MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Overdrive speedo drive

Hello, I'm installing a 1976 OD into my 1970 MGB. The standard transmission has a right angle drive that is not installed on the OD for the speedometer cable.

Is the OD supposed to have a right angle drive? Thank you in advance.
TM Byers

Moss shows one for the late overdrive, I had a 71 parts GT with overdrive and it had one. That said, I didn't use it when I switched the OD into a roadster. Works fine without the angle drive but you do need to route it carefully so there are no sharp bends in the cable.

Clifton
Clifton Gordon

TM,

The angle drives do not change the ratio. They are an unnecessary evil. Use a 72" cable so as to round out the curve in the cable under the car as gently as possible. Angle drives are a PITA; they wear out and they're expensive. Similarly, if there's an angle drive on your speedo, take it off. The cable will screw directly up to speedo and to the transmission without problem.

BUT... If you're putting a '76 OD in a '70 B, you'll need an (interchangeable) '75-76 speedo. This is NOT because you're switching to O/D. It's because you've gone from a '70 to a '76 transmission. At the bottom of your '70 speedo face, you'll see that it is marked "1280 tpm" - that's 1280 turns per mile. Beginning with rubber-bumper production (or is it '75 model year production?) all speedos (with or without O/D) were switched to 1000 tpm. If you prefer, a good instrument repair shop can convert your old speedo to the 1000 tpm specification so that it will register accurately with your new O/D gearbox (which you will love). Otherwise your old speedo with your new O/D box will register about 22% slow.

FWIW,
Allen
Allen Bachelder

It was rubber bumper production, or blue-label ODs to be precise as it may not be original to the car you have got it from, that need the 1000tpm speedo, before that 4-synch gearboxes used 1280. And that assumes the new OD you have has the original ratio speedo drive! As Allen says the right-angle drives are 1:1 and irrelevant. It is easy to replace the gearbox end with a longer cable, although I thought the LHD cars really do need one at the speedo end as there isn't space for a gentle enough curve. You need a 80mm 1000 tpm speedo from a 74 1/2 to 76 to fit a 72 with a blue-label OD, from 77 on they went back to 4" same as the 3-synch.
Paul Hunt

Paul,

Yes, the 72" O/D cable works well on LHD cars. I haven't used an angle drive at either end of the cable in many years. The cable goes through a hole in the bulkhead on the left side and makes the gentle curve around to the right side of the transmission. Now my '69 MGC uses an angle drive at the speedo and the cable goes across to the right under the dash and goes through the bulkhead presumably using the same hole as found on the home-market cars. Somewhere between '69 and '73 they eliminated this angle drive on LHD cars.

LHD cars (or all of them?) may have used an angle drive at the transmission up to the end of production - I don't know. I know only that angle drives are an expensive PITA and they're unnecessary if the 72" cable is used. The angle drive at the speedo on my C was all but seized and I'm pretty sure I'll reroute the cable to eliminate it. The hole now used for the windscreen washer hose will work just fine.

FWIW,
Allen
Allen Bachelder

According to the Parts Catalogue all LHD cars used two angle drives i.e. one at each end and RHD cars only used one at the gearbox end. Neither of my cars have them but, the cables are long enough to cope without, even though it is a little awkward. I can see the routing it would take if there *were* angle drives, as there are various notches/bent flanges to avoid the sheath rubbing against sharp edges. I was under the impression that the speedo cable came across the inside of the bulkhead on LHD cars before dropping down through the same hole as RHD cars i.e. on the right of the tunnel, hence the tight curve behind the LHD speedo if an angle drive isn't used. I have heard of people drilling through on the left-hand side so it can go straighter, maybe that has been done to yours, maybe the factory adopted it after a while and didn't update the catalogue, which isn't unknown. I think the important thing is that if a specific LHD cable *does* go across the car inside the cabin before dropping through the hole on the right, then doing away with an angle drive there could cause problems.
Paul Hunt

Thanks to everyones opinion. I have a 1976 blue plate OD and purchased a 1000tpm speedometer. I have the OD speedometer cable so I will see how it will fit without the angle drive at the transmission. I also have a copper washer which I understand will help the cable from "bottoming out".

Thanks for the advice and now on to installing the engine/transmission and trying my luck at the crossmember installation!
TM Byers
TM Byers

This thread was discussed between 26/08/2008 and 30/08/2008

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now