwise, some for anticlockwise, some 2;1 some 3;1.

I would consult a professional such as Russel at Chronometrics.


http://www.chronometrics.co.uk/

[Black Sheep]

I got Russell at chronometrics to do it for me. Excellent job. In student days with chronometrics at 30/- we did the DIY method outlined above. It worked.

[Tomcat]

Another way I heard of was putting the tacho face in a speedo, fit a cable and hide the other end which wasn't connected anyway. Well the rivet counters won't check that the speedo and tacho actually work!

[Peter Gee]

Very useful stuff and actually an old fashioned clockmaker should be able to work with  Chronos to Tachos as long as he had a source of drive.. Thanks folks

[BSARGS650]

Mmmm...Yes easily done, the movements are the same,  As per previous post regarding calibration by adding/subtracting weights to the "bob" wheel - the initial size of this can various with different speedo ranges and if the original one is not suitable foe the calibrated range needed, it is easily swapped out.  The driving worm and wheel may be changed for direction if you have one in the scrap box.  The problem is that if one does not have sufficient old scrap speedos to rob parts, the specialists are reluctant to sell spares - if any member knows one that is agreeable to sell parts, it would be nice to know.  Also, it makes calibration easy if done on a lathe with the speedo held on a bracket fixed to the lathe slide and a shortened old cable in the chuck.  I have a precision hand held Smiths tacho that I can accurately measure lathe spindle rpm and with the speedo mechanism partly exposed for adjustments it is easy to do.  I think faces are still available on the dock of the bay?  I bought several along with bezels and glass a couple of years ago.

[BSARGS650]

Sorry, meant to have added: consideration should be given to ratios upstream of the tacho input.....

[Peter Gee]

Thanks BSA RGS, very useful... I have quite a lot of old spae ones in various stages of disarray and a new mechanism without Odometer drums, so with luck I should be able to do all of this not myself but with a suitable clock repairer who are fairly common around here. One could also use a variable speed drill and various fixtures, as long as one could get the drill speed accurate, maybe using a magnetic  tacho first  to  note it?

Absurd prices on ebay for chrono tachos, well over US500 and often US$800. Various Indian copies about but they seem to be built too thick so won't match a chrono speedo in looks..and I wonder if they are chronometric or just magnetic look alikes? Range from 70 to 17 pounds so who knows what you will get... because you wont get what you pay for from India in many cases.

Faces you can still get on ebay, sometimes NOS. There was a guy doing great silk screened decals of them a few years ago, maybe he passed on. An Oz guy used to do Grey Face and Chrono vinly discs, but seems to have dropped off ebay.

[Bsareg]

Indian clocks are definitely magnetic not chrono. Also although the cable screw fitting is correct (12 x 1mm) the inners are  usually not. Most are 2.4mm whereas Smiths are 3mm

[BSARGS650]

I would like to add regarding my previous comments: I referred to the "bob" wheel - this is really the (mass) balance wheel.  From memory, I think the diameter was the same at half of one inch in all?  And, only came across two thickness or widths in my dabbling - I think the most common was five sixty fourths, the thin calibration weights just slipped on/off.  For changing speedometer ranges, such as 120/80mph one needs to be aware of the odometer arrangement regarding the pawl driven ratchet wheels via a eccentric cam - I do not have the knowledge whether it is sufficient to swap the eccentric cam and/or ratchet wheels too.  I was lucky that over the years I had accumulated a box of junked chrono stuff that sorted me out.....

[JulianS]

If you want a tacho which actually works then worm gear indicated in first photo is pivotal to any conversion from speedo to tacho, they vary according to the input direction and the ratio. If you do not use the correct one you will not get a correct reading whatever you do to the balancing wheel.

[Bsareg]

My memory may be playing tricks on me but surely the worm only drives the odometer, no?

[chaterlea25]

Hi Julian and All
The worm itself has no function in a tachometer, it is only used to drive the odometers
The brass gear at the base of the worm drives the speedo / rev mechanism
When reverse cable drive has to be accommodated as in the A10 units driven from the oil pumps
an idler gear is fitted between the  gear on the worm shaft and the chronometric mechanism

The  tacho unit on my SR gave trouble (more than once ) and I repaired  the  unit by making a new pivot for the idler gear as it had worn enough to lose tooth mesh

John

[Peter Gee]

All this stuff pouring in is highly useful, even when members contradict each other!