The MGA and MGB 4-cylinder engines were Austin B-series engines. Austin may not have been so vigorous as MG about post-war adoption of the Unified thread standards. Most of the fasteners (screws, bolts and studs) would be Unified threads, while many of the threaded ports in the MG/Austin engines retained some pre-war design using Whitworth threads, or BSF threads (based on Whitworth thread form). This article endeavors to detail all of the threads in the B-series engine that are NOT Unified thread standards.

Part No's	Description	Where Used	thread	Ref.
...	Plug w/ Cu washer	LR above oil pressure port	...	...
...	Plug w/ two fiber washers	LR oil pressure relief cap	...	Can sub. one Cu washer
...	Special Adapter, oil outlet, w/Cu washer	RR side of block	...	...
...	Thread for tube coupling	Outlet end of Special Adapter	...	...
...	Adapter w/ Cu washer, oil inlet	Into filter mount or adapter	Same thread as port 7	...
...	Plug, dipstick port (standard engine)	Right center lower block near sump flange	...	...
...	Plug, dipstick port (Magnette engine)	RF lower block near sump flange	...	...
...	Plug, dipstick port (Twin Cam engine)	LR lower block near sump flange	...	Machined for Twin Cam only
2F4249 2A380 88G257	Plug, sump drain (except Twin Cam)	Right, low on sump	1/4-BSP or G1/4	Same thread as port 2
2F4249X	Plug, sump drain, magnetic (except Twin Cam)	Right, low on sump	1/4-BSP or G1/4	Same thread as port 2
2H4685	Plug, sump drain (Twin Cam only)	Right, low on sump	3/4-14-ADM	Larger thread than std. eng.
...	Coolant drain petcock	Right block, aft of dist.	...	Same thread as port 2
...	Coolant drain petcock	Bottom of radiator	...	Same thread as port 2
...	Crank dog nut w/ locktab	Front end of crankshaft	...	...
...	Bolt, square head, and Square Nut	distributor clamp plate	...	...
...	Thermostat cover studs	Bottom end into head	...	May be BSF threads in head
...	Flare nut for temperature sender	RF of cylinder head	5/8-18-UNF	DT-102D DT-102H
Moss Mot's 361-065	Flare nut Adapter for temp. sender	RF of cylinder head	5/8-18-UNF	DT-102D DT-102H
...	Manifold Studs	Left side in cyl. head	5/16-24-UNF	Inboard 4 of 6 need sealant
...	Water Pump bolts	Front of block	5/16-24-UNF	3 of 4 may need sealant

Separate Care page for all double threaded studs in the car.