MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Transmission Interchangability 3-Main 5-main

My TD is fitted with a 3-Main engine and transmission. The transmission is fitted without any modifications to the unit. The current transmission grinds when downshifting from third to second. Up shifting to second is generally not a problem nor are there any other problems that I am aware of.

Since I am going to have the transmission fixed, I was thinking that it might be wise to just to change to a later transmission with 4-syncros.

My questions are:
1. If I buy a transmission that includes the bell housing, can I switch the two and use my current bell housing? I want to be sure that the starter motor fits. (it is a TD starter motor)
2. Is there a particular range of MGB model years that offer a preferred transmission?
3. Is the gear shift position the same in the two transmissions? In other-words, can I use my existing transmission tunnel without modification?
4. Will the transmission mounts be the same style and be in the same location?
5. Are there any other areas that could present an interchangeability issue?
6. Am I being foolish? Should I just stay with the current (repaired? transmission. Any words of wisdom If I get the current transmission repaired?

All advice will be appreciated.

Frank Grimaldi
Frank Grimaldi

Frank.
Are you certain that you have an original MG-TD transmission and bell housing. If so you must have a totally modified rear engine plate.
The MGB transmissions are housed in a singular unit , not split into a bell housing and transmission case like the TD. If all you have is a syncro problem it would be easy to install new syncros.
Can you supply photos of your present unit?
Sandy Sanders
SANDY SANDERS

I think Frank means he has an MGB 3main eng & Gbx.
So, the 4 synch gbx is from 68 on. You need the rear engine plate, flywheel, starter, and gearbox. Bell housings are integral as Sandy says. Mounts are the same. There may be some minor issues with the tunnel, depends on what was done and how in order to fit the 3 main setup in the first place. Starter motor seems like an odd thing to worry about here! I think the shift lever position is slightly different, but not much. I never swapped 3 & 4 synch boxes myself, but plenty of people have.
Might help to post a picture or two of this creature.

FRM
FR Millmore

Frank

The starter motor is in a diff position between the boxs and i think it fouls on the chassis legs with a 4 synchro box so i would just get someone o rebuild your own box.

Ste
Ste Brown

A friend of mine installed a 4 synchro box in his '64 3 main engined car. He reused the starter but had to drill out holes to mount the old style starter to the transmission housing. The pilot bushing needs to be drilled out to accept the larger diameter input shaft. The shift lever sits back 2 inches farther but it can be installed 180 degrees out to bring it close to the the 3 synchro position. He was able to use the original backplate and flywheel. You will also have to drill a 1 1/2" hole for the bendix drive in the bell housing. When I converted my '67 to a 4 synchro, I used the later backplate, flywheel and starter. Everything bolted right up except for the shift lever being 2" aft. RAY
rjm RAY

Ray

Franks doing it to a TD not the MGB

Ste
Ste Brown

Frank, if you have a three bearing B motor and trans in your car then you should be able to fit the later 4 speed without to much trouble and you will love the result. RIC
RIC LLOYD

Thanks guys for your input. After reading all of the comments, I think that rebuilding the original 3-synchro unit is the best choice. The additional cost for the necessary parts to make the change is not worth the gain of a first gear synchro.

Frank Grimaldi
Frank Grimaldi

This thread was discussed between 16/09/2009 and 18/09/2009

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now