Ebay speedos

[tomkilde]

The time has come to replace my reproduction clocks with vintage Smiths instruments.  The repros currently on the bike look great, but are wildly inaccurate.  I was lucky and found a reasonably priced speedo in good condition on eBay, but the tachs seems to be much more expensive and I'm hesitant to take the plunge until I'm sure I've got the right model number.  I understand that a RC1303/03 with a 3:1 ratio is the "correct" model for my 1961 Super Rocket, but I intend to drive the tach off the magneto, not the oil pump, and I don't know if this makes a difference.  The tach drive fitted to my bike is marked BG1508/05, which corresponds to a 1:1 ratio and is "reversing".

[Billybream]

The Road Rocket version was off the magneto and is 4 to 1 ratio
The Super Rocket from 1960 was off the oil pump and is 3 to 1 ratio.
If you can find a speedo, the specialist can convert them to a rev counter.

[Black Sheep]

Converting a speedo to  a rev counter is easy. Just take out the trip mileage mechanism and put on a rev counter dial. Used to do that when we were peniless students.

[Klaus]

The Road Rocket version was off the magneto and is 4 to 1 ratio
The Super Rocket from 1960 was off the oil pump and is 3 to 1 ratio.
If you can find a speedo, the specialist can convert them to a rev counter.

Still  in mind, Magneto is turning 2:1 two engine revs are 1 Magneto, the tachdrive is 1:1 so the tach had to be 2:1.


cheers Klaus

[limeyrob]

Yes, which is why the BSA's eat up the tacho cable as they are doing up to 3000 rpm.  Annoyingly mine has a 2:1 drive and a 4:1 cheapo tacho so its a big hit to correct that.

[groily]

This is heresy, I know, and also tangential to the main point here about tacho g'box and instrument-head gear ratios, but I'm not sure that the chronometric mechanism is ideal for a tacho.
I've preferred the magnetic instruments I've had on a few machines despite their not being as pretty.  So I put up with the chrono tacho on the Norton (which is in good shape) often staying at 800rpm when the engine is stopped  . . . and taking its time to adjust to the actual rpm being employed out on the road.

Was also looking at Scitsu tachos for bikes with mags, but somehow the conversation with the guy that does them petered out and I never resurrected it.
Maybe I will look again  . . . Anyone got one on a magneto bike? And if so, any good? They eliminate the need for mechanical drive, which has a certain appeal if the pick-up from the HT leads is reliable, and they're not ugly. Which may make them of interest to anyone pondering the how and how much of fitting one.
https://www.scitsu.co.uk/common-fitment-20-w.asp

[tomkilde]

Thank you for the comments, but it's unclear whether the magneto drive requires a 2:1 tach or a 4:1 tach.  Also, am I correct that the anti-clockwise RC1303/3 tach originally specified for the 1961 Super Rocket was driven off the oil pump, but because I intend to use a "reversing" drive fitted to the magneto, I should be looking for a clockwise tach instead?

My main point of reference for this exercise has been the excellent table found at http://www.chronometrics.co.uk/html/tachometers.html

[Rex]

Was also looking at Scitsu tachos for bikes with mags, but somehow the conversation with the guy that does them petered out and I never resurrected it.

I asked this very question on the Velocette pages, and there was no answer. Easy on coil-and-points engines, not so much on mag engines.

[mark]

RC 83 Tacho with a 1-1 reversing drive is what I have on my 54 Road Rocket and 53 Super Flash

[limeyrob]

Yes 1:1 reversing is the std drive for a 2:1 tacho off the mag.  Same as a Goldie

[Klaus]

I have the Skitsu tach on my Racebikes.
The new ones had an external 9v Accu and can be used on bikes without  12 Battery.
They pick up the signals from the HT leads.
On  Racebikes they are ok, but on others bikes, its my oppinion they are ugly.

cheers Klaus

[Black Sheep]

1959 A10 R/c is 4:1 clockwise, drive from the magneto is 2:1. I thought they were all like that prior to driving off the oil pump.
No problems encountered with chronometric rev counters in the last 50 odd years.

[groily]

I have the Skitsu tach on my Racebikes.
The new ones had an external 9v Accu and can be used on bikes without  12 Battery.
They pick up the signals from the HT leads.
On  Racebikes they are ok, but on others bikes, its my oppinion they are ugly.

cheers Klaus

Thank you Klaus, much appreciated.

Poking around again in various places this afternoon, Access Norton, Britbike forum etc, I can see that there has been a bit of discussion, some of it not very complimentary. When I was first looking I was minded to order up a 'Black' version with an 8K range, for a mag-fired twin, just to see how it went. I'd been put off by the cost of tacho-drive gearboxes and correct instruments, and was looking for easier / cheaper options. At that time they had rechargeable Ni-Cad batteries and a special charging lead, a dummy plug to put in the thing when not in use (to preserve the battery's charge), and the need for a bit of sticky tape to stop water ingress in use. I didn't go there. It seems now though, that they come with (lithium) batteries which last about a year, and are easily and cheaply replaced. That sounds more like.

But, coming back to the complaints from people who have had problems - and apparent difficulties in getting responses from the makers - it doesn't look quite so rosy. Previous 'some and some' experience of not dissimilar after-market tachos on cars also makes me a trifle wary. 
Still tempted  . . . but it's just gizmo-lust at the end of the day (and probably a bit silly).
If I could find anyone among my local friends using one on a classic race bike, that might nudge me into doing something  . . .

[limeyrob]

I did look into putting a trigger on the timing cover and using a programmable digital tacho.  What put me off was that it was still going to cost £2-300.  The odds are that a lot of the internal electronics are 5v so it "should" be possible to run of 6v. But to be honest i was out of my depth.