Magnetic oil drain plugs for MGs
Moss # 328-282 MGA, MGB, MG Midget Replaces 2A380, 2F4249, 88G257 328-280	Moss # 328-461 MGA Twin Cam, MGC, AH-100, AH-3000 Replaces 2H4685 328-460

This is an engine accessory that I personally recommend. It is a direct replacement for the engine oil drain plug, and it contains a small but powerful magnet. When you change your engine oil you can clean the fuzzy collection of iron particles off the magnet. This is especially beneficial during the initial engine break in period when freshly honed cylinder walls and new piston rings are wearing in together.

For the standard engine drain plug (left in photo), the thread size is 1/4-BSP or G1/4.

1/4-inch BSPP (British Standard Pipe Parallel thread)
1/4-inch BSPS (British Standard Pipe Straight thread)
1/4-inch BSPF (British Standard Pipe Fine thread)
1/4"BSP (19 threads per inch)

Whitworth form parallel fastening thread with 55 degree thread angle.
19 threads per inch -- major diameter 0.518-in (13.157-mm)

British Pipe Thread (Whitworth Form) -- Parallel (BSPP/BSPS/BSPF)
G1/4	Thread Designation
1/4"	Nominal Size
BSPP	Thread Form Type
13.157	Major Diameter mm
1.337	Pitch mm
19	Threads per inch
12.301	Pitch Diameter mm
11.445	Minor Diameter Male Thd.
0.856	Thread Height
11.60	Tap Drill Diameter mm 

References:
CO-201
Moss Motors
uk.rs-online.com