new valve problem?

[amajamar]

I purchased a new Thermo Pro 3442 PSI DIN\yoke convertible valve to install on my PST 3442 E7 80cf instead of the Blue Steel convertible that I have on there now. The Blue Steel valve is not "shaved" on the sides as my other Thermo valves are and one of my yoke regs doesn't fit on it.

I went to replace the valve and found that I could not thread the new valve on. It would only go on about two turns then bind up. I compared the threads on the two valves very closely and they seem to match up perfectly. I also looked for any damage on the threads of the new valve and see none. The old valve spins right on, yet the new one doesn't.

Any ideas what could be wrong (or what I may be doing wrong!)?

Thanks in advance!

 

[Scared Silly]

Hmmm, without looking at the valve my only thought would be a burr on the threads or perhaps a damaged thread. How hard does it bind? I have a DIN/Yoke plug for one of my cylinders that has a bit of a bind to it that I give it a bit a push to get past. I certainly would not force it but it just might need a little encouragement.

 

[rjack321]

pictures would help alot.

 

[amajamar]

Hmmm, without looking at the valve my only thought would be a burr on the threads or perhaps a damaged thread. How hard does it bind? I have a DIN/Yoke plug for one of my cylinders that has a bit of a bind to it that I give it a bit a push to get past. I certainly would not force it but it just might need a little encouragement.

I was afraid to "encourage" it very much at all! I was really afraid of crossthreading it. I really can't see or feel any thread damage on the new valve. As I said before, the old valve spins right in. It isn't possible that it's the wrong thread, is it? (I don't have a 7/8" valve to compare it with) The old valve threads and new valve threads fit together perfectly, so I'm pretty sure that the size and thread is correct.

I'll try to take pictures later, but I don't know what you will be able to see...

 

[Scared Silly]

Thread damage can be very hard to see. On my plug it took me a while to see that one of the threads had a flat spot on it.

BTW you mentioned 7/8" threads - they should be 3/4"

 

[amajamar]

Thread damage can be very hard to see. On my plug it took me a while to see that one of the threads had a flat spot on it.

BTW you mentioned 7/8" threads - they should be 3/4"

I mentioned the 7/8" valve only for the purpose of stating that I don't have a 7/8" one to see if I might have received the wrong valve. As far as thread damage is concerned, I looked at the valve again yesterday afternoon and the only thing I saw as far as damage is concerned is a small spot about a half inch long on the top of one of the first few threads that the chrome plating is off; the underlying brass is showing. I didn't see any crossed or deformed threads. If I have some time this afternoon, I will try to reinstall the valve. One thing I was thinking, is it possible that the neck of the tank was really cold from releasing the air and was a little smaller that it would be at a warmer temperature? It seems kinda' logical but I would be surprised if it would affect the threading that much that the valve wouldn't thread in. What do you think?

 

[JKPAO]

It could happen (temp expansion and or contraction), but it should spin in without any binding. you should be able to compare the threads side by side, touching crests of one valve to the troughs of the other valve. Threads should match exactly, if they are the same thread pitch. good luck, i hope this helps!
See you topside! John

 

[Scared Silly]

The 7/8" UNF will fit in a 3/4" NPT - different standards for measuring. Yeah it is odd ball but true just the difference in reference.

 

[amajamar]

The 7/8" UNF will fit in a 3/4" NPT - different standards for measuring. Yeah it is odd ball but true just the difference in reference.

Could that be the problem? Would the the threads match up? I didn't think that the 3442 psi DIN/Yoke valves came in a 7/8"

 

[amajamar]

Update...

I got it! I returned yesterday morning from a west coast trip and decided to give it another try. I pulled the old valve off and again compared the diameter of the valves and the threads and determined that they matched. I then looked very closely at the threads of the tank and I noticed a small burr or some kind of embedded piece of of debris. I could feel it with my fingernail. I took a small screwdriver to it and it dislodged. I put a drop of christolube on the new valve threads and it it threaded in all the way (it was a little stiff the first time but I spun it out and the second time I threaded it, it was smooth as silk!)

Thanks to all!