Does 21mm refer to the size of spanner you're using?
As far as I know they are 1/2" BSP thread, which is the bit that matters.
Vin
Never thought to think outside the proverbial box Vin.
My socket set goes up the 24mm and the next size up is a 1". The 1" is a tight fit.
I've measured the diamt of the threads and that is approx 21mm. So 1/2" BSP is nearly 21mm. According to Google 20.955mm.