There is a thread here just recently where one of the proficient mechanics found there was a difference in a pair of Ate calipers. His is apparently the only instance of this known to happen. I can't explain it. So I will now give you the stock answer we have used for so many years. Just use your eyes and report if this looks like it has problems when you do this, OK? The calipers are two sizes. A single disk set up should have a 38mm caliper and said caliper is not marked with the number. A dual caliper set up should have two (2) 40mm calipers and they should be marked with the number 40 on the outside round portion of the caliper. This is visible with them mounted on the bike. Very few exceptions to this note, if any. How ever since the bikes are now what we call vintage many times the OEM equipment has been moved around. You can even occasionally see a dual set up with one of each. Since you are building your own you could use 40 or 38 calipers I suppose. The calipers are movable from side to side. There are two (2) holes in each caliper. One for the brake line, on the bottom. And one for the bleeder screw on top. Swap this function to make caliper work on what ever side you like. Th problem another inmate had was the taper at the bottom of one of these holes did not accept the bleeder screw so he couldn't move a left caliper to the right side. He fixed it. But it is also the only time any body remembers hearing this problem I think.