Sealing PVC fitting in cast iron

[MillWeld]

I have a cistern jet pump w/ 1" and 1.25" PVC fittings screwed into cast iron and they have developed pretty good size leaks. I screwed then in all the way in and used Teflon paste on the threads. I exerted almost all the torque I could to seat them and STILL got a leak - on both. What is the proper way to seal this?

 

[PsRumors]

I've always liked using tape over paste and the gas rated tape (yellow) seems to do a better job than the thin white tape.

 

[Gunny]

Agree, I never use paste on PVC fittings, always tape

 

[High Compression]

I agree with the tape (I like the blue monster tape), but also there could be a crack in the threads. Switch to a brass fitting. Get a 1" female npt by 1.25" IPS pack joint and you shouldn't have any problems.

 

[radioman]

You shouldnt have to use gorilla strength with pvc in cast iron. Chances are the iron pipe is distorted out of round and plastic threads are stripped a little from overtightening. High compression has the right idea - use brass fitting as its more malleable and is more forgiving when using against dissimilar materials. I only use paste on metal to metal, never on plastic mating.

 

[ozley]

I have a cistern jet pump w/ 1" and 1.25" PVC fittings screwed into cast iron and they have developed pretty good size leaks. I screwed then in all the way in and used Teflon paste on the threads. I exerted almost all the torque I could to seat them and STILL got a leak - on both. What is the proper way to seal this?

A lot of tape but not on the starting threads then some paste. Make sure the tape is on tight and go slow.

 

[spruce Deere]

+1 on the blue monster tape and brass fitting. Wrap it on fitting so when you screw it in the tape does not 'wipe' off, edge off sealant tape not facing oncoming threads and go slow. Should not require bruit force but some effort.

 

[quicksandfarmer]

When connecting threaded metal pipe to plastic the metal should always be female and the plastic male. The plastic yields more than the metal and you want it compressing rather than expanding.

 

[MillWeld]

Well, I put new PVC fittings in the cast iron ports with Teflon tape sealant and the pump works again. I will wait few weeks before I declare any success - after all the paste worked for a while, too.

The interesting observation is that when I cut the PVC pipe to remove the old PVC fittings I could unscrew them with my fingertips and I had heavily torqued them when I installed them. I read (maybe on this forum) that the paste is really a lubricant and not a sealant. I would have never suspected that screwing in pipes could be so complicated. I did find an interesting comment on another forum: "Also, some of the compounds and chemicals in the paste can react with certain plastics, including PVC, and cause them to swell or soften and eventually degrade and then they crack and leak or fall apart." Lasco, a PVC fitting manufacturer, recommends neither Teflon tape or paste, but rather a sealant but didn't specify which one. I've learned far more than I ever thought I would need so I'm dropping the subject.

The pump was a new obsolete one and it could have distorted threads as Radioman suggested. If my current solution fails I will go the brass route as Highcompression suggested. I like having a plan B.
Thanks,

Bob

 

[MillWeld]

Well, like everyone said, Teflon tape is the way to go for PVC threads. No leaks I will NEVER use pipe dope on them again!
Thanks for all the help,
Bob