Measure the od of your collet and subtract .0541, this will give you
your hole size .

To find id thread height mutiply lead x .54127
to find od thread height mutiply lead x .61343

A 5c collet has a 1.250 x 20 thread
1/20 = .05 lead
..05 x .54127=.02706
..02706 x 2 =.0541
1.250 -.054 =1.196 hole size
http://www.cox-internet.com/drspiff/...ts/Collets.htm