It is difficult to determine which rear engine plate should be used for a particular application. This is compounded if a conversion to a different engine is contemplated. The following is a listing of the common B series rear engine plates for reference.

3 Main Bearing Engine Rear engine plates:

The MGA and early MGB engine with 3 main bearings did not use a rubber real oil seal. These backing plates are easy to identify as the hole in the center is 3.95” which is smaller than that used on the later cars with rear oil seal.
There are three common rear plates for these 3 main engines. All are shown from the rear. The indicated measurements are all from center to center of the holes.

MGA low mount starter. 1500 to (c)61503

Note that the starter is almost in line with the crankshaft, just above the oil pan.
(The plate shown is a new adapter plate that has the larger center hole for use with a 5 main engine. Otherwise it would have a slightly smaller center hole and four more counterbores as shown below.)

MGA high mount starter. 1500 from (c)61504, 1600, 1622

Note that the starter is higher than the crankshaft. To clear the drive of this high mounted starter the transmission tunnel and firewall have a steel bulge which can easily be seen from inside the car. You can install a low mount engine into a later car, but a later high mount engine can not be installed in an early car without modifying the transmission cover and firewall.

MGB 3 Main 1963-mid ‘65

This plate can be easily identified as it has the smaller 3.95” crank hole like the MGA plates, but extends higher to just below the head. Note that the starter motor has been moved down slightly. The relocation of the starter is why the MGA and MGB gearbox bell housings can not be interchanged.

5 Main Bearing Engine Rear engine plates

These plates have a larger 4.12” center hole to fit the rear main seal.

MGB 5 Main, 3 synchro transmission mid 1965-67

Note that the starter location is the same as the 3 main backing plate. The 3 main and 5 main transmissions use the same bell housing. The input shaft diameter of the later transmission is much larger though. A 3 main transmission can be used on a 5 main engine if a conversion pilot bearing is used.

5 main, 4 synchro transmission 1968-80

The bell housing of the 4 synchro transmission is much larger than the earlier 3 synchro design. This necessitated a lot of changes to the transmission tunnel of the later cars.

Many, but not all of these late back plates will have a hole at the bottom right as shown above. This normally is fitted with a steel backed rubber plug. This hole provided access to the automatic transmission torque converter bolts when fitted.

Austin/Morris Marina 1.8 L

Engines with this backing plate sometimes show up. None of the MGA or MGB transmissions will match up to this plate. The Marina 1.8 L engine is virtually identical to the MGB 18V motor, however the crank shaft has a smaller pilot bearing bore requiring machining to fit the 4 synchro input shaft. Some engines fitted to the Marina did have MGB cranks fitted with steel sleeves. (Note: The dimension shown below is incorrect. It should be more like 3.4 inches, but the exact dimension is still to be determined.)