Production taps:-
Gun taps are the toughest, primarily designed for driving the swarf to the bottom or thru the hole --idealy thru hole.
Spiral taps idealy for blind holes --screws the swarf out of the hole
Roll taps idealy for sheet metal --forms a thread rather than cutting one.
Hand taps --a handy desperate last resort
UNC or NPT taps give the most drama with chipping, galling or breakage -- just a PITA
Pipe threads BSP (or NPT if you must) use "gun" taps
straight ( the fitting is tapered for a jam fit) Most Ford stuff is NPT.
BSP and NPT look similar , but -- 1/8" - 3/8" is one pitch / " different, 1/2" - 3/4" same pitch, 1" - 2" half pitch / " difference - not to mention incl.thread angle, BS is 55°, UN is 60° along with metric
Metric taps -- for drilling size, just minus the thread pitch from the nominal O.D. eg M10 x 1.5 -- 10 - 1.5 = 8.5
Dave W --"Trefolex" (green death) is laced with asbestos or other carcinogens is it not ?
