Just to be sure I wasn't clear above, the hole that is drilled out to 31/64" is NOT the part that the larger bolts thread into but the part that the little cup threaded into at the far side of the chain breaker. This has to be drilled out so that the cotter dumb end fits into it and has somewhere to move into when it is pressed out.

I drilled mine out to 31/64 because it it looked "about right."

The mouth of the big bolt has to be drilled out to 21/64 because that is the size of the collar of the "pushers" which will get put back into the outer bolt from the other end. If the hole is left small the pusher will not go in far enough because of the collar and there isn't enough room to fit onto the crank.

DO NOT try and just use the pusher in the way the chain breaker uses it. It will fall off the threaded part of the cotter and destroy the threads. I tried doing this while I was messing around and almost ruined the cotter in one of my bikes before I noticed what was happening. One you drill out the receiver cup threads it is ruined for use as a chain breaker anyhow. Might as well finish out the mod and drill the big bolt too.

The pusher has a dimple in it because it is designed to use that little ball bearing with the chain breaker. It happens to fit over the cotter perfectly and many of the better cotters have a dome built into them so they work with a cotter press with a dimpled end on it.