Other than making one is there any way to get a 3C collet in 5/8? I
know I could have one custom made, but that's silly. It wuld be handy
for a repetive job I am working on to do the heavy operations on the big
lathe and some lighter operations on the smaller lathe. The problem is
I have it setup with 3C collets and a closer and I absolutely hate
bolting on and unbolting a chuck on the small lathe or the mini lathe.
Its just a big pain.

Anyway, Is there any chance 5/8 is one of those out of spec sizes that
just happens to exist anyway. Like 7/16 for ER16 or 5/16 for ERll.
They are outside the spec, but they are commonly enough made that you
can hunt around and buy them for about the same size as spec size
collets. There is a enough meat on a 3C for it. It wouldn't go all the
way through, but my 1/2 inch 3C collets don't go all the way through
either.