End the guesswork over the correct thread and ball sizes for your probe system. Our thread card has ten tapped holes between M2 and 3/8-24 to check the thread size, and ten holes ranging from 1.0 to 6.0mm to gage the ball size. A built-in 40mm engraved scale confirms the stylus length. We make these in the USA from aircraft grade aluminum that is anodized and laser engraved. Protective slip cover included.