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MG MGB Technical - O/D Conversion

I have just installed an O/D from a 1976 MGB to my 1972 Mgb.

Speedometer is off and wanders up and down. The original speedo is #1280. Replaced the cable with a new OD one.

I haven't found any information on which speedo I should use or if I can use another #1280 to fix the problem. Also, I am unable to locate a new speedo that is listed for the OD. What number would be proper.
A Huggins

The 1976 transmission should have been a side filler rather than a top filler with dipstick. I believe that the speedometer was changed with the change over of transmission styles.

Your #1280 is actually 1280 turns per mile, the number of rotations the speedo cable makes when the car travels one mile. That ratio was used from the introduction of the Mark II series in late 1967 through, I believe the end of the 1974 models. After that, the turns per mile were 1,000 and both the gearing in the transmission speedo cable take off and the gearing in the speedometer were changed to reflect this.

Most of my RB car experience is with the 77 and later models. Thus, I am not sure if there would be speedometer used in the 75 or 75 model which would both have the correct gearing for your new transmission and would fit into the existing mounting.

As to the jerking around of the speedo needle, it would be wise to remove the inner cable and make sure it is greased. The one I got from Moss, several years ago, did not come with grease applied to it, causing it to seize and damage the plastic piece at the tranny. Such is an expensive lesson.

If it is not lack of lubrication on the cable, you will probably have to have the speedometer itself examined by a competent restorer of instruments.

Les
Les Bengtson

My information is that the 1000tpm speedo was used from the start of rubber bumper production (Sep 74), and the OD changed at that time from a black label to a blue label. I would think that a speedo from an RB car with the rectangular oil gauge should be right, SN5230/13. I think they changed from 80mm back to 4" for the new 77 model year dash with the six round dials.
Paul Hunt 2

Thanks to everyone on their comments. I have articles on the subject, but I think YOU answered all the questions. Made a call to pick up a 1000, will let u know how it comes out.
A Huggins

Hey Everyone,

Back to square one. The 76 OD requires a 1000 TPM speedometer. The normal size for the speedo is 4" but the 72 takes a 3". Does anyone have any idea where I could acquire an OK speedometer 3"/1000 TPM?
James Huggins

james
why dont you just get your speedo recalibrated to suit the new gearbox?
ste
Ste Brown

You could maybe use an earlier speedo:

11 BHA4466E Speedo - Smiths - 1040 TPM
(square pointer) . . . . . . . . . .
11 BHA4468E Speedo - Smiths - 1020 TPM
(square pointer) . . . . . . . . . .
11 BHA4478E Speedo - Smiths - 1040 TPM
11 BHA4480E Speedo - Smiths - 1020 TPM

Sorry, I don't know the sizes of these but I thought the earlier speedos were larger than the later ones?
Steve Postins

The earlier model speedometers listed above are too big for a North American 72 model car, we lost the steel dash in 1968. Try Tony Barnhill at http://www.theautoist.com/

Clifton
Clifton Gordon

Have a read here. It covers lots of info that I think will be useful:-

http://www2.mgcars.org.uk/cgi-bin/or17?runprog=mgbbs&access=&mode=archiveth&subject=71&subjectar=71&thread=2001040405353022145

Derek Nicholson

You need a 1000 tpm speedo from any '75 or '76 American market car. overdrive has nothing to do with it. Beginning with '77 models, the restyled instruments will not fit your '72 dash. But the dash remained the same from '72 till '76.

And yes, an alternative is to have a shop like Nissongers recalibrate the speedo.

FWIW,
Allen
Allen Bachelder

If a 1280tpm speedo is hooked to a later (1000tpm) box, will it read 28% high, or 28% low? Where my thinking is going would be to have made up some vinyl, stick-on speedo faces that would read to 150 mph (120mph + 28%), if that'd work! Great for boasting rights. Of course that would only work if it's reading slow. And if you're not worried about originality! But I think a 1280 speedo would read slow. If your speedo is expecting 1280 tpm but the gearbox is only sending 1000 tpm, then it will read slow, right? Or have I got it bass ackwards?

Derek Nicholson

Derek,

' Takes a bit of thinking doesn't it! But it sounds to me like you have it right. I've been there, but I can't remember. I'm sure the speed could be corrected with the face cover you're proposing, but the odometer would still be way off. That might not bother a lot of folks, but it would bother me.

Part of this lesson: if you're intending to rebuild a late-model 4-sync box (one without the dipstick) for use in a 1280 tpm car (or vice-versa), change out the speedometer drive gear on the transmission while you're at it. I found a 1000 tpm speedo for my '73 conversion, but it bugs me that I thus missed out on the turn-over to 200,000 miles. 8^(

FWIW,
Allen
Allen Bachelder

Thanks everyone for your comments. Good info.

The bottom line is: the easiest part is to send the speedo off for recalibration. They also completely go through it.

There is a procedure to check the accuracy. Mark off a distance in a parking lot etc of 52 feet 9.5 inches. Have the cable open so it can be seen, and mark it so you can count the rotation. Mark the starting spot and center your left front tire on the mark, push the vehicle to the 52 feet 9.5 inch mark and center the left front tire again. Count the rotations of the cable and any part thereof ie 9.25 - 10.5. This is the number the speedo shop uses to calibrate the instrument. Considers tire/wheel size, differential and tranny. Eliminates the guess work.

I found a shop in Los Angeles and New York. Recommendatiion is the NY shop is quicker.

Anyone needing shp info, email.
James Huggins

This thread was discussed between 04/12/2006 and 14/12/2006

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