REV Counter

My -59 40M before restoring.

Hello again Bob - I got a really good copy of the ATRC from Andy Molnar for my Manx, with mount and cable and they suit Smiths spec stuff .   They are excellent reproductions and new movements so it's worth a punt. Let me know if you need an image or two.  Regards, Howard

12,000 RPM ! No idea what it would have been fitted to. Came with a box of Commando parts from a sidecar crew.

Robert a 2:1 tacho head will complement a 1:1 reversing gearbox.  The camshaft rotates once to two revolutions of the crankshaft and it goes backwards.  The usual heads on heavy twins are 4:1 ratio clockwise rotation so a 2:1 reversing gearbox is required.  Cheers,  howard

So , perhaps a little assistance is required.  The camshaft rotates backwards at half engine speed as you know,  so with a 1:1 ratio tacho gearbox the drive cable is rotating at half engine speed, so the head needs to double the cables input.  The cable also need to rotate clockwise if the tacho reads c/w (and usually Norton versions do) so the tacho gearbox must be reversing.  Hence a 2:1 ratio head complements a reversing 1:1 tacho gearbox. Now,  if the tacho gearbox has a ratio of 2:1,  then four crankshaft revolutions rotate the drive cable once.  So the cable is rotating at a quarter of engine speed. Hence the head needs to have a 4:1 ratio.  Good luck, howard

It doesn't really matter Bob as long as you have the correct gerabox and keep the cable well oiled and have no tight bends.  The inner cable spins twice as fast with a 2:1 head and with the slower speed set you get some flickering of the instrument needle if the cable is rubbing on the outer - so you can get some oil down there pronto.  With the faster spin rate, when dry,  the cable just snaps away and no amount of oiling will help !!   ATRC heads cost a fortune these days.   Cheers, Howard

For many of the Heavy Twins the camshaft drives the rev counter gearbox.  This is already turning at half engine speed.  The standard Smiths Rev drive 1508/05 gearbox for Dommies is generally also rated at 1 to 1. (you can get 1508/06 with 2 to1)  Plus the Smiths Dominator clocks are usually 2 to 1. 

No wonder people buy the wrong replacement parts by mistake!!!

While we're discussing tachometers, I need one for my magneto ignition 650SS restoration. A genuine  original Smiths Chronometric instrument would be prohibitively expensive so I was wondering about one of their Electronic jobs. I believe they can be "programmed" to work with (four stroke) engines from a single cylinder up to 8 cylinders by taking a pulse from the ignition coil. On a single cylinder engine, that's one pulse every two crankshaft revs. 

Ok... I'm hoping that it will also take pulses from the same pulse sender unit that the electronic speedometer uses, but instead of screwing it into the the speedo drive gearbox, I'll screw it into the 1:1 ratio tacho drive gearbox on the timing cover. That should give me one pulse every two crankshaft revs as it's driven from the camshaft. Has anyone tried that?

I know about wrapping sensor wires around the magneto ht leads and I'm aware that there are cheap cable driven instruments available. The magneto cable is messy as the exhaust pipe is in the way and I thought an electronic option would be a "cleaner" option.

Any thoughts, please?

I have a Smiths electronic tacho on my Commando (green globe, not chronometric, style but I think the guts are all the same). It will trigger from a mechanically driven pulse generator or CB points. If you have a drive for a tacho cable, screw the pulse generator in there, work out how many pulses are generated per engine revolution and set the tacho to match. The pulse generator you use for the speedo should work for the tacho too.

My tacho picks up from the Rita connection to the coils ('cos the main purpose of the exercise was to eliminate the cable oil leak). I don't know whether there's an option to trigger the Smiths tacho from a HT lead.

I have a Skitsu on the Dommie.  This had a single trigger per rev from a wire wound a few turns round one plug lead, for lost spark electronic ignition.  When I returned to magneto, I extended the wire round the other lead.

Thank you, Stan.

That's encouraging... :)

I also wanted a rev counter on my 1963 650ss but could not afford a genuine clock or drive even if I could find one.

So I purchase a pattern system from RGM, the saga is in this month Roadholder.

As I say in the article it is not for the purist but 

I now have a fully working presentable rev counter

Today stumbled on a nice chronometric tacho lying in the workshop. Marked RC1309/00. Seems I had tested it and a note says it's quite accurate and 2:1 ratio. Anybody knows if it is for a Dommie?

Yes 1309/00 is correct for a 1963 Dommi.

Gearbox is listed as BG.1508/05

See attached photo from 1963 Smith's catalogue.

Hello again Bob - I think that I am correct in 1961 88SS,99SS and 650SS Models had the 150mph speedo.  These were retained on the 650 until the magnetic speedo came in in 1964.  I am unsure whether the 88SS and 99SS kept the 150mph instrument in 1963 because my 1963 88SS had a 125mph fitted.  Of course it could have been swapped by the previous owner.    Cheers, howard

My 1963 88SS has a 120mph chronometric speedo.  I can't believe someone changed it.

According to Smiths motorcycle fitment catalogue for 1962, the rev counter listed for 650 is   RC.1302/00

(40M  and 30M are listed as ATRC.2652)

Gearbox for 650 was BC.1506/05.  Cable length 2'8"

Hi all, thanks for all the input on the general rev counter subject. I have now managed to convince myself that my 1962 650ss with a camshaft driven 2:1 gearbox does indeed need a 4:1 rev counter clock unit . 

The next problem however is the drive rotation  of the new clock unit . Looking into the rear of the clock the cable will be running anti clockwise as it attaches . Looking through from the dial side of the clock unit the  drive rotation will be clockwise . 

Sorry about all that but it is relevant to the question I have finally got to. 

When the clock unit is stated as clockwise or anti clockwise , does anyone know which viewpoint the rotation is taken form . Thank you , Bob Cooper

When describing the rotation of the instrument Smiths describe the action of the finger as viewing the face of the instrument.  So for a clockwise rotation at the tachometer with a ratio of 4:1,  I believe you need a Reversing 2: 1 ratio tacho gearbox.   regards, Howard

Thanks for the input Howard . I am much more confident on what I am now looking for to put the finishing touches to my project . Regards Bob .

As below was destined for the DSNs but the question of copyright arose.

I could not load this before because it was 8.2Mb in size and the file rules said......NO WAY!!!!!

Slightly shrunk version with less Mbs as below.