I always used threaded rods for cork grips. Clamps as has been posted ( double nuts and fender washers) and when it came time to turn them Id unbolt the washers and set them aside so that my "shaping" tools didnt run into them.

When Id first buy them Id center drill one end and that way the live center on my Jet lathe would always center up just fine.

I didnt make it a habit of turning grips from glued rigs because the price of AAA quality rings was about $2 less than a completed Struble AAA quality grip PLUS the struble grip was already conuter bored for the reelseat nose.

Reelseat inserts I used a "pen mandrel" which fit the hole I had to deep bore in the wooden seat insert.

Here is my grip and insert lathe...