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MG MGB Technical - Overdrive on 3 Syncro, 3 Main Bearing Engine

I would like to convert my 1964 3 Bearing engine MGB with a 3 Syncro gearbox to Overdrive.

Am I right in thinking that an O/D gearbox off a 5 Main Bearing engine will not fit a 3 main bearing engine? What other parts such as relays etc will I need to complete the job?

Rob
R J Collier

Rob

Have a look at I D Cameron's post a few threads down. There is already a lot of discussion that might be relevant. If not, the poster there might be a source of information since he has already made the conversion and is at the installation stage.

Good luck,

Barry
BJ Quartermaine

Rob

You either need a gearbox from a 3 bearing car or you need to machine the first motion shaft from a 5 bearing gearbox.
Although you can presumably use the first motion shaft from your original gearbox.

I was planning to fit a 3-sync OD 'box in my '67. It was originally a 3-sync non-OD but was converted to 4-sync non-OD in the '70s. My intention is to now fit a 4-sync OD 'box, so my 3-sync OD will be for sale, if you're interested?

Dave

dave(at)o138(dot)freeserve(dot)co(dot)uk
Dave O'Neill 2

Rob,

There are two choices. You can go for a 3 sync OD box (with the D type OD box) or the later 4 sync OD box (with the stronger LH type OD box). However, fitting the 4 sync is not an easy option as it doesn't fit in the tunnel (the later cars have a wider tunnel). It will also have the larger input shaft as mentioned above.

Now if you choose to go for the D type OD then there are still choices. The more common of these is the slightly later one that also has the larger diameter input shaft. I think this came in with the switch to 5 main bearing engines.

Apart from that, then you will need a different length prop shaft, or a spacer to extend the current one. A new speedo cable, and OD switch and the wiring. Some of the wiring is already in the loom making it a little easier. You'll also to fit a vacuum switch. These are NLA and difficult to get hold of. I am choosing to go without. They are designed to stop you switching out of OD under high stress conditions (overrun). Oh, strictly speaking you should get a different speedo as the TPM is different. However, it is a small percentage and not really worth bothering about (1020 tpm vs 1040).

Before starting doo check that the required holes are there for the gearbox X member as mine were missing (through previous restoration) and this caused me loads of porblems! See my thread further down for details.

Good luck
Iain
67 BGT
I D Cameron

If you go the 4-sync route, you will need to modify the tunnel. You will also need the backplate, flywheel and pre-engaged starter from a 4-sync car, I believe.
Dave O'Neill 2

Barry Iain Dave

Thanks for your comments, it would seem that fitting an o/d gearbox is not as straightforward as it would seem and considering my existing non o/d gearbox works perfectly (including 2nd gear syncro!),I would probably put this job off until a clutch is needed.

Dave, thanks for the offer but I assume your 3 syncro o/b box is 5 main bearing? I dont really want to cannabalise my existing gearbox to remove the 1st motion shaft. If anyone has a 3 bearing, 3 syncro O/D gearbox for sale though I may be interested.
Sounds like a winter project!!

Rob.
R J Collier

Rob - Consider doing what I did to get an O/D for a ZB Magnette what we had for a number of years. Since the MGA transmission (used in the Magnette) is the same as the early MGB transmissions, other than some differences in the housings, I just removed the tail piece from the original transmission, swapped out the main shaft for the one from a 3 synco overdrive transmisson and put the overdrive and remote shifter on in place of the original tailpiece. This was a fairly easy modification other than fussing round with the rear mounting, which you would not have to do, since the mouning is the same on all the early transmissions up through 67. This is still something you wil want to put off until you have to do other work requiring removal of the engine/transmission, but it is an alternative to be considered. Cheers - Dave
David DuBois

Rob,

Worth remembering that 3 sync OD's are VERY expensive! £250-£350 for used in unknown good nick. £500 for used and known to be good from a breaker and £1000 for a reconned one. I paid £350 from eBay and am hoping my luck holds! It all looked fine when I inspected it but the car won't be on the road until tomorrow night (I hope!).

