The screwed joint (threaded male and female ends screwed together) is the most common method of joining pipes. A high degree of standardization is maintained in the threading of piping materials. American Pipe Thread Standards guide the piping industry.

Use a wire brush
Since dirt can make a good fit impossible, the first thing to do is to wipe both male and female threads clean. Wire brushing is recommended, especially if threaded pipe has been exposed to weather. Running a tap or die over the threads will usually straighten any that may be damaged and will help to ensure a good metal-to-metal joint.

Lubricant is good - but not inside a line
Next, use a bit of good thread lubricant or "pipe dope", but put it on the male threads only. This prevents any excess from squeezing into the pipe and cause harm to valve seats or other mechanism.

Lubricant reduces friction when pulling up a joint. It is not expected to seal the joint, or to compensate for poor workmanship.
With lubricant applied, start the joint by hand. If the thread engagement "feels" right, turn it up as far as it will go. If the threading job is up to standard... and if the threads are free of dirt... the joint is now started right. Futher turns with a wrench will complete the job.

Pipe run in too far
But be careful with the wrench. Never use an oversized one that will tempt you to "lean on the joint". Too much pull up can cause damage, especially in the case of valves. Don't try to run all the male threads into the joint. The lead of the die always leaves a few imperfect and unusable threads which should be left exposed.



First published June 1999