Originally Posted by
Drew Eckhardt
First position cogs have a builtin spacer so they have good spline engagement overhanging the end of the freehub, and serrations to prevent lock ring rotation. While you can probably get away with no serrations, poor engagement with the freehub splines is a bad idea especially if they're aluminum like Campagnolo spline freehub bodies and many aftermarket Shimano/SRAM pattern parts.
Just buy a cassette with a 13T first position cog. The lock rings are the same for all 12T+ first position cogs.
The point about spline engagement is a good one. I've noticed this firsthand when putting the cassettes together. On one setup I did not need any spacers and the lockring torqued down nicely with the spline engaged. On another set it was a little tricky and I needed a very thin spacer between the lockring and high cog. Will give these setups a try, but certainly something to keep an eye on.