DFL dynamo rev counter drive

[Ianfrancis]

I have  DFL dynamo on my magneto engine and would like to know if its possible to fit a rev counter drive to this dynamo. I already have a mechanical rev counter but no way of driving it?
Also, am I correct in understanding that a modern rev counter which uses a cable round a spark plug lead wont work well with the magneto engine ? 
Could I connect the rev counter to the speedo drive on the 3 speed gearbox as having a rev counter is more useful that the speedo?
Any suggestions would be appreciated.

[Ruairidh Dunford]

Willie McKenzie makes very nice, period looking, electronic Rev counters - you will need to ask him if they are suitable for magneto ignition.

[stuartu]

Ruairidh's suggestion is by far the straightforward and probably the most economical solution if it is magneto compatible, which I regret I do not know.

If you could contrive a drive from your DFL dynamo you would need to know at what rate it rotates with respect to engine speed. Then you would need a rev counter calibrated to that speed or, if yours was inappropriate, a little intermediate gearbox to convert the drive to suit your tacho.

If I were dead set on a mechanical drive I would try to contrive a drive from the front of the camshaft a la Gordon England Brooklands, probably using an appropriate tacho gearbox such as you can see if you scroll down here:

http://www.timelessmotorcycles.co.uk/html/drives.html

If you really wanted to use your gearbox speedo drive you could make a new face for your speedo. You would need to know the drive ratios in each gear, the final drive ratio and the appropriate tyre constant, then you could have concentric calibrated rings on your instrument face.
I wouldn't do this because I am lazy but I might follow 2cv practice and work out the speeds in each gear at the rev limit of your choice and mark the existing speedo face 1, 2, 3 (and 4 if you have such a gear) at the appropriate speeds.

Best phone Mr Mckenzie first, I think.

Regards,

Stuart

[Austin in the Shed]

It is possible to fit a revcounter drive off the end of the camshaft using a BSA gold star type drive.I did one once.
They are available in r/h and l/h and 1:1,2:1 from memory.You would need to slot the end of the cam though.It is similar to the GE brooklands idea.

[Charles P]

I used the camshaft drive to achieve this because I couldn’t find a suitable version to run with a magneto.
I made a replacement camshaft nut that had a long 3/8“ (I think) extension. This went through a brass bush in the timing cover. The end of the 3/8” extension had a square hole in it (created with a needle file). The end of the brass bush was threaded to take a 90 degree angle drive, as suggested by Dave above.
The general arrangement is in this picture without the angle drive. You can see a grease nipple to add a little lubrication to the camshaft nose. 
If you start with a coil engine cam you can machine the 3/8” extension directly onto the cam in place of the taper for the fan belt pulley, but you need to make a new front cam bush.


   

[Ianfrancis]

Thanks for all the suggestions, I will ask Willie Mckenzie if his are compatible with my magneto engine as this sounds a much simpler solution.