I have the same issue as FastFrontier. Last year I bought a couple of oil drain plugs from Speedy Spares (who said they will fit), won't fit oil drain but they fit transmission drain. BSP and BSPP threads are not the same. BSP is a tapered thread. BSPP is a parallel thread. See link for explanation. Today's a good day, I learned something.
David,
Pegasus Racing has this article about BSx nomenclature.
https://www.pegasusautoracing.com/
Knowing the Parts List identifies the plugs as BSP and I've never seen a tapered plug in a sump or transmission, I'd think the Pegasus Racing info to be more reliable. Any confusion probably stems from a long term general usage of the generic BSP instead of the more descriptive BSPP for the more common straight threads.
BSP ~ BSPP = (parallel)
BSPT = tapered
https://www.pegasusautoracing.com/document.asp?DocID=TECH00098
Hope this helps,
BSP Fittings: Identification and Logic
Identification and Logic
Ordering and using BSP fittings can be confusing because the nominal sizes have no relation to thread size. They are also much less common (outside the UK) than other threads, so many people are unfamiliar with them. Fortunately, BSP sizes are easy to identify.
Background
BSP stands for
British
Standard
Pipe. It is sometimes called BSPP (Parallel) to differentiate it from the even less common BSPT (Tapered thread). Like most other plumbing fittings, BSP threads are straight or parallel, not tapered. The outside diameter of the male thread is the same from the base to the tip.
BSP fitting sizes refer to the nominal ID of a standard pipe. This is where the confusion comes in. Most plumbing fittings are named for the thread size, but BSP sizes are not. A BSP fitting with a 3/8" OD thread is actually a 1/8 BSP size, not 3/8 BSP. Use the chart below to identify BSP fitting sizes.
Sealing
BSP fittings may seal at a concave flare at the end of the male fitting, or at the port face. Female fittings which seal at the flare must be a 2-piece design with a swivelling nut. This allows you to tighten the nut without damaging the flare. One-piece female fittings (without a swivelling nut) must use a sealing washer at the base of the male fitting. Male fittings without a concave flare also require a sealing washer. In any case, BSP fittings are designed to be used dry, with no thread sealant.
A specialized sealing washer known as a
Dowty Seal (Pegasus Part No. 3240-Size) has been designed specifically for sealing BSP fittings. These washers are made of steel with a rubber insert bonded to the inside. They are thicker than Stat-O-Seals, and they are sized to match BSP thread sizes. Note that some female BSP ports are chamfered to accept a plain rubber O-ring instead of a Dowty seal. Dowty seals should only be used on ports without a chamfer, while O-rings should only be used on chamfered ports.
BSP Size Identification
Male Thread OD.......
Threads per Inch .......
BSP Size
0.38".................................. 28
...........................1/8 BSP
0.51"................................... 19...................................
1/4 BSP
0.66"................................... 19..................................
3/8 BSP
0.81"................................... 14
............................1/2 BSP
0.90".................................. 14 ...................................
5/8 BSP
1.04"................................... 14
.............................3/4 BSP 