Not sure if the earlier boxes carry a further premium or whether they are simply harder to come by.

Iain
I D Cameron

rob
go for the 5 bearing later d type box as it is stronger than the early 3 bearing overdrive box
early box has 3 bearings on the laygear and the later one has 4 which is stronger
either strip your box or get a spare first motion shaft
ste
Ste Brown

Rob,

I should have my 3 sync OD up and running this evening. Assuming it all works I will have a spare non OD box. It has a badly worn 2nd syncromesh, so really it is only fit for spares. I'd be happy to part with it for a token contribution to the MG fund! You could take the first motion shaft from that box and get it turned to the correct diameter at your leisure. Then you could pick up any OD box without having to worry about which shaft it has fitted.

I am located 5 miles South of Warrington. Let me know if interested.

Cheers
Iain
I D Cameron

I have an article which I wrote 20 years ago about putting the later OD gearbox in the narrow transmission car. I will cut and paste it here but for some unknown reason the beginning of the article is a few paragraphs down because it was scanned.

It actually starts

MG GEARBOX CONVERSION

Anyway the information is all there and may be of some use. It is also contained somwhere in the archives.

Ian Buckley


****************************************************


Firstly, the gearbox physically fits in the transmission tunnel without any trouble whatever . Once in place, it is not really close at any point to the body. In fact, my gearbox is one with the filler plug on the side, which I expected to have to drill a hole in the tunnel to gain access to, but had no need as there is plenty of room from underneath to fill the gearbox using an oil gun.
To complete the job I made the following modifications:
1
My car is a three main bearing model which necessitated removal of the first motion shaft from the gearbox and turned the front down to suit the small three bearing spigot bush (in the back of the crankshaft). I bought a new bush and machined the shaft to suit. This is really a straightforward job with a lathe. Note: Later models with 5 bearing engines will not require this operation.
2.
Mark II gearboxes have a different spline for the clutch driven plate to operate on, so you will need a Mark II clutch plate. The thrust bearing is the same but I am sure you would renew this in any event.
3.
I had to make a small cut from the top of the transmission tunnel immediately behind where the gear lever surfaces. After removing the tunnel carpet and the black plate covering the top of the gearbox I relieved the back of the hole about 25mm. The Mark II gear lever is about 28mm further back than the Mark I. I used an air operated hacksaw and took out a moon-shaped piece about 70mm by 25mm.
At the top of the starter bulge in the bell housing you will need to file off a lug of aluminium which when completed is almost invisible. This can be easily done with a dreadnought file. This lug is the only real point of contact between the gearbox and the body.
The gearbox cross member on the later models, I found out by accident, allows the gearbox to sit about 15mm lower and is offset fore/aft {I forget which). You could modify your own but a later one is easier. It all bolts straight into place.



MGB GEARBOX CONVERSION

The Mark I is plagued with cluster gear and layshaft/needle roller failure, especially if driven hard. Many years ago I bought a really low mileage Mark II gearbox with the intention of fitting it to my car, but changed my mind after being told that I would need to change the car to the wider transmission tunnel of the Mark II. This modification was out of the question for me as I was not prepared to modify my car to that degree. Incidentally, being tall, I have always found there is more sideways leg-room alongside the steering wheel with the narrow transmission tunnel than with the wider one. The L model MGB with the console even further restricts this room for me and I have trouble driving one unless it has a smaller steering wheel than standard. .
The idea of improving the gearbox with a minimum of modifications has always appealed to me. I did read an article in Safety Fast on putting a 5 speed Rover SD1 gear box into an MGB which did sound appealing if not a little radical. I am sure the SD1 gearboxes don't grow on trees in Australia which lessens the viability of that proposition. I am not sure now if the article describing the conversion referred to a Mark I or II MGB.
5.
4




