So you're looking to buy a set of lowering spindles for you 1973-1987 C10 or 1988-1998 C1500 pickup and the parts guy wants to know if you have 1" or 1.25" thick rotors? It's easy to measure them, don't guess! These trucks were built with two different spindles that have different brake rotor thicknesses along with different bearings and calipers so you need to make sure you get the right ones. If not you'll either need to return them for the right ones or buy a bunch of extra parts that you may not have needed otherways. The correct way to determine your rotor thickness is to measure the rotor on the brake pad surfaces (see picture). They should measure 1" or less for the 1" (LD) brakes and 1.15" or more for the 1.25" (HD) Brakes. If your wheels have spaces in them you might even be able to do it without taking the tire and rim off. Your measurement doesn't need to be exact, just close enough to tell which you have.