Danny D said:

No.

The way I read that Lasco article, you should not be using EITHER pipe dope OR teflon tape on plastic pipe threads.

You should be using a thread sealant meant for plastic threads ONLY. Don't use it with teflon tape because that teflon tape on the male threads is going to cause more stretching of the female threads around them. With metal threads, that teflon tape doesn't matter, but with much weaker plastic fittings, three or four wraps of teflon tape is enough to cause significantly more stretching of the female thread than they were designed to handle. AND, way more compression than you need to prevent leaks.

From my reading of that Lasco article, you should apply thread sealant meant for plastic threads to your male plastic threads, and then screw on the female plastic fitting. Tighten to finger tight and then no more than two full turns after that. Then stop, and move on to the next plastic threaded joint. That's all you need to do, supposedly, to get a leak free threaded plastic joint.

If you've already used teflon tape on your threaded plastic joints, and there's no splitting or leaking, I would just leave them as they are. But, in future, just use the red bottle that says it's meant for plastic and don't use it with anything else.

At least that's what I got out of that Lasco article when I read it. Maybe re-read it with what I said in this post and my previous post in mind, and see if it makes more sense. Ignore that part where he starts talking about doing stress calculations. The part that reads:

"Stress" (tensile stress) is the force exerted by the strain of the male thread multiplied by the resistance of the PVC. The resistance of PVC is 400,000 pounds per square inch (psi). The strain per turn past finger tight for one-inch PVC pipe is .00447, so the stress per turn is 1,788 psi. Thus, a one-inch threaded PVC joint that is tightened four turns past finger tight will develop a tensile stress of 7,152 psi. The joint is bound to fail since the stress exceeds the 7,000 psi tensile strength of PVC, without even adding the tensile stress caused by the pressure inside the irrigation system (up to a maximum of 2,000 psi).


Ignore that part. It's hard for anyone who hasn't taken engineering to understand, and he's not even using proper terminology. He's saying the "resistance" of PVC is 400,000 psi, when he should be calling that the "modulus", or more correctly, the "Young's Modulus" or "E" of PVC is 400,000 psi. Also, he's saying that the strain is ".00447" without explaining what he means by that. Most people would be expecting a unit of some sort after that number, but strain is a dimensionless number, like percent. It's actual dimensions are inches per inch, or centimeters per centimeter or miles per mile or lightyears per lightyear. If you stretch a 8 foot long board by 27/64ths of an inch, you've put a strain of 0.00447 into it cuz 27/64ths of an inch divided by 8 feet is 0.00447 (within round-off error).

All he's saying is that each turn past finger tight of a 1 inch tapered male NPT thread into a female NPT thread will stretch the female thread plastic by about 1/2 of one percent (actually .447 of one percent), which when multiplied by 400,000 psi per 100 percent is enough to put about 2000 psi of stretching force into the female plastic thread. So, four turns will give you 4 times as much, or 8000 psi of stretching force, and that's gonna split the female fitting because PVC plastic breaks at 7000 psi of stretching force.

That's all that gobbeldygook is saying.

So, just read the rest of that article with the horse sense in mind that the more tape or dope you apply to a male tapered thread, or the more you tighten a male tapered thread, the more stretching force you'll cause in the female tapered thread it's screwing into. THEN, that article should be easier to understand. And, keep in mind that when he says "thread sealant", he's talking about a special sealant meant for plastic threads, NOT pipe dope.

Read it again and tell me if you don't agree that, according to Lasco, you should be using the stuff in the red bottle labeled "For Plastic" and ONLY that stuff. If you're still confused by something in that article, quote it and I'll try to explain it.