6. To mount the starter motor you will need to bore a hole for the bendix spring through the bell housing in the same manner as is done on a Mark I gearbox.
7. You can also fit a modified Austin Kimberly pre-engaged starter which I have done and can show you. We made studs to mount the starter as the bolt holes in the gearbox do not line up with the engine backplate. The top one we threaded through the engine backplate and into the bell housing. This was easy. The bottom stud hole was bored to 1/2" in the engine backplate only and tapped, and a stepped down stud made to the original size of 3/8". There is not enough aluminium to tap into the bell housing for the bottom stud. Mark II MGB starters were hard to come by at the time I did this and the Kimberly was plentiful. The only difference is the location of the solenoid. I modified this starter to orientate the solenoid between the distributor and steering column by relocating the mounting lugs. A Mark II will do the job without any modification. Note: There is no need to change the ring gear despite the bendix gear entering from the opposite side. After 15 years of use there were no problems at all. The early inertia starters of course, chew up ring gears. .A later suggestion I have is to use a gear reduction starter motor
8. You may or may not require to modify the tailshaft, but ensure that it is not fully compressed when installed. On my car we removed 1" from the tailshaft in the lathe and had it mig welded and balanced. I went to this trouble as it was an almost new shaft. The alternative is to buy a secondhand unit the right length. The workshop manual says there are only two different length tailshafts but I can assure you there are more and it is important to have the right one.
9. Where the speedo drive exits from the Mark II gearbox I did make a small, unnoticeable dent to give a little more room to connect and disconnect the speedo cable, this is definitely optional.
10. Once everything is done and the gearbox is at its final resting place, cut the metal plate on top of the transmission tunnel in half on the straight piece about 2" in front of the gearlever turret. Bolt the front half in place, fit the rubber boot to the back piece and the chrome ring and fit the rear piece by tapping the metal to fit the new location which is obviously further back on the tunnel, then PK screw this in place ensuring that it is correctly located and that selection of all gears including reverse does not stretch the rubber boot unduly. I then gaswelded a 65mm piece between the two halves of the plate shaping in to the plate as I went. This ensures the rubber boot finishes up in the right place and looks professional when finished. Some of the original bolts can then be refitted, but I did use 2 PK screws at the rear of the plate. Trapped nuts could be fitted in lieu of this as I plan to do later when I do a rebuild.
11. Your transmission carpet will then need to be remade, as the step-up to the gearlever is further back. You can have a new carpet custom moulded to the transmission tunnel. Once this is done you won't be able to tell the difference.
12. Don't forget the gearbox is not held back from moving forward during harsh braking or hitting a bump by the retaining ring underneath it, so I suggest fitting a couple of those simple Mark II engine mount brackets which you could make yourself or buy. These brackets are the later method of stopping the engine fan hitting the radiator.
When I started to write this article I thought I would show everyone how easy it is, but as I go along it seems there are endless things to do. The jobs are mostly simple and the gain in driveability is really worth the effort. The job is easier if you have a 5 bearing engine because you do not need to machine down the first motion shaft. You can remove this shaft without dismantling the gearbox by being very careful and having the gearbox upright.
Synchromesh on first, and the higher ratios on 1st and second, really are worth having. Not to mention the long term reliability of the overdrive (if fitted) and gearbox. No one knows the difference visually afterwards except eagle eyes might spot the later starter motor and engine mount brackets. When the new carpet is in no one picks the gear lever is a little further back.
The gearbox I installed in my car was an overdrive unit to replace my existing overdrive unit. Of course, if you want to install either type of Mark II gearbox, it is no more difficult. The Mark II overdrive does not require the relay and vacuum override switch of the Mark I, but you could easily leave them fitted if you wish to maintain originality if you had overdrive as standard.
PS: I knew I would forget something! You will have to change or rethread your gear lever knob as the Mark I is UNF and the Mark II is Whitworth. Also, you will need longer bolts (2 3/4") to bolt the engine to the gearbox.
IAN BUCKLEY – originally written about 1988


Ian Buckley

I think the following information is missing from the information above.

