Overdrive install problems

[Torsten]

Hello,

I bought a nice, working overdrive gearbox in the UK (online) for my 1970 USA-spec MGB GT. I asked the seller twice if it is the correct 'black label' LH-overdrive, which is used in these cars until mid-74. Both times he answered yes, but after receiving (and thoroughly cleaning it, long story on the delivery...) I discovered that this is a blue label LH-overdrive, which is used in the rubber bumper cars.

My problem now is that my speedo doesn't match the 1000 tpm of the overdrive gear. The speedo is a 1280 tpm, which is used in combination with the black label OD's.
I now have several options:
1. returning the gearbox: no, because it is in a state way too good for the money I payed for it and it was a real 'struggle' getting it 'all the way' from the UK to the Netherlands...;
2. buy the right speedo: yes, but I'm looking for a matching one (see my pic);
3.calibration of the current speedo: no, way to expensive over here!
4. change the speedo pinion and speedo gear in the OD: yes, I have a set of matching pinion and gear from a broken OD.

At the moment my favourite choice is number 4. But this means I would have to take the OD apart, which kinda creeps me out... Is there a possibility to replace the speedo gear 'from behind' by taking out the rear bearing?

What's your opinion about my little project? Keep in mind that I'm just an enthousiast who works on this case with a friend who luckily has some years of experience working on classic British cars. But this project is a novelty to him too...

Best regards,
Torsten

 

[Stewart]

I think you can just swap out the drive pinion which is just involves 1 bolt and sliding the old pinion out.

 

[Deleted member 8987]

Or, get a reduction gearbox for the speedo cable.

https://www.speedograph-richfield.com/html/minature_gearbox.html

https://www.westnet.com/~mfrank/Speedometer/Smiths.html

We've used these drive gear up/down units for EVER in all sorts of applications to correct for rear axle and tyre size changes.

 

[PSJ]

Another option: I had my speedometer re-geared to match my overdrive and the exact tire/wheel size I am running. (Actually, I bought the wrong speedo on Ebay and went this way just to be done with messing around with it!).

Phil

 

[Torsten]

Well, I found someone who was willing and capable of changing the speedo pinion and gear from my '76 rubber bumper overdrive for a set from a chrome bumper OD. Luckily I didn't decide to swap the speedometer, because when we opened the OD and removed the annulus, I was shocked... One of the planet wheels' bearings was broken, so the wheel carved into the outer ring of the annulus. The result is shown in the pic below. I wouldn't gave gotten far with the OD in this condition... Luckily the good man had a broken OD with a fine looking annulus for me, so we swapped this and the speedo gear and pinion. After some other checking and replacing some gaskets and bearings it seems that the gearbox finally is ready for installation.

Regards,
Torsten