Threads are risky business in vacuum. On 1/8 NPT if you are careful you can get a good seal with teflon tape. Thread goops are not good (although there is one that supposedly sets up and has a low vapor pressure).
Where I have had to use threaded fittings I use clear 5-minute epoxy: fill the first 3 threads of the male fitting level with the top of the threads (a toothpick or similar can be used as the applicator and squeegie). Then screw the fittings together and tighten about 1 turn beyond hand tight. For smaller fittings (up to 1/2 inch) you can easily break the seal if you want to take things apart at a later date.
On tubing sizes, in viscous flow (about a Torr and up) conductance of the line degrades proportionally with length but varies by the diameter to the fourth power. That means a 1/2 inch line with have 1/16th the conductance of a 1 inch line. In molecular flow (under 1 milliTorr) it's to the 3rd power. Between 1 mTorr and 1 Torr (transition flow) it's, well, in between.
In short, don't skimp on your tubing diameters.
For reasonable length forelines (mechanical pump to chamber) try to use at least 3/4" lines.
Steve
Created on Sunday, March 11, 2001 11:10 PM EDT by Steve Hansen