© 2017 by Ashford & Faversham MGOC

Rebuilding the MGA gearbox

As part of the work on my MGA project I decided that I would rebuild the gearbox myself. Most people tend to shy away from doing this themselves as they consider it is something that should be carried out by a specialist. In my youth, when I was considerably younger and impetuous, I had dismantled and rebuilt several gearboxes including putting close ratio gears in my friend’s MG Magnette gearbox, which is the same gearbox as that fitted in the MGA. Luckily I had the body off the car at the time of the rebuilt so removal was a simple matter of taking out the engine and gearbox as one lump and separating the engine and gearbox on the floor. It should be noted that the gearlever had to be removed by taking off the large circlip to enable it to pass under the cross member on the chassis.  It is also best to make sure there is no oil in the gearbox prior to removal!


Once the gearbox was out of the car the first job was to separate the remote selector arm assembly. This involved removing the main bolts connecting it to the main gearbox assembly and taking off the plate to expose the selector arms, which had to be removed before separation and removing the prop shaft flange by undoing the large nut on the end. It is also necessary to remove the speedometer drive pinion from the side of the casing. This unscrews and incorporates an oil seal retaining ring and washer which may need replacing if they are leaking. Once apart this exposed the selector rods going into the gearbox. This also exposed the main shaft with the gear that drives via a cable the speedometer. These parts can be removed from the shaft by bending back the lock washer and undoing the large nut.


The cover plate on the selectors needs to be removed very carefully as each rod incorporates a spring and ball bearing, which enables accurate gear selection. If it not intended to replace these springs and ball bearings then the rods and plate can be removed as a complete assembly. However  in my opinion these should be replaced as this can be one reason one the car will jump out of gear. To gain access to the main gearbox parts the side cover plate needs to be removed by undoing the ten bolts and washers. Note the order of these as some of the bolts are different and go in specific places. If these are put back in the wrong place oil leaks can occur.


Once the selectors have been removed the next stage is to remove the clutch selector forks together with the hydraulic clutch slave cylinder. This will then enable the engine end plate to be removed from the gearbox. Once this is done tap out the lay shaft, allowing the gear cluster to rest in the bottom of the gearbox. Unscrew the retaining set screw and remove the reverse shaft and gear. Withdraw the main shaft assembly to the rear. Withdraw the first motion shaft. carefully from the front. Note that where the shaft splits there are 18 spigot needle roller bearing, which may fall out into the gearbox. Then lift out the layshaft gear cluster and the two thrust washers.


The next step is to inspect the baulk rings. Be careful if removing the syncromesh sleeves as this will release the three locating balls and springs. These springs and balls should ideally be replaced as any weakness could cause jumping out of gear. Generally inspection of the baulk rings will indicate wear, as those on 2nd and 3rd gear are a noted weakness. Hardened baulk rings are available but these need to be matched to uprated laygears, which are not currently available.  So the only option is to replace these with original brass items. The next item to inspect is the laygear shaft, which will normally show signs of wear and should be replaced together with the 3No laygear bearings. Check thrust washers for signs of wear and replace if necessary. As a matter of course all main bearings and oil seals should be replaced. There is an oil seal and bearing on the propshaft end of the gearbox extension, which will probably need a puller to remove it. There are also bearings on the front and back of the main gearbox housing.


Reassembly of the gearbox as all good manuals say, is a reversal of the dismantling procedure!


Firstly once all the parts have been removed from the gearbox, the casings and parts should be thoroughly cleaned to remove all old oil and sludge deposits. The  laygear should placed in the bottom of the gearbox before replacing the main shafts from either end of the gearbox, ensuring the needle roller bearing between the two halves of the shaft remains in place with liberal applications of grease. Bearings should be tapped into place from either end. Once this has been installed the layshaft can be installed from the clutch end of the gear box. Sometimes this is made easier by using a drift from the back of the gearbox to hold the lay gear in the right position so that the shaft can be drifted into place. The reverse gear and shaft can then be installed. Then the gear selector forks can be placed into position prior to reinstalling the gear shifter fork shafts and shaft locating block together with springs and ball bearings if these have been removed. Once this is reassembled it is a good idea to try operating the gearbox to see if all gears can be selected by moving the rods backwards and forwards. This may be helped by liberally oiling the component parts with the correct oil. Once you are happy with the gearbox operation the bolts on the block should be rewired though the holes in the bolts to ensure security in operation.


Before replacing the  back casing on the gearbox the rear flexible bush will probably need replacing. This is a tight fit in the casing. The easiest way to remove this is to carefully cut through it and then drift it out. A new flexible mounting can easily be purchased, however my experience shows that this can be manufactured slightly too large and may need to be machined to fit properly. It is supposed to be a tight fit but trying to hit it too hard to get it in may cause the aluminium casing to break and should be avoided at all cost.


It is not generally necessary to dismantle the remote control shafts, but smooth operation should be checked and liberal amounts of gear oil applied.. The gear lever fits into a rear selector on the shaft and incorporates a bush which should be checked for wear and replaced if necessary.


Refit the speedometer drive and spacer onto the main shaft using a new lock washer and tightening the nut to the required torque. The rear section can then be refitted onto the main gearbox using a new gasket and ensuring that the two casings mate tightly together. This may require some juggling of the selection levers. Then replace the interlock arm and bracket. Once the gearlever is in place check for good operation of the gearbox selecting all gears in turn. If this is satisfactory replace the extension side cover and main gearbox cover using new gaskets and ensuring the correct bolts are used.


Then replace the propshaft flange on the end of the gearbox, tightening the nut to the correct torque.


The front cover should then be replaced carefully using a new gasket and ensuring that the plate is centralised to prevent operational leaks. The clutch operating forks and clutch release bearing can then be installed, using new parts if required. Clutch slave cylinder can then be reattached to the casing.




This text and photographs do not necessarily represent all of the work that may be required to be carried out, but show the major items that will probably need replacing.


It is also related to a 1959 MGA 1600 gearbox. Earlier and later gearboxes may incorporate subtle differences, which may need further investigation. This description will also largely apply to early MGBs up to about 1965, which have the four speed 3 synchromesh narrow tunnel gearbox.


Ideally this should therefore be read in conjunction with following reference manuals, where additional information can be found;-


The BMC MGA Series Workshop Manual Section F & FF.


Haynes MGA Workshop Manual.


The MGA Gearbox Parts 1,2 and 3 by Charlie Kaelin published by the MG Car Club.


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