The MGA With An Attitude
MGB D-TYPE OVERDRIVE Gearbox in the MGA - GT-304

Here are a few comments about installing the early MGB D-Type overdrive gearbox in the MGA. Thanks to Adam Prince for the pictures.

You can refer to the welded rear mount in the previous article GT-303 to see what may have to be done to the MGA frame to mount this gearbox. That article shows the later and larger 4-synchronizer gearbox with the L-Type overdrive unit. This smaller 3-synchronizer box with the D-type overdrive will require less extensive modification to the MGA tunnel, but still has similar problems with the rear mount. The OD unit hangs down behind the frame cross member, so to remove the gearbox for servicing you would have to remove the MGA seats, carpet, floorboards and tunnel. Otherwise you would need to modify the frame even more to include a removable cross member for the gearbox rear mount.

With the increased width of the overdrive unit the rear part of the MGA tunnel will need to be widened some. For the D-Type this widening requirement is not excessive, and may be accomplished with a little metal bashing or maybe a mechanical spreader. With the D-type OD gearbox the hand brake lever may be retained in the original position, and the MGA seats should not need any modification.
MGB D-type O/D gearbox right
This gearbox is a little longer overall than the MGA 1600 gearbox, so you need to use the MGB propshaft to match. There are three different length MGB propshafts, so get the right one for the D-type unit.

MGB D-type O/D gearbox left The MGB hand shift lever sits about two inches farther forward than the MGA lever, so you might want to bend the lever backward some to avoid barking your knuckles on the heater control panel. Or if you were real particular and energetic you could cut and extend the MGB remote shift extension a couple of inches to put the lever in the correct position for the MGA.
MGB D-type O/D gearbox top
The remote shift housing on this gearbox extends considerably farther forward than the original MGA part. As such, some modification of the top of the forward tunnel section is required. This is where there is a removable tunnel cover (except for very early production 1500's). The mounting bolts for the new shift extension housing are farther forward, so the access opening and tunnel cover on the MGA need to be extended forward to accommodate it.

If you install this gearbox in the MGA 1500 (production prior to January 1959), you will need to modify the tunnel at top right and front end, and also the forward toe board, to accommodate the high starter position. This also requires installation of the engine rear plate with higher starter position from January 1959 or later MGA.

MGB D-type O/D gearbox front The splines on the input shaft of the MGB gearbox are 23 splines, same as to the late production MGA MK-II, so use the appropriate clutch disk to match whatever splines you end up with. The spigot end of the input shaft for the MGB through 1964 models (with 3 main bearing engines) will be the same as the MGA. For 1965-1967 gearboxes, which were mated to the early 5 main bearing engines with small flywheels, the spigot bearing is larger diameter. It may be quite inconvenient to modify the earlier 3-main bearing crankshaft to accept the larger spigot bearing. But it is easy enough to switch the input shaft for an earlier part with the smaller spigot end, either 10 spline MGA, or 23 spline from the late MGA or early MGB.

If you want to use the MGA clutch parts, you would need to swap the MGB gearbox font cover for the MGA front cover, which has the release arm pivot point a littler farther back. Also use the MGA clutch release arm and release bearing. If you do this, please save the MGB front cover and release arm for the next person who wants to install the MGB clutch in their MGA.

Photos and notes for a welded rear mount to fit the D-type overdrive gearbox to the MGA frame can be found here:

Note: There seems to be a reoccurring problem with many of the MGB OD gearbox installations in the MGA. When the overdrive unit is positioned above the MGA frame cross member (without cutting the frame) the tail end of the gearbox sits a bit high, about level. This leaves about a 5 degree drop angle for the propshaft. This amount of misalignment is not likely to give any noticeable vibration, but it may lead to accelerated wear of the u-joints, and possibly also the sliding spline joint in the propshaft.

This configuration of welded mount on top of the frame also makes servicing of the gearbox somewhat difficult. Since the overdrive unit is positioned behind the welded mount, the tunnel has to be removed before the gearbox can be removed from the the car.

The MGB has a removable cross member to solve this problem. Installing a similar removable cross member in the MGA requires more cutting and welding to clear the original frame and install bolt-up plates, but it would also allow better (lower) position of the gearbox. This has been done, but I do not have the documentation or pictures. If anyone who has done it would like to contribute the notes and pictures, I would be happy to post it here.

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