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MG MGB Technical - Overdrive Transmision Compatability

I've got a friend who's switching to a Ford Sierra 5-speed gearbox. He's GIVING me his '77 overdrive gearbox to put in my early '74 BGT.
My question to you is - will it fit and, just as important, is it compatable. I looked at the Moss Motors cataglog and see 4 OD trannys listed:
1962-1964, 1965-1967, 1967-1974, and 1975-1980. However, there is just 2 non-OD trannys listed: 1962-1967 and 1968-1980.
The only difference I can see in the 2 Type "LH" overdrives is the speedometer gear and pinion. Do you guys know of any other reason why I shouldn't take this generous gift and run my early '74 BGT with it?
~Dan
Dan Neu

None at all. Except, if I am not mistaken, his will be the newer 'blue label' LH, which is 1000TPM, whilst your speedo is 1280TPM. You can either replace your speedo, have is re-clibtayed, or replace the ouput pinion in the tranny (not sure how hard that is!).

I'm sure the pros will correct me if I have made any errors...


Go for it!
Dave
Dave Whitehead

Dave you are absolutely right - they are the options and you have to replace the driving gear on the OD output shaft as well as the pinion - a complete tear down of the OD unit to get to it!

Much easier to get the speedo recalibrated - the later 1000 tpm speedo is a smaller unit than the early 1280 tpm one but there is a midget early speedo that is 1040 tpm which is as close as dammit!
Chris Betson

Nort American MGB's for 75 & 76 have the smaller 1,000 tpm speedometer so you should be able to find a used speedometer. Clifton
Clifton Gordon

Only other point I can think of is that the blue-cover overdrive units aren't as sturdy as the black-cover ones... if it's near 100k miles or so, it's a good investment to have the later type 'seen to'. See Cars The MGB GT Gearbox 3 on my webble site for more info.

Speedo in my '67 car says 1080tpm... don't know how useful that info is, 8% error isn't really acceptable, unless the speedos are guaranteed to over-read more than the modern amount (wouldn't be surprised!).

hth,
--
Olly
Oliver Stephenson

I'm using a late O'drive in a '67 with the 1080 odometer. I'm also using slightly smaller diameter tires (195 55 15) The difference should be about equal. In the real world the odometer is about right but the speedo calibration is about 10% hi.
Barry
Barry Parkinson

This might be an unpopular opinion, but if you'll be pulling the engine and tranny anyway, why not do the same (or similar) conversion that your friend is doing? The Laycock overdrive is nearly impossible to fix properly should anything goes wrong with it (just ask Volvo owners with this unit installed). My US$0.02.
Adam Birnbaum

I've gotta disagree on that one Adam... the overdrive is superbly easy to work on and I think all the parts are available from Overdrive Repair Services. Plus, the LH type is pretty bulletproof in black-cover form. Most problems with them sticking can be solved with a flush-through with ATF.

If you're disassembling the OD unit to have the thrust washer replaced, it's not that much more work to swap your speedo drive gears out. That would be my choice of action... 50 quid and a couple of hours' work gets you a sorted transmission that's compatible with your existing speedo...

Anyone know if there's a 1000tpm speedo in the early smiths style that goes up to 160mph?

fwiw.
--
Olly
Oliver Stephenson

Olly,

Nothing as far as I know 0-160 MPH but SNT 5372-06C is 0-170 MPH and SNT 5372-09C is 0-140 MPH. Both available new from http://www.gower-oaks.co.uk/

Geoff
Geoff King

Many thanks Geoff, I'll bookmark that page for when I can afford to know how fast I'm going. Which, at the moment, isn't very fast :o)

ttfn,
--
Olly
Oliver Stephenson

Thanks for all the helpful info. My early '74 GT is an orginal non-OD car. While looking in the Moss catalog, I notice that there's a non-OD speedometer and a OD speedometer listed for '72-'76. Does anybody know the difference between the two? How many TPM is my non-OD speedo? Dave tells me that my blue label LH OD is 1280TPM. If I take my non-OD speedo to Palo Alto Speedometer, will they know how many TPM it is and how to recalibrate it to the blue label LH?

