[denbigh1974]

I have a heating oil tank which is filled up by connecting to a
threaded male inlet at the top. I need a new lockable cap for it. The
thread is parallel-sided (i.e. not tapered)

I assumed that the thread would be 2" BSP. Having bought an
appropriate cap (SpinSecure) on the internet, however, I discovered
that it would not screw on. The cap female thread was too small.

So I measured the tank's inlet thread dimensions using some digital
calipers. These dimensions are as follows:
Outer diameter (thread crest-to-crest) = 60.4mm
Inner diameter (thread trough-to-trough) = 58.0mm

I estimate the measurement error to be ±0.2mm.

I also measured the distance from the 1st crest to the 9th crest to be
19.12mm so there are eight threads in 19.12mm (= 10.63 threads in
25.4mm or 1 inch).

Anybody any idea what thread size this could be?

When I looked up the 2" BSPP date it said
Outer diameter (thread crest-to-crest) = 59.6mm
Inner diameter (thread trough-to-trough) = 58.0mm