Bolts, fittings, pipe threads and spark plugs with US or metric threads are easily identified. Aluminum gauges identify: 1.) US threads from #5 to 3/4� pitch of ~10 TPI to 44 TPI. 2.) Metric threads from 2.5 to 22 mm ~and pitch of .45 to 2.5 mm. 3.) Pipe threads 1/16� to 1/2� with ~14 TPI to 27 TPI. 4.) Spark plug threads 10 to 18 mm.