Drilling and tapping a main

cn30

I need to drill and tap a 1 ½” main to put in a vent; there are no plugs, no T’s, nothing, and old rusty pipe.



My question is, how small a hole can I drill and tap and still get decent venting? I ask because I’ve read that a Gorton D, at 1/8” (or ¼” straight), vents as much as a Gorton 1, with a ½” connection. Is that accurate? Is it all the orifice size? If so, why have such large connections?



If I can tap for ¼” I can get 18 threads per inch, or 27 threads per inch at 1/8”; given that the 1 ½” main is somewhere around 3/16 to .20 inches thick, but is curved, I could get at least 3 threads cut for a ¼” tapping, and maybe 5 for a 1/8” tapping. I would do the latter if I trusted that I would get adequate venting.

cn30

going to steam section

Sorry, I meant to put this in steam

bob young

vent

forget about drilling.  weld a boss on the main. with a ips tapping.  perfect job.