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MG MGB Technical - Gearbox rebuild

Afternoon all,

Looks as though I need to get the (three synchro, non-OD) gearbox on my GT rebuilt. I have been recommended first motion transmissions in Letchworth by a non-MG friend, I just wondered if any of the UK crew on here had ever used them? Part of the appeal is that within the price they will collect and deliver the broken gearbox from my garage. But I thought I should canvas in a few places in case anyone had an MG-specific experience of them.

Alternatively, if anyone has had successful gearbox rebuild recently I'd be delighted to hear any other recommendations!

Many thanks in advance,

Piers
Piers Colver

quality of rebuild and parts used is essential otherwise it could be that later it turns out to be a low value very expensive rebuild

I had a rebuilt box on my previous Spridget that vibrated from installation between 50-70mph and to cut a long story short I gave up on it and had another rebuilt box installed that was fine

search the Archives in MGB, midget and Sprite and MGA forums on here for recent recommendations or reports of good work

only one that comes to my mind at the moment is Hardy Engineering as in the useful b web site notes - http://www.hardyengineering.co.uk/
Nigel Atkins

Good reports for Hardy Engineering.

There is also www.klassiktrans.co.uk who get various recommendations on the Sprite & Midget board...and not too far away, in Wolverhampton.

Dave O'Neill 2

Thank you, chaps, I will check out those two companies.

Piers
Piers Colver

Piers,
You could also take this opportunity to fit the later MGB all synchro gearbox with or without overdrive.

Maybe even consider the 5 speed conversion using the Sierra 5 speed gearbox and kit from Hi Gear Engineering.
Ian Buckley

Hi Ian,

Yes, I had thought about that, trouble is at the end of the day I decided that I'm too much of an originality freak, so I'm going to stick with non-OD 3 synchro and hope I remain an originality freak so don't regret the decision! Thank you for the thought, though, just the sort of point of view one needs when too close to the project.

While I'm on the subject of originality, what do people think of rebuilding an existing box against fitting a different (albeit identical) box that has already been reconditioned? I have been offered a reconditioned box from a good source, but as I had planned to have my own rebuilt I'm suddenly torn. The money is pretty similar.

Piers
Piers Colver

same thing it's about the quality of the work to yours or the other box, what was done, how well, what was the condition of the box before and now after

what do you know about the history and condition of your existing box and the other box

originality has limits otherwise you'd never replace anything only get it repaired, what and how much can you guarantee is original on your car now - all very subjective, is a body panel that's been repaired still original - up to you how much you value or worry about originality
Nigel Atkins

That's kind of how I view it - as far as I know my current box is the car's first and we have no record of it ever having been opened up (although obviously that's not to say that it hasn't been, although I have a lot of history with the car going back to nearly new). The rebuilt 'box was rebuilt recently by Heathrow Transmissions, who are well known for doing a good job, so I am leaning towards that. Thank you for the response!
Piers Colver

Piers

Are you aware that there were two types of 3-synchro 'boxes?

The first-motion shafts were different diameters.

You would need to make sure that you are replacing like with like.

Also, the later 'boxes had stronger layshaft bearings.
Dave O'Neill 2

Hi Dave,

Yes, thank you, the garage I am dealing with mentioned the difference. Nice catch, though, thank you for the thought!

Piers
Piers Colver

Piers,
You seem to have done your research and it is good that you know there are 2 different versions of the early gearbox. One of the shortcomings of the early gearbox is the longevity of the layshaft needle roller design. It would be disappointing to have a nice quiet fully reconditioned gearbox, only to find it becomes noisy again after a few years or not a very high mileage. I have read that the case hardening on some of the replacement layshafts for these gearboxes is not to the standard of the original parts.
Arguably the biggest plus for the later all synchro gearbox is its longevity compared with the earlier design.
Ian
Ian Buckley

Piers,

Be aware that the 4-synchro gearbox requires a wider gear tunnel, so you may need to doctor the MK1 GT's body to accommodate the newer unit. I'm sure some member can advise you better on this.

Do you know if it is feasible or possible to fit the newer bearings to replace the aforementioned weak 3-synchro needle bearings? This may help overcome the longevity problem already identified with the older unit.

