Rev Counter Gearbox - 1958 SR

[Terryb]

I have the chance to acquire a Rev Counter Gearbox part no. 65-9164, which fits onto the Primary Outer case and is driven off of the magneto. Am I correct to assume that this drive assembly will only fit Magneto's with manual A/R?

[Bsareg]

It's thought the drag caused by the cable and rev counter would cause the automatic ignition to keep the timing retarded.

[Klaus]

No you can also fit with the automatic unit. I use this for many years with out problems.

cheers Klaus

[JulianS]

The gearbox has a tongue which connects with a slot in the pinion retaining nut on the magneto.

You would need to replace the a/r unit nut with one of suitable length with a slot. I have a vague memory of such a nut being advertised, years back, by George Prew.

If such a nut is available then the usual a/r unit problem is that the centre nut will not screw out of the unit because the threads are damaged.

[Bsareg]

Would you need to fit stronger springs to keep the same advance curve ?

[Radlan2]

I recently bought one of those elongated nuts from Dragonfly, but I then found a problem that the machined face holes in my timing cover do not allow the rev counter drive  to correctly align (not the fault of the nut). It will be a tricky thing to correct as there is no meat on the drive to elongate the holes, plug and re-drill I suppose.
       Chris

[CheeserBeezer]

The magneto gear in the ATD turns at the same speed as the camshaft gear. The magneto gear is connected flexibly to the armature by springs and bob weights. It is the job of the bob weights to pull the armature into its advanced position as revs increase. The springs return the ATD and armature to the retarded position as the revs drop. Any additional drag on the armature spindle caused by something attached to the ATD nut will increase the amount of work the bob weights have to do. Therefore, theoretically, the attachment of a tachometer drive will cause the ATD to be sluggish due to the additional friction in the system which is acting against the effort applied by the bob weights..... in my opinion!

[Rex]

It's the opinion of the Velocette factory/guru's too.
Four feet of cable and a chrono head exerts quite a bit of drag on the ATD.

[Joolstacho]

I agree, conventional wisdom in the Velo community is that a tacho needs manual A/R but I can't speak with any personal experience.
Then again, Klaus has hands-on experience. Interesting.

[Black Sheep]

You really need a manual A/R for a timing cover driven rev counter. Don't know how you would do it with an auto-advance. Anyway, I prefer a manual advance. Spot the A/R lever on the bars.