Check if any markings on the collets to see if imperial or metric, bet they are imperial. typical sizes are 1/4 3/8" 1/2" and 5/8" m If metric they are generally 6, 10, 12 and 16mm
Found another link that may be of use.
For S type not C but think the collet sizes are the same. http://www.oldengine.org/members/diesel/Tables/Clarkson4.htm
Adrian
Found another link that may be of use.
For S type not C but think the collet sizes are the same. http://www.oldengine.org/members/diesel/Tables/Clarkson4.htm
Adrian