On Sat, 12 Jun 2010 18:13:36 -0700 (PDT), Vernon Tuck
wrote:



I'm not a machinist and I don't have any measuring tools. *So I'm not
sure about the diameter of these holes but I'm pretty sure they're
M20x1.5 or M21x1.5. *I have found European sources for blind plugs in
these dimensions but have not yet found a source in the USA.


These seem to be intended for use as conduit fittings, but the threads
are right:
http://www.sealconusa.com/strainreli...0_1_2_npt.html
http://www.novamaxindustrial.com/adapters.htm

McMaster has some as well, but a quick look showed only very expensive
high pressure SS fittings there.

4822T45
Extreme-Pressure SS Threaded Fitting M20 X 1.5 Male X 1/2" NPT Female
Adapter, 5000 PSI
In stock at $46.63 Each

--
Ned Simmons