When an MGB 1800 5-main bearing engine is installed in an MGA, you need to use the engine rear plate that matches whatever gearbox you want to install. When mating a 5-main bearing engine (1965 or later) to a 3-synchronizer gearbox (pre-1968), you need to bore out the large hole in the rear plate to accept the rubber seal that is required for the 5-main crankshaft.

The original seal is Silicone rubber, commonly red color, and has a variety of BMC, Austin, Rover part numbers. There is also an upgraded seal in Viton material, commonly black. This seal is often sold with a thin tubular assembly tool to help boost the seal lip over the end of the crankshaft without seal damage. This is particularly helpful if you have installed a Speedy-Sleeve on the crankshaft. The seal might also be available in kit form including the steel retainer plate 12H1547 (as seen in article PP-201).

88G621 Oil seal, MGB 1965 to 80, BMC, Rover
Inner Diameter = 88,9 mm (3.500-in)
Outer Diameter = 104,7 mm (4.125-in)
Width = 9,5 mm (0.375-in)
Mat'l: = MVQ (Silicone Elastomer) (Left-hand Twist)

Ajisa 15038800 Austin, Rover ADU9143 Austin, Rover AHU2157 Austin, Rover AHU2242 Austin, Rover AHU2242Z Austin, Rover CAM9991 Austin, Rover K418FF01 Austin, Rover LUF10002 Bars INA51727 BMC, Rover 88G621 BMCS/PRIMA 131021 Corteco 12011270B Corteco 15510201B Fai Autoparts OS772

Glaser 77127 Goetze 5031942000 Land Rover EER6130 Leyland 13H2457 Malo 1153RSSI Moss UK LUF10002 (Silicone) Moss UK LUF10002/2 (Viton) Moss USA 120-820 (Silicone) Moss USA 120-821 (Viton) Mercedes-Benz 6039970146 Perkins K418FF01 CNB123 NB772 NB772X Victoria British 5-816

99347 - Speedi-Sleeve for rear seal on 5-main bearing MGB engine