wrjjrs@aol.com wrote:

Dale,

We have been using these taps for years.

Bill,

   I've been using them for more than 20 years, myself.  I needed smooth
strong threads in a rear sight I was making for a mini-14 rifle.  Since then
I've been using them on nearly all aluminum parts I make for aviation use.
As soon as my new "tap" comes in, I'll be doing them even for the NPT
threads in my oil filter adapters.

You should check in Machinerys Handbook or with the manufacturer for the tap drill size. I always ask for a tap with a air/lube relief groove. Hand tapping can be tough with the roll tap as these are called.

Bill Jepson 

For those without a copy of Machinerys Handbook, a quick rule-of-
thumb that works is to drill the hole halfway between the size needed
for a conventional tap and the size of the thread.  For example, for a
1/4"-28 thread, I use a #A drill, rather than the #3 recommended for
a cutting tap.

Best Regards,
Dale R.
COZY MkIV #1254