"Emergency" Spring collets - question.

[Highpower]

Is there a "rule of thumb" for how far down a spring collet should be compressed before boring it to a desired size?  

I have a set of Snap-On metric stud removers that I use, but I want a collet in 12mm x 1.75mm and Snap-On doesn't offer that size. I just bought an old collet off of eBay and want to bore it out and re-thread it to my desired size. But I don't know how much to squeeze it down before boring it.  

   

   

[f350ca]

Don't know how far in they pull, but machinable 5C collets have spacers in the slots that you knock out after boring to desired size.

[Highpower]

Yes, I was thinking of filling the collect with Cerrosafe alloy to support the fingers while boring it out and threading but I want to get it sized correctly of course. Don't want it so large that it won't grip the stud threads tightly, but still need it big enough to screw onto dirty, rusted threads too.

[Dr Stan]

I'd measure the slots at their extreme bottom to determine the thickness needed for the spacers. Insert same and machine away.