Heater Element Thread Type

I'm building a RIMS tube with a 1" heater element. My stainless end cap has a 1" NPT thread, but heater elements have "Straight Pipe Threads" I brought my end cap to a couple of big box stores and a plumbing distributor store, and tried to fit their elements in my end cap. I could never get more than 1 or 2 threads on to it before it jammed. I know many of you have had no problem with this, but I could not get a definitive answer as to why. Finally tried a friend's element from his tube. It slipped on perfectly even though it too had NPSM threads. Here's the link:

www.plumbingsupply.com/elements.html

Search for: 120-1500-ELD

NPT means tapered threads....the threads get narrower as you go farther in. NPSM (straight pipe threads) have no taper, same width the whole way.

This means that NPSM will only thread a bit into NPT threads, until the NPT threads taper too small to allow the NPSM to go further. Most of us just use a lot of teflon pipe tape and it doesn't leak.

There seems to be some variation in how straight NPSM threads are, and how tapered NPT threads are, so some elements fit into some NPT bushings/caps better than others. It's a crapshoot.

Yes, there is a variation, and the only way to be sure of a good fit is to try them out before purchasing.