Just tried ordering a tap for 1/4 x 32 tpi. I thought it was a standard UNF thread, but apparently 1/4 x 28 is the UNF standard! The only thread I know of that is 1/4 x32 is a model engineering thread. Assuming this is the correct thread form, it seems odd that an unusual British thread standard was used for miniature spark plugs, that were made and used mainly in the US! Anyone got an explanation?
Regards,
Andrew.