Speedo drive gear issue

Iain Warner · Posted: Fri Apr 22, 2016 10:33 am Post subject: Speedo drive gear issue

I am currently running my 3 litre with a B type gearbox I picked up at Beaulieu autojumble about 20 years ago, in place of the original A box that currently sits on the garage floor.
My axle ratio is 3.53 but the box came from a car with a 4.23 axle, therefore the speedometer reads some 20% slow.
I have sourced a set of speedo drive gears to correct this but my problem is getting the original driven gear free of the keyed and tapered shaft it is stubbornly attached to. Drawing A87 at the back of the technical facts shows the set up very clearly.
A large amount of heat and pressure on the driven gear have achieved no movement at all, and I am concerned that much more pressure will result in severe damage to the apparently fragile cast aluminium housing. I am beginning to think that my only option (if I am to fit the new gears) is to machine/drill into the old gear until it frees up, thereby destroying it.
Does anybody have any suggestion or experience with this issue?

Stephen Blakey · Posted: Fri Apr 22, 2016 10:42 am Post subject:

With Jaguar rear hubs, folks sometimes have to tighten up the puller as tight as possible then leave it there. A day or two later the thing comes off with an almighty bang. Could you try a variation on that approach? If you're using a hydraulic press, you would have to check it was maintaining pressure.

Good luck and do let us know how you get on.

Chris Card · Guest

Before I replaced it with the "proper thing", my rev counter drive was a little gearbox (Speedograph Richfield?). Would it be practical to insert something similar into the drive cable for your speedo?

Chris

Stephen Blakey · Posted: Fri Apr 22, 2016 11:25 am Post subject:

That might be a pragmatic and less destructive solution.

Paul Spencer · Posted: Fri Apr 22, 2016 6:55 pm Post subject:

My car was switched from 3.78:1 to 4.23:1 but still has the original speedo. Or at least a speedo that is out by about the same amount as my overdrive - which is 28%. We could consider swapping - it might get mine right and yours closer. On the other hand. I have just had my speedo rebuilt, so I am not 100% sure I would want to do it - I am considering a gearbox to drive the speedo driven off the overdrive. Will you be at Concours? We could always discuss it if you are interested.

Chris Card · Guest

Paul - I am in Namibia at the moment, so can't look underneath my car until we get home, mid May. I have an overdrive and the speedo drive is geared to give a more, or less, accurate speed reading. If I remember correctly, there is a little gearbox adjacent to the speedo take off.

Chris

Paul Spencer · Posted: Sat Apr 23, 2016 8:26 am Post subject:

Thanks Chris. I would be interested in knowing where yours came from. I know Tim Payne will do one for me, but it is pretty expensive.

Chris Card · Guest

Paul - Elmdown supplied it when thet fitted my overdrive but, I think that it came via Tim Payne, because I got a spare plastic drive gear from him (fits into the overdrive unit), when the first one failed due to the cable seizing.
It's possible that Tim gets them from Speedogragh Richfield.

Chris

Iain Warner · Posted: Sun Apr 24, 2016 1:19 pm Post subject:

Thanks for your replies and suggestions. I am pleased to report that the gear has finally succumbed to a mixture of brute force and heat.
With the nut tightened against the inside of the casing and the gear heated, several heavy blows on the outside of the case finally caused the gear to come off the taper. Thankfully without destroying the fragile looking casing.

Paul, I may now have a surplus pair of 'B' box speedo gears for a 4.23 axle if they are of any use to you. Although as you have an overdrive fitted I cannot see that they would produce a correct reading.

Paul Spencer · Posted: Sun Apr 24, 2016 7:02 pm Post subject:

Thanks Iain. I am probably better off talking to Tim Payne or Speedogragh Richfield. As you say, just changing the gears is only of limited use - it would make the speedo right without the overdrive switched in, but wrong when it is. Currently, it is the other way round and I interpret my speed from the tacho. I am pretty good at this in 3rd, 4th and overdrive.

Chris Card · Guest

Paul - my speedo is driven from the overdrive unit and reads correctly (more or less!), whether the overdrive is engaged, or not. The longer cable bypasses the gearbox drive, which I have removed as, without a cable attached, it leaked oil. I made up a blanking plate to cover the aperture in the gearbox.

Chris