On Aug 15, 4:11 pm, (DoN. Nichols) wrote:
....
In particular, check import collets. All that you have
mentioned above are good ones which will be made to good standards. My
offending one was an import 4" 3-jaw chuck on a C5 shank from a later
eBay auction.

Enjoy,
DoN.
....

I've milled the slot deeper to make several Enco and other inexpensive
5C collets fit my index fixture, after confirming that the slot wasn't
hardened with a riffler file. They worked fine in the lathe in an old
collet adapter that's had its pin sheared off.
Jim Wilkins