3 speed speedometer drive

[Howard Wright]

Hi All

I’ve just rebuilt a spare 3 speed box with Andes gears from Andy Bird. I’ve replaced the old box and so far everything seems fine and I think the ratios are an improvement.

However the speedometer now under reads and the odometer records lower miles than actually travelled.  I’ve taken out the speedometer drive spindle and found the gear to have 9 teeth.  On the original box the gear has 8 teeth.  I’ve put the spindle with the 8 tooth gear back in (replacing the 9 tooth gear) and turning the axle by hand turns the speedo drive gear smoothly.

My question is from what gearbox is the 9 tooth speedo drive, AND is it ok to simply change the drive spindle or should I change the gear on on the main drive shaft as well? I hope it is the former otherwise the back cover has to come off  .

Final question, is anyone making new speedo drive gears as both gears I have seem a bit worn?

Cheers

Howard

[Alan]

There are at least two different speedo drive gear sets for the 3 Speed Box. The late RN type has different gearing to earlier cars and Ulsters etc with different axle ratios will be different too. Sorry, but I’d be surprised if they are interchangeable except in sets.

[Ian McGowan]

Some while ago, Tony Betts had a selection of new speedo drive pinions; worth a call.

[Howard Wright]

Thanks Alan and Ian

I’ve contacted Tony.  I wondered if a 9 tooth gear could be a rare Sports variety but I guess any back axle ratio change would lead to a change of speedo gears.

Looks like I’ll have to take the rear cover off. Hopefully I can do this with the box in situ.

Cheers

Howard

[Mark R]

Page 15 of the gearbox section of the index cards of the a7ca archive may help.

It lists different parts for both parts of the drive, suggesting that they should be used as a pair. It does list both 8 and 9 tooth examples. Clearly a blank lug where the handbrake pivot would be may suggest a later box which may have the later ratio?

[Alan]

Howard, the difference between the two gear sets should be about 2mph at 30mph. I’d be inclined to live with it. I may be wrong but I don’t think you could change the gears in situ because I think the 3rd motion shaft has to come out.

[Howard Wright]

Hi Mark and Alan

Thanks for the input.  According to Tony Betts it seems that most 3 speed boxes have 7 teeth speedo spindle gears.  I need to check that I have counted the teeth correctly before getting a replacement.

Alan.... I found that it wasn’t the speed that concerned me but the change in the odometer reading.  I found it was reading about 0.8 of the true distance.  Having just rebuilt the box I think I could remove and replace the back plate (yes it does involve removing the 3rd motion shaft) but as this is almost the last operation you do when assembling the box you should be able to do it in situ.

However I did a 12 mile run with the mismatched speedo drive gears and everything seems to run smoothly. I may just chance it  . 

Cheers

Howard