Duncan,
There are two types of BSP threads:
BSPT (British Standard Pipe Tapered)
BSPP (British Standatd Pipe Parallel)
The tapping drill sizes are different.
BSPT is designed to seal on "wedging" (metal - to - metal contact WITH a sealing medium; 569 loctite is best). Previously we would have used teflon thread tape.
By contrast, with BSPP thread form the thread retains the fixture (bush or tap), and a sealing washer (traditionally fibre or copper; now a days metal backed viton washers are available), seals the petrol (or oil) in. The tapping size is slightly larger, and the fitting will spin in easily, until it bottoms out against the washer. Both surfaces must be smooth and parallal, and free of scratches, paint, etc.
A BSPT tap has a distinctly tapered appearance, whereas a BSPP tap has is "straight" (parallel thread form).
If you run a tapered tap too far into the hole, the fitting will be too loose, and will probably leak. Usually, tapping 2/3 depth of the tap will be good. It is recommended that the hole has a tapered reamer run through it before tapping, but you can get away without doing this.
Richard