What size is that nut you're trying to replace? What about that brake fitting you need to adapt, is it SAE or metric? And that suspension bolt... how can you tell M10 from 3/8 UNF without a micrometer or a trip to the hardware store?
Thread Detective® to the rescue!
The "Standard" Thread Detective has a total of 25 special steel fittings, each with a different SAE (inch) or metric (mm) thread size. Each size has both male and female threads so you can identify nuts, bolts, and plumbing fittings. The SAE sizes are all strung together on a wire lanyard, and the metric sizes are all strung together on a second lanyard. Both lanyards are connected to a small metal tag with an 85mm (3 3/8") ruler with both metric and SAE markings. The fittings are separated by shatterproof, color-coded plastic dividers (blue for metric, red for SAE). Each fitting has its thread size permanently printed in bold numbers. Loops at both ends of each lanyard let you hang them near your workbench or in the trailer. The extra-long lanyards let you reach into tight places when removing the mystery hardware isn't practical.
This Standard kit includes the most common thread sizes found in racecar hardware and brake system plumbing.
SAE (inch) sizes: 6-32, 8-32, 10-32, 1/4-20, 1/4-28, 5/16-18, 5/16-24, 3/8-16, 3/8-24, 7/16-14, 7/16-20, 1/2-13, 1/2-20
Metric (mm) sizes: M4x0.7, M5x0.8, M6x1.0, M8x1.0, M8x1.25, M10x1.0, M10x1.25, M10x1.5, M12x1.25, M12x1.5, M12x1.75