Neiko Digital Caliper [measure fittings, o-rings and more]
Electronic Digital Caliper by Neiko. Easy to operate. One button switches between inches and mm. Hardened stainless steel shaft. Resolution: 0.01mm/0.0005 inch, Accuracy: 0.02mm/0.001 inch. Large LCD display. Take measurements from 0 to 6 inches. 1 year warranty.
I use calipers around my home brewery to… measure things. 🙂 Including… fitting sizes, o-ring sizes [See: Bulk Keg Orings and Keg Repair Part Numbers]
Update: Since this post was published the price, availability or promotion has changed. Check the product page for current description, price and availability. Also: Today’s Deals on Amazon
Neiko 01407A Electronic Digital Caliper with Extra-Large LCD Screen, 0-6 Inches
Also: Nut & Bolt Thread Checker (Inch & Metric) – via Amazon