Regards,
Brian McIlvenna
Brian McIlvenna

"Be aware that the 4-synchro gearbox requires a wider gear tunnel, so you may need to doctor the MK1 GT's body to accommodate the newer unit. I'm sure some member can advise you better on this."

This is actually not correct, but is often written and requoted.
The wider tunnel was to accommodate the Borg Warner 35 automatic transmission.
The Mk I tunnel does need modifying to accommodate the gear lever which exits about two inches further rearward with the full synchro gearbox, and a further "slight gentle persuasion" with a mallet is needed near the starter motor.

What is indeed correct is that obtaining good quality parts for rebuilding these earlier gearboxes has become very difficult. The steel second gear synchro rings, which were much more robust than the earlier brass rings are essentially unobtainable (although I've heard that Brown and Gammons will occasionally commission batches of them) and the layshafts from Moss are inadequately hardened (Rockwell hardness around 50 instead of the required 62-63).
The last of the three synchro gearboxes (from about February 1967 had a larger diameter layshaft, and four instead of three supporting bearings.
T Aczel

The 4 synchro gearbox does fit as advised above. I did it to my narrow transmission tunnel, 3 bearing car almost 30 years ago.
In the starter motor area I used a body file to remove some aluminium from the bell housing in that vicinity and did not then need to make the small modifications to the body T Aczel refers to. These modifications are almost invisible.(Either option)
I did create a little extra space for the speedo cable to be attached to the gearbox with about 2 strikes with a ball pein hammer.
I have not done this, but with a non OD (only) 4 synchro gearbox it may be possible to shorten the remote to achieve the original gear lever location. I extended the black cover over the transmission tunnel and I am very happy with the results. A new moulded carpet is required, of course.
I concur with the information provided about the problems with 3 synchro gearbox parts. That is well worth noting. I have read about this many times over the years.
Ian
Ian Buckley

The layshaft hardness issue is not juts confined to the early 3 synchro. All the other layshafts have the same problem: late 3 synchro, 4 synchro, A series. We have tested several examples of them all and the hardness varies from 50 to 59. See photo for results! We gave up and had them made locally right up to spec, though at a cost.

Some of the needle rollers around are 4 thou undersize so these need checking too.

It's easy enough to convert to that later layshaft - machine the layshaft hole in the casing out to 17mm and get a different laygear and thrusts (or modify your existing one).

I fitted a full synchro box to my '63 B at about the same time as Ian, and lengthen the top of the tunnel in the same way. All dead simple to do, it's surpising just how many things line up especially if the original was non-overdrive.


Paul Walbran

This is an excellent thread and contains a wealth of information to help owners of early MGBs make the best decision about how to approach their gearbox dilemma.
4 synchro gearboxes are relatively cheap these days because so many owners have fitted one with overdrive.

Paul's advice about the undersized needle rollers is well worth knowing. It would be nice to know where on spec ones are available. The same applies to layshafts.
Ian
Ian Buckley

Ian my last sample came through on size, but I think there are some problem ones in the system still so it pays to check. They should be 3.0mm.
If you can't get the right size, here's an alternative:
http://www.mgparts.co.nz/advice/technical-notes/aau3052-layshaft-bearings/
Paul Walbran

Agreed, Ian, this has turned into a helpful exchange of thoughts.

I have gone with a rebuilt, 3 synchro, non-OD box, supplying my existing 'box on an exchange basis. It is as original, with one upgrade - stronger material for second gear synchro to aid longevity.

So thank you all for the various thoughts!

Piers
Piers Colver

I just wanted to add a belated postscript to this thread.

In the end I went for a rebuilt gearbox from Heathrow Transmissions, and picked the car up today. Just wanted to thank everyone for the input, and give a hearty recommendation for Heathrow Transmissions, both in terms of service and product. Over an extended test drive it drove like a dream.

Piers
Piers Colver

well done for reporting back and giving details of Heathrow Transmission work and service
Nigel Atkins

Piers, It is great you are happy with the job. Happy motoring.
Ian Buckley

This thread was discussed between 09/06/2014 and 05/08/2014

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