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MG MGB Technical - Overdrive swap?

Aside from the driveshaft lengths, are the LH and D type overdrives interchangeable?
Greg Hayes

I believe the later OD can be used in earlier cars but the tunnel needs to be bashed out a bit as the older cars were narrower thru there.
Bob Ekstrand

Greg,

It depends. As a whole unit with gearbox, then most likely as Bob indicates, but only if the input shaft is the correct size for your engine. 18G and 18GA had a smaller pilot bushing in the crankshaft and all the rest have the larger shaft requirement. Are the overdrive units themselves interchangable, I don't believe so--you'll need the matching intermediate boxes and gearboxes.

The only other thing I remember to look for is a reverse light switch on the gearbox if your car requires--not all D types came with a reverse switch. Be sure the backplate to the engine and gearbox match to your aplication, I'm pretty sure they're not all interchangable. Lastly, the driveshafts do not have to be swapted unless you have a banjo axle and are installing a D type overdrive. In this case, although many reference books say that you don't--they always forget about the earliest cars--you will snap the driveshaft as it will over extend.

I always cringe at the thought of bashing an early tunnel. Narrow tunnel cars are getting rare and the added syncro isn't that big a deal--just my opinion.

Paul Hanley

I will add to Pual thoughts...that the gear stick (shift) is slightly differnt on the two boxes and i think (I may stand corrected on this) that the hole in the floor may have to be enlarged too. Now where is Chris when you need him????
Cecil Kimber

Go to Doug Jackson's web site, British Automotive at http://www.mgbmga.com/tech/index.html (this URL should take you directly to the tech index), scroll down to transmissions then select the article that matches the year of your car. These articles tell you everything you will need to make the swap. Good luck - Dave
David DuBois

Swapping the complete gearbox/overdrive assembly requires changing the plate on the back of the block, flywheel and starter plus different driveshaft lengths. Shifter locations are different and the early tunnel must be bashed out as previously stated if that is the direction you are going. As for swapping a D for an LH on an existing gearbox it cannot be done (from a practical standpoint).
David

I have done this conversation and installed an all synchromesh overdrive gearbox in my narrow transmission tunnel car.

The information from myself and Ollie Stephenson (and others) is all in the archives.

The only clearance problems are around the starter motor mounting point on the bell housing. I over came this by filing some aluminium from the actual bellhousing.

Where the speedo cable exits the gearbox, it is helpful to make a small dent which makes connection of the cable easier.

It is simple to machine the first motion shaft down to fit the smaller crankshaft bush. You can even remove this shaft without dismantling the gearbox.

The modification to allow the gear lever to exit correctly is also quite simple.

For driving pleasure it is well worth the swap.

cheers
Ian Buckley

Ian

Re: machining the 1st motion shaft - Did it go through the hardening? Does it matter?

I came across that when I machined a Marina 1st motion shaft shorter to fit a midget 1500 box, but counterbored the shaft and fitted hardened steel pin to fit the pilot bush.

Cheers
ian
Ian Fraser

Ian

No, machining the first motion shift didn't make any difference. I did 25,000 miles and then made the big move to a Rover V8.
Ian Buckley

I guess I didn't make my problem clear enough. The D overdrive didn't disengage and I backed up in overdrive. Now the overdrive is shot. I know where I can get a rebuilt LH overdrive at a good price from a local mechanic, and that's cheaper than a D overdrive, but I'm not sure if the LH overdrive from the 4 synchro gearbox will fit on my 3 synchro transmission?
Greg Hayes

As Mr. david said above the two OD are completely different and not interchangeble on the same gearbox. D is much smaller and lighter and splines are diferent not to mention mounting. Bob/International auto
Bob Thompson

If I was faced with your situation, I would have the D type rebuilt. Contact John Twist at University Motors. http://universitymotorsltd.com

Paul Hanley

There is a place here in Jax that specializes in Laycock OD's. The name is Vinco and the fellow has been doing them for years. The # is 904-757-6300. I've never done business with him but I believe Bob T has.
David

Hi:

Have the D type overhauled. These units are becoming rare, but replacements parts are available. Try Quantum Mechanics (Connecticut) for overhaul and spares. John Esposito is top notch regarding LBC OD gearboxes.

Good luck: Rich Boris, MGCC-LIC member

Rich Boris

One step further than sending your D type to Twist, is to take it (and the whole gear box if you like) to Twist's transmission rebuild seminar. I did this last year and highly recommend it.

While this is not Greg's problem, when I drive a metal-dash car, I expect that tight "snick" of the three-synchro boxes. It's part of the charm. I spent five years looking for a three-synch O/D box because of this. It's really more fun to drive. The four-synch is just a little too refined for the grass-roots bare-bones simplicity of the pre-smog cars.

My 2¢ worth,
Allen
Allen Bachelder

This thread was discussed between 16/12/2004 and 19/12/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.