I can tell you one reason for the small bores.
When machining a bore, it is common practice to shoot for a dimension on the lower side of the tolerance. What that does is allows for maximum material to be left in case rework is necessary from a broken tool. AKA reduces scrap.

Say your machining a 20ga bore of .615 nominal with a tolerance of +/-.015. Most machinists would use the tolerance to their benefit, and shoot for .605. That leave .025 for any problems to be corrected yet still makes a good part.

Just the opposite on an OD. Leave it big yet still in tolerance.


Last edited by ithaca1; 08/25/16 12:33 PM.
Bill Johnson