On another note - the oil that came out of it wasn't the recommended 20w-50. It was gear lube. You know - the stuff that stinks. It was kinda cloudy too, which makes me think that there might have been some additives in with it. Is that going to muck up the OD clutch friction material? Is there anything else I should think about before I clean the screens and filters, fill it with good oil and install it in the GT? (That Ford Sierra box is sounding like the better option all the time. Except you can't beat the price of this OD.)
~Dan
Dan Neu

The blue label OD should require a 1000 tpm speedometer. I assume it has a side fill plug rather than the dipstick as you have on your 74. The speedometer drive cable drive gear should be red. The tpm should be marked at around 5 o'clock on the outer edge of your speedometer. I have not noticed any difference between an OD and a non OD speedometer.

Why not ask your friend if he had problems with the gift overdrive. Why is he spending around 2k $$ to replace a good OD transmission? If it were going in my car I would like to know why it's free and why my friend is changing it. FWIW, Clifton
Clifton Gordon

I have a '65 MGB With a '72 black label OD, it has a side fill plug. Which tpm speedometer should it require?

Ray
Ray Gordon

Wow, once I get all this data together, I'll post it - TPM, Year, Model.

He's got a early '74 roadster that he's building. He was given a '77 roadster by his wife for his birthday as a donor car. He drove it around the block once and started pulling it apart. Now his wife doesn't like having two half disassembled cars in the driveway, so he's liquidating the parts he doesn't need for his '74 project.

Now about this oil I found in it. . .
Dan Neu

the problem with gl 5 gear oil is that it has sulpher that will attack the brass synchronizers. gl 4 is low sulpher and will not damage the synchros. The problem with the heavy oils (gl 5 or 4) in o'drives is that the o'drive clutch/band doesn't work well with heavy oils. Modern auto trans fluid is made for radial gears and band/hydraulic clutches. It is a little thin for the conventional tranny, although my local transmission shop says it works fine. Most modern manual transmissions are factory spec for auto trans fluid. The thinner fluid has less friction drag and helps with better gas mileage. Red Line manual trans fluid is about the same viscosity as auto trans fluid. For my o'drive I put in a about a quart of 70-90 gl 4 lube and the rest auto trans fluid. The synchros work great and the o'drive works great.
Barry
Barry Parkinson

Ray, I thought the side fill transmissions were supposed to have a blue label but it cold have been changed at some point. My guess is that it should be 1000 tpm, but it's probably best to verify what you have with some type of odometer check like interstate mile markers. Clifton
Clifton Gordon

side fill = blue label = 1000tpm

there's two types of side-fill though... after '75 there's less bolts to hold the side cover on. Everything else is the same though.

Bizarrely, standard engine oil comes out smelling like gear oil. I took the oil out of my trans yesterday and it stank of sulphur... must be something to do with the temperatures you get, as I've changed the oil myself several times using duckhams 20w50 so I know what was in it.

hth,
--
Olly
Oliver Stephenson

Sorry Clifton
my '72 OD is blue label not black label :-/

Ray
Ray Gordon

Ray
68-74 North American overdrives were 1280 tpm, the same as the non-overdrives of the same years. These were top fill transmissions with a dip stick. The late model transmissions both overdrive and non-overdrive had a side fill plug. The color of the labels could change as PO did work on the transmissions. Your speedometer is most likey a 1040 tpm if the car was a non-overdrive car. If your car had a overdrive originally, then it would have a 1010 tpm speedometer. If your transmission has a side fill plug, then it is most likely a 1000 tmp and would be close to your existing speedometer. If you have a 1280 tmp transmission you can get a 4" speedometer from a UK car 68-74 and have a correct speedometer. I have a 1280 transmission from a 69 in my 67 B. I got a 1280 speedometer (68-74 UK car) from Chris Betson and that solved my problem.
Jim Lema

I have a similar problem, I have replaced my 72 OD with one from a later RB car. The speedo is now laughably inaccurate. Can I remove the gears from a 1000tpm speedo and use them to replace the gears in my 1280 speedo?
philip may

Jim

My '65 was non-overdrive car originally W/dip stick. I replaced it with a '72 OD W/side filler plug.

Then I likely a 1000 tmp speedo?

Ray
Ray Gordon

This thread was discussed between 24/08/2003 and 07/09/2003

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