Ian Buckley

I had to make a small cut from the top of the transmission tunnel immediately behind where the gear lever surfaces. After removing the tunnel carpet and the black plate covering the top of the gearbox I relieved the back of the hole about 25mm. The Mark II gear lever is about 28mm further back than the Mark I. I used an air operated hacksaw and took out a moon-shaped piece about 70mm by 25mm.
At the top of the starter bulge in the bell housing you will need to file off a lug of aluminium which when completed is almost invisible. This can be easily done with a dreadnought file. This lug is the only real point of contact between the gearbox and the body.
The gearbox cross member on the later models, I found out by accident, allows the gearbox to sit about 15mm lower and is offset fore/aft {I forget which). You could modify your own but a later one is easier. It all bolts straight into place.
Ian Buckley

Ste, Ian, Iain

Thanks for all the advice including Ians (which will take a bit of digesting). As my non o/d box is in perfect order I am not in a rush, but I had hoped that I could find a 3sycro, 3 brg o/d gbox at a reasonable price. I am surprised at prices of up to £1000 for a recon box. If this is the case I would probably not bother!

I want to keep my car as standard as possible as it is a pull handle car. When Ian says a Mk2 gearbox will fit does he mean a 4syncro and if so will all 4 syncros gearboxes fit a 3 bearing engine withthe 1st motion shaft changed?.

I would prefer the easiest solution and if 3syncro gearboxes fitted to the 5 bearing engine are as Ste says better and are also easier to get hold of (and therefore cheaper!) possibly this is the way forward?

Rob
R J Collier

Rob,

Ian is refering to fiting a 4 syncro box. These are cheap as chips! You can pick them up from eBay for as little as £25. However, they are not easy to fit and the only person I know who had one in an early car eventually went back to a 3 sync OD box as he simply could not stop it banging on the tunnel with spirited driving. I looked into the 4 sync method and decided that it wasn't worth the extra work. I didn't like the idea of the tight fit and I really didn't want to be disappointed at the end! Today I drove the car to work for the first time with OD, and it works beautifully. I have been lucky as the box is excellent with no noise in any gears (including the notrious 2nd gear syncro) and the OD switched in and out crisply in 4th (I have modified it so it won't work in 3rd gear as I am fitting a supercharger).

The prices I quoted came from Chris at Octarine (I think the recon one was £900 and something), so I am confident that they are fair. Also, they are not exchange. You need to watch for that as many places quote a price for a recon box on an exchange basis, which is of no help to you as you don't have an OD box to swap! At £500, a box that has been checked over by Chris with some kind of warrenty is not so bad! Alternatively keep an eye on eBay and wait patiently. They come on there reasonably often, but not frequently! The other option I considered was buying a complete early car with OD, swapping the box with mine and selling on (or even breaking). If you buy a rough early BGT it should not be more than a few hundred so it should be easy to recover the cost if breaking the car, or limiting the loss if selling on.

Cheers
Iain
I D Cameron

rob
the later overdrive boxs fitted to 5 bearing engines go for more money it seems than a 3 bearing as theres not that many left on the road.
they are out there cheapish but the may need rebuilding that is were a second hand none overdrive box comes in handy for spares as they are the same apart from the first motion shafts and the laygears.
ste
Ste Brown

Rob

My car is a pull handle which I have owned for 40 years. I was reluctant to change the gearbox when I did, but the increased driveability and reliability convinced me. My gearbox has never touched the body or made any noise.

In my car no one knows I have the later gerarbox unless they look underneath and see the slightly different removeable cross member.

Yes, when I say Mk 2 I mean 4 speed all synchro.

If you send me your email address I will send the article in a more readable format.

I have not seen a post from Ollie Stephenson for a while, but he has some excellent material on the conversion in the archives.

cheers
Ian Buckley

For the information from Olli Stephenson go to http://www.funkster.org/ and click on cars on the left hand link and then MGB BGT Gearbox 2 and that is all about it.

This looked a lot of work to me compared to simply replacing a non OD box with an OD box! And to be honest, as I can't weld, I think it would have cost me as much or even more to do!

Iain
67 BGT - now with OD!
I D Cameron

http://www.mgbmga.com/tech/mgb1.htm is another link worth reading.

Iain
I D Cameron

This thread was discussed between 10/06/2007 and 15/06/2007

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