The flow control valve body on my PTO driven hydraulic welder had a stuck fitting on the outlet side. I was starting to round off the 1-1/16" fitting when torquing it to remove the blown hose. Initially, when installed, I used teflon pipe dope to seal the threads and put it in my-tight. The outlet hose blew on startup the other evening (bloody mess of ATF fluid everywhere in the freshly painted box) when it let loose of the fitting crimp. After about 5 years, that fitting would not budge to loosen up, even with a long torque cheater. The fitting was starting to round off. The only way I could see to get it off was to apply heat. I used my Mapp Gas torch to gently heat the outlet of the valve body until the fitting broke loose. After getting a new hose from Rural King (Sunday repairs), I was installing the new hose but it never felt like the taper was seating properly on the 3/4" MPT fitting. Good thing I noticed the lack of torque to seat it. Turns out the female threaded port on the valve body had a very small crack in it. A new one is on order but why did it crack after gentle heating? It never got red hot. Is this just the nature and risk of heating cast or did I do something basic, wrong? Might as well learn from my mistakes rather than repeat!
Pipe thread port on the valve? My guess is you may have used a little too much tape, and then probably a bit overtight. Usually about 3 wraps is enough, and a light smear of pipe dope over the tape works well for hydraulic fittings. Some of these Chinese parts fit a bit loose, those are a real sucker to get sealed. I hate pipe thread ports, ORB is much better.
I agree on NPT ports vs other types but that's all that was available when I built it. Fairly low pressure system. No tape but I did use dope on the original install. ATF has started leaking on many of the joints so now using both tape and dope to stop the leaks. Must be ATF affecting the dope. The stuck valve connection was not leaking prior to blowing the hose. I used to do gas piping so I tend to make joints very tight with NPT. I never had an NPT seize like this one did.
Any chance the fitting bottomed out, instead of getting tight? That would mess the threads up, and then your torque to try to take it out of the ruined theads, could have cracked it when you went to take it off. Or the threads were bad in the first place. I could see that being more likely than the heat.
I recently bought chinese 3/16 brass pipe thread fittings that were so poorly machined, they would not seal. The thread was right, but the taper was too straight. Bottomed out before the threads came tight. Not impressed, I don't mind sayin'
I had an overtightened NPT fitting crack while trying to remove it. So, on the plus side I know it's not just a me thing now. For NPT fittings, use hydraulic thread sealant and do not exceed turns past finger tight for the size.
It wadded material while being removed & put a hair line crack in it, the installation finished it off.