Speedometer drive gear

[Roger]

Finding my speedometer not working I used a drill in reverse and proved the speedometer and cable  from the gearbox was all working.
The gear fitted has 8 teeth, on a spare gearbox I found one with 9 teeth, neither appeared to turn when rotating the drive shaft by hand.
The worm wheel on the gearbox output was not visible, the one on the spare box was far more visible.
Does anyone know how many teeth there should be, it is a 4 speed synchro box in a 1933 RP.

Roger

[Speedex750]

Finding my speedometer not working I used a drill in reverse and proved the speedometer and cable  from the gearbox was all working.
The gear fitted has 8 teeth, on a spare gearbox I found one with 9 teeth, neither appeared to turn when rotating the drive shaft by hand.
The worm wheel on the gearbox output was not visible, the one on the spare box was far more visible.
Does anyone know how many teeth there should be, it is a 4 speed synchro box in a 1933 RP.

Roger

Hi Roger,

A standard 4 speed syncro box should have an eight toothed pinion and a two start worm. The nine tooth pinion was in the Nippy type box, is your spare more exotic than you thought! The pinion unscrews easily from the tail housing, the gear coming loose on the spindle isn't unusual.

Dave

[Roger]

Dave,
It looks as though I have two 4 speed and two 3 speed, how do I tell them apart, they don’t all have tops, although I do have loose tops .
The spare 4 speed box has what looks like a single start drive gear, there was not one in the original one fitted to the car!
I take it that the 3 speed ones would not be compatible with my box.

Roger

[Speedex750]

Dave,
It looks as though I have two 4 speed and two 3 speed, how do I tell them apart, they don’t all have tops, although I do have loose tops .
The spare 4 speed box has what looks like a single start drive gear, there was not one in the original one fitted to the car!
I take it that the 3 speed ones would not be compatible with my box.

Roger

Hi Roger,

Mmmm, sounds like you've a good collection of bits as we all do! The three speed boxes are easy to spot, the lid flange is held on by four bolts, one at each corner. The lids have a brass plug for oil filling. Four speed gearbox lids are held on by six bolts and the oil filler is a casing bulge near the back left hand side with a plug in the top. The very first four speeds had a 'crash' box with no syncro on any gear. These can be spotted by having four nuts just behind the lid securing the fork rods - the syncro boxes only have three. Crash lids are different as well with a large nut just in front of the lever. If you can't visualise this browse through the photos of gearboxes for sale on Ebay. Nippy's had a four speed syncro box but with different internal gearing - including the speedo pinion. You can spot Nippy boxes by an S stamped next to the small oval flange at the back. Internals are better preserved by keeping the lids on.

It is possible to swop between three and four speeds but they're not a direct swop, other parts and things have to be done in parallel. If your car has the right box in and it works I would suggest you stick with it.

The speedo pinion can be removed by unscrewing the large brass hex where the speedo cable goes on underneath. It's not a very deep hex so make sure you have a well fitting spanner or socket.

There's more specific gearbox information on the Speedex website http://myweb.tiscali.co.uk/speedex/index...ge1138.htm 

Dave

[Roger]

Dave,

Thanks for your reply, a tremendous amount of information on the Speedex site.
I have now sorted the spare 4 speed synchro box, the worm drive is fitted but it is only a single start thread. The 8 tooth pinion meshes well with the worm, the 9 tooth pinion does not mesh so well.
I notice in the Speedex formulas the numbers are divided by either 8 or 9 so that Is obviously the pinion teeth value. I will use the 8 tooth pinion as a starting point.
My reasons for swapping boxes was first gear jumping out also difficult to engage, the spare box felt better when selecting gears on the bench, it also had the worm wheel fitted to start with, it just needed a good clean but was in good condition.

   

I was wrong, all the boxes I have are 4 speed, one has an”S” as you suggested.The other does not.

   

Thank you again for your help

Roger