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MG MGB Technical - Scored an Overdrive Transmission

I got lucky and found an overdrive transmission out of a crashed 70 BGT that I'll eventually put in my 73 B roadster. It was crashed 30 years ago and been sitting ever since. I think it has about 60k miles on it.

I have the transmission/overdrive unit. Once I get it cleaned up I'll post some photos with a few more questions.

At the moment though I'm just trying to clarify two things I've heard over the past few years.

1. Is it true I'll need a different drive shaft? Some have said the trans. with the overdrive is the same overall length as the one without and some have told me it is longer with the overdrive. Which is correct? I can't find anything in Moss that specifies a different drive shaft for one as versus the other.

2. Does the car use the same rear axle whether there is an overdrive or not? I've been told that the overdrive used a different rear end gear ratio.

Just two general theory questions for now. I'm sure there will be more to talk about later.

Thanks,
BH
BH Davis

BH,

It is a striaght swap. No problems. The axles is the same and the prop is the same (it is different for 3 syncro cars, but a 70 box is 4 syncro). You will need a little wiring, which is pretty straight forward, especially if you go for a dash mounted switch. It will tranform the car!!

Enjoy,
Iain
67 BGT
I D Cameron

BH. Iain is correct. The different prop shafts only apply to the older, three syncro transmissions. All of the four syncro boxes interchange without problem.

Well, there are some problems. Getting the bolts into the transmission to attach the mounts is a real bear. Oliver Stephanson, who used to post here, made a modification to the transmission cross member which allows the overdrive transmission to be assembled to the cross member in a somewhat different, far easier manner.

You will need an overdrive length speedometer cable and the angle drive is not used with an overdrive transmission.

I have found that swapping in an overdrive transmission of unknown reliability can be an interesting experience. The only overdrive transmission that I have installed that had no problems at all was one rebuilt during John Twist's "Four Synco/Overdrive Transmission Rebuild" class, offered every year in February. I do not plan on installing a non-rebuilt overdrive tranny again. Having had to pull two of the three installed do to problems found when the car was put back on the road, was the deciding factor in this decision.

Les
Les Bengtson

BH - I was going to send you link to the web site that had the rear cross member modifications on it - unfortunately the person who has that web site is doing something with it and it has only cutesy pictures on it. I do have the modification downloaded and if you would like to have it, send me an e-mail and I'll sent it to you. Cheers - Dave
David DuBois

An angle drive was originally used on at the gearbox ends of *all* systems as far as I am aware, certainly the Parts Catalogue shows them for 4-sync gearboxes of both non-OD and OD type right up to the end of production. It allows the cable to lie alongside the length of the gearbox before it turns upwards to go through the firewall, instead of taking an awkward loop past the crossmember. LHD cars had an additional one at the speedo end of LHD cars due to the tight radius there as the cable still came up the right-hand side of the tunnel and not the left.

Many cars have lost these over the years as they are relatively fragile and expensive to replace (neither of my cars has one), and a longer speedo cable allows for a gentler curve at the *gearbox* end which makes one less necessary. Get the original length of OD speedo cable and use it without the angle drive and you may have problems. For USA without service interval counter the OD cable was 1829mm or 6ft, compared to a non-OD of 1372mm or 4ft 6" - quite a difference.
Paul Hunt 2

BH,

Congratulations on your find. I love the OD on the 70 MGB, and when I get around to building my 70 BGT, I'm going to hunt one down for that.

dave
Dave Braun

BH-
The same rear axle ratio (3.909:1) was used with both overdrive and non-overdrive transmissions.

I agree with Les on the subject of installing an untested Overdrive unit. After sitting for thirty years, the friction material on the sliding clutch will almost certainly be ruined and it will need to be relined. The closest rebuilder to you that I'm aware of is Quantum Mechanics at 505 Wheeler Road, Monroe, CT. Phone # (203) 549-9612. They have a website at http://www.quantumechanics.com/ So far I've sent two friends to them and the results were excellent, no problems since.
Steve S.

X2 on Quantum mechanics...I bought a OD box from him for my 78...best upgrade to date and he really knows how to rebuild these properly. Use ONLY 30W engine oil in the OD box.
P J KELLY

For some info on the cross member modification see http://www.kewengineering.co.uk/upgrades4mgs/Engines/modifying_the_gearbox_cross.htm

I carried out this modification and it helped greatly. I think I have a pic of my fully modified cross member. If that would help let me know and I will dig it out and post it.

Basically it allows you to bolt the Y piece and mounts to the box, and then offer up the gearbox X member. You can the use the modification to get access to the studs on the mounts and tighten the nuts and then tighten the "top hat" piece to the X member. Finally I bolted the X member to the chassis. Much easier than the original way.

Cheers
Iain
67 BGT
I D Cameron

Oh, and the original spec oil for an OD box is simply the same as the engine oil. I use 20W/50 which is the normal oil for the cars in the UK.

Iain
I D Cameron

This thread was discussed between 05/04/2008 and 09/04/2008

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