There were five different gearboxes used in the MGA during seven years of production. Most of these units can be readily substituted, given the correct interface parts. Most of the internal parts between the front and center main bearings are virtually identical and interchangeable, including early production MGB gearboxes. The main variations are in the rear housing, rear coupling, and the third motion shaft (mainshaft) to match, and some changes in the outer configuration of the bellhousing.

Early 1500 gearbox - May 1955 to Sep 1956

Low starter position (about level with mainshaft), 1-inch 10-spline input shaft, 1-inch 10-spline output shaft, external rear seal. Low on the right side of the main case there are two tapped holes which are closed with screw plugs. In other applications (such as Nash Metropolitan) those holes may hold acorn caps which carry cross shafts for the side shifter mechanism (not used in the MGA). This gearbox model was used in about 10,653 cars with minor variations in very early units.

Addendum April 2009: Notice above four bolts, two long and two short, in top of rear housing for attachment of the remote shift extension housing. These bolts changed from fine thread to coarse thread, most likely with the transition to the next style gearbox (at Car No. 20753). If you change from early-1500 type to any later type gearbox (or vice versa) you will need to change these bolts to match the female threads. The starter motor mounting bolts also changed from fine thread to coarse thread at the same time, so swapping a gearbox might require changing the starter mount bolts. Since these female threads are susceptible to wear or damage any time the starter is removed, I highly recommend installing stainless steel Heli-Coil inserts for the starter mount threads.

Mid 1500 gearbox - Sep 1956 to Jan 1959

Same low starter position and 1-inch 10-spline input shaft, but 1-1/16-inch 10-spline output shaft and internal rear seal. This is the most common MGA gearbox, covering about 40,751 cars. Notice also a small reinforcing rib added on the bottom of rear housing at the rear mount point. Internally the mainshaft is slightly larger aft of main case, using a different "spacer" (actually the oil pump), speedometer drive gear and retaining nut. This minor change is not often reflected in current parts catalogs. For interchangeability, the rear housing can be swapped only with matching change of the mainshaft (and matching propshaft).

Early 1500 external seal

Mid 1500 internal rear seal

MGA Twin Cam gearbox - Apr 1958 to May 1960

High starter position (about 1-1/2" higher than the input shaft), 1-inch 10-spline input shaft, thick reinforcing rib along the bottom of the rear housing, and flanged output shaft with ball bearing carrying the rear flange. This required use of a 2-piece splined propshaft, which was not used on other MGA until January 1959. This gearbox was used for 2111 MGA Twin Cam cars. Notice the large "square" bump on top of the bellhousing, and the (unused) 2-hole bolt pad for mounting a slave cylinder on top. In other applications (possibly the MG ZB Magnette and Lotus Elite 14) this gearbox would have a top actuated clutch release. Notice also low on the right side of the main case the cross shaft bosses are not machined, left blank in the casting, so no screw plugs required.
 

Addendum: Dec 18, 2004: It appears this Twin Cam type gearbox was also used in some early MK II cars. At 05:47 PM 12/15/04 -0500, David Lentinello wrote: "I bought my car from the second owner who has owned the car since 1965 (it was his Mothers car). He was 100% sure that the gearbox was original to the car. The numbers being M1551 come very close to my cars production of 1409. Maybe the factory started a new renumbering sequence with the MK-II's. John Esposito said to me that he knows as a fact through his experience of rebuilding gearboxes that the early MK-II's had the Twin cam cases. .... He is the world renown MGA gearbox (and other British Marques) rebuilder."

Late 1500 and all 1600 gearbox - Jan 1959 to Mar 1961

High starter position, 1-inch 10-spline input shaft, flanged output shaft. This is the point (other than the Twin Cam) where the propshaft changed to the 3rd model with sliding spline connector just aft of the front universal joint (see below). Notice the ribbing on bottom of rear housing identical to the Twin Cam gearbox, and the blank un-machined projections on the side of the main case, but no square bump on top of the front housing. This is the second most common MGA gearbox, covering about 31,500 cars. Even though this is commonly called "the 1600 gearbox", about 7000 late production 1500 cars were built using this model in early 1959. The rear housing could be swapped with earlier units using the matching mainshaft and propshaft. To use the high starter position in earlier cars you need to install the later front tunnel section and frame bulkhead panel (or modify the early parts accordingly).

1600-MK-II gearbox - Mar 1961 to May 1962

This model is similar to the "1600" gearbox shown above with high starter position and flanged output. The input shaft changed to 1-inch 23-spline with rolled involute splines in October 1961. Here there are added strengthening ribs on the outside of the bellhousing to reduce vibration and noise, and also a thicker front flange on the bellhousing for the mounting bolts. There is also an added rubber shaft seal in the gearbox font cover, which is interchangeable and often retrofit with earlier units. Also some indication that this front seal cover may have been factory issue sometime earlier. This model of gearbox ran through end of production of the MGA, about 8719 cars.
 

Addendum: Dec 18, 2004: With reference to the addendum note above for the TC type gearbox. It is possible that early production MK-II cars (some or all) were fitted with the TC type gearbox. In that case this later type with ribbed bellhousing may not have been used until possibly as late as October 1961.

There are three different part numbers and casting numbers on the rear housings:
Early 1500  Part No. 1H3084 -- Casting No. 1H3085
Mid   1500  Part No. 1H3149 -- Casting No. 1H3150 AM  -- Com. (G) A101
Late  1500  Part No. 1H3273 -- Casting No. 1H3274 VAF -- Com. (E) GD101
All 1600, 1600-MK-II and Twin Cam are same as Late 1500 

In similar basic form (with change of rear mount, remote shifter, and front cover) this gearbox was continued into MGB production with the 1800 engine through October 1967. This included installation of the 18GB engine, first of the 5-main bearing B-series engines, using a small flywheel unique to the 1965-1967 model years. That flywheel can be used to install a 5-main bearing engine in the MGA. Many of the internal parts of the early MGB gearbox are the same as the MGA parts, everything between front and center main bearings and the laygear (and the MGB gearbox may be easier to find if you need some good used replacement parts for the MGA). Also the late 1500/1600 rear housing can be installed on the early MGB gearbox (1962-1967) to have the correct rear mount to install in the MGA (along with the late MGA propshaft).

MGA engine rear plates

Two different engine rear plates were use in the MGA. Low starter position on left goes with the early and mid 1500 gearboxes with the sliding spline output shafts. High starter position on right goes with all other MGA models. The low position put the starter motor very close to the bottom flange on the engine block. The high position put the starter motor very close to the distributor housing. Change to the high starter and flanged output shaft on the gearbox occurred in late 1500 production at car number 61504 in January 1959 (April 1958 for all Twin Cam cars).

Please refer to propshaft tech for variations in three different propshafts used to accommodate these various gearboxes.