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MG Car Repair/MG TD Gearbox


QUESTION: In my 1951 MG TD RHD the 3/4 sliding hub & dog can be assembled in either of two ways. Depending on how they are assembled, when viewed side on, the hub projects more on one side of the dog than it does on the other. If the hub is then removed from the dog & reversed, then reinserted, the projection when viewed side on now appears to be the same on both sides. I have seen this on more than one assembly.

My questions are: Which assembly arrangement is correct? Is this asymmetry due to wear?  Can an asymmetric or symmetric assembly of these two components affect the operation of the gearbox?

ANSWER: Hi Peter.
A good question. The synchro hub is not symmetrical.  

It must be assembled so that the plain side of the hub is to the rear of the gearbox.  Fitting it the wrong way round will cause problems with assembly and operation, because the 3rd/4th fork will not be located correctly with the shifter in neutral.  

Regards, Barrie

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QUESTION: Thanks Barrie. It seems I've not made myself clear. I realize the synchro hub is not symmetrical & that there is a fore & aft position of the hub.

If you take a hub & a dog & assemble them, view the assembly side on, the hub will either project slightly from the dog an equal amount or significantly more on one side than the other. This is at the perimeter NOT at the centre!

In the first case, this is just a matter of maybe 15 or 20 thou. Now take the hub out of the dog, reverse the dog, then reassemble it. The projection will now be asymmetrical. The hub will be almost flush on one side & project about say 35 or 40 thou on the other. You will need to do this to understand what I'm describing so you can then address my initial questions.


ANSWER: Hi Peter

If the hub is assembled incorrectly neither side of the assembly will be flat.  It must be assembled so one side is flat, and the assembly must be mounted onto the mainshaft such that the flat side sits towards the rear of the gearbox.  

So, there are two details which can go either way making 4 possible ways to fit it, and only one of these 4 ways is correct.

This is nothing to do with wear, and incorrect assembly will cause problems such as inability to select 3rd gear whilst the engine is running, or worse.


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QUESTION: Hi Barrie. I think I understand your reply now. Only one of the 4 possible assembly methods is correct. A final couple of questions. Do the 6 balls wear on the inside of the dog & if so does shifting the hub /dog relationship to the adjacent spline make any difference? An examination of a number of different 3/4 dogs does seem to indicate some wear where the 6 balls have made contact, while the adjacent spline seemed much less worn. Prior to disassembly I marked the adjacent faces of the hub & dog to ensure I could reassemble the pair in the same position that they were in or consciously alter this relationship. Is this precaution necessary? I also took them apart in a clear plastic bag in case the balls hadn't been peened. When the balls let go they sure do want to fly!

Thanks for you advice thus far.

Hi Peter

There was a problem with excess movement of the early TD 3rd/4th gear assembly.  In order to improve it the factory made several alterations.  

1) They fitted a longer shifter shaft with a circlip on the end to prevent the balls from ejecting if the gear lever was forced past its correct operating position.  

2) They also peened the 6 holes to stop the 6 balls from ever flying out.

3) Finally, they came up with the idea of drilling one of the 6 holes right through the sliding hub and modifying the mainshaft with a small indentation.  This allowed them to insert an extra ball bearing, followed by the spring and one of the 6 ball bearings.  The result was a positively located hub.  

With this last modification, there is only one way it will go together correctly with the drill hole lining up with the indentation.

I have never worked on a gearbox without these modifications, so the thought of re-aligning the sliding hub on the mainshaft has never entered my head.  

However, I see no reason why you should not re-align the striking dog on the sliding hub, especially if the dog has wear marks.

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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