DougC dcimper norcom2000.com wrote:

John Doe wrote:

2017 aluminum collar/rod
64 mm length (about 1" will be used)
22 mm outer diamete
8 mm inner diameter
http://www.flickr.com/photos/27532210@N04/

Is the collar's 8mm inner diameter appropriate for 3/8 inch 24
thread?

Not really.

This tap & drill chart online-
http://www.newmantools.com/tapdrill.htm
says that the standard drill size for 3/8-24 is a #Q drill bit

which this drill chart online-
http://bobmay.astronomy.net/misc/drillchart.htm
says is .332" inch.
Note that it also indicates that a #Q drill is 8.4328 mm
diameter, so an 8mm hole is -probably- too small to attempt with
a cheap tap.

Can anyone specify a 3/8-24 tap that might work without enlarging
the hole?

http://www.use-enco.com/CGI/INLMK32?PARTPG=INSRAR2

The 3/8-24 H1 Hertel spiral point tap is back ordered.

In taps, the "H" number is the measure of looseness, or oversize
of the tapped threads. Increasing numbers mean increasing
looseness.

Okay, thanks.