tight valve knob

[halocline]

Good point and although it makes sense I hesitate to interchange them. The picture of the stem is how it came out of the valve when I first opened it (and I am pretty sure it was the first time it was dissembled since it left the factory). Also all the pictures I can find on-line (eg google) have the stem as shown here. Finally in the book "scuba regulator maintenance and repair" Vance Harlow describes it as: "The o-ring goes on first, so the washer is on the outside of the stem". Kind of vague but I assume that by "outside of the stem" he means the low pressure side i.e. as I have it.


The said teflon ring is not shown here because it goes inside the bonnet nut. I think its' purpose is to prevent metal to metal contact when the valve is fully open (BTW I think that this is somewhat related to the famous quarter turn that can kill the diver)



First I will remove, inspect and lube the stem (much easier as it can be done even with the tank full). If that doesn't help I will then have a look at the seat. If all these fail, I will have to try @halocline 's and @Angelo Farina 's suggestion (have the washer on the high pressure side of the stem o-ring). I am a bit skeptic about this, because I don't know if this will have any side effects.

Thanks a lot

I didn't suggest that you swap the plastic spacer and o-ring. I would find a diagram and just assemble it the way the diagram says. It looks in the photo like the spacer is on the low pressure side, not the high pressure side, and that's the way I would assume it goes. Pressure pushes the o-ring into the spacer; the spacer is there to prevent the o-ring from getting extruded. At least that's the way I would guess it's supposed to go.

The reason I asked about the difficulty in turning it without pressure is because if it is hard to turn with an empty tank, it means something just doesn't fit or there is real damage to the threads. It's not unusual for a valve knob to be a little stiffer with a full tank because there is some friction caused by the stem getting pushed on. But that might just be a lubrication issue.

 

[Jack Hammer]

I had a valve that was hard to turn and occasionally creaked. It ended up the teflon washer for the spring was missing. Put a new one in and problem went away.

 

[rjack321]

The said teflon ring is not shown here because it goes inside the bonnet nut. I think its' purpose is to prevent metal to metal contact when the valve is fully open (BTW I think that this is somewhat related to the famous quarter turn that can kill the diver)

start by confirming it is inside the bonnet nut

then:
1) with the bonnet nut outside of the valve just stick the stem through - is it hard to turn at all? If yes replace the stem. If no - now you know the stem/bonnet nut connection isnt the problem

2) is the Teflon washer under the knob intact?

If none of those things the threads on the HP seat carrier need lube

 

[stepfen]

Thanks for all the suggestions, I will consider them all.

Today I breathed the tank down to 30 bars. The knob is still as hard as it was with full tank.

One of this days I will find some time to investigate. I will start with the low pressure side (stem, bonnet nut etc). then move to the seat, threads etc.

I will report back my findings

Thanks for all the suggestions.

 

[rsingler]

So here's a variant on @stepfen 's question. After reading this thread I decided to service my shop valve on my air bench. It's an old homemade bench with a cheap no brand valve. Upon disassembly, there was the usual Teflon spacer but no o-ring! Instead there is a second thin Teflon washer. I couldn't believe it seals! Does that thin Teflon washer act like an O ring? Does it actually extrude enough around the shaft to seal? I couldn't find any schematics that showed a design with 2 plastic washers instead of a washer and an o-ring.
Surprisingly, I had an old kit in my parts box that must have been for a similar valve. It too, had a thick and a thin Teflon washer and no o-ring. I popped that in, lubed it, and it seals just fine! What made the shaft seal?

 

[James79]

So here's a variant on @stepfen 's question. After reading this thread I decided to service my shop valve on my air bench. It's an old homemade bench with a cheap no brand valve. Upon disassembly, there was the usual Teflon spacer but no o-ring! Instead there is a second thin Teflon washer. I couldn't believe it seals! Does that thin Teflon washer act like an O ring? Does it actually extrude enough around the shaft to seal? I couldn't find any schematics that showed a design with 2 plastic washers instead of a washer and an o-ring.
Surprisingly, I had an old kit in my parts box that must have been for a similar valve. It too, had a thick and a thin Teflon washer and no o-ring. I popped that in, lubed it, and it seals just fine! What made the shaft seal?

I know Sherwood used the thick and thin teflon, here is page 9 of their valve manual addressing it (circa mid to late 90s, I believe).
Respectfully,
James

 

[divad]

Some knobs are just harder to manipulate...


My Halcyon and Thermo valve knobs are easy to manipulate (with the Thermo requiring less turns than the Halcyon, but my DiveRite valves are stiffer (maybe to thwart roll-off?).

 

[in too deep]

My OMS manifold’s 3 valves are incredibly tight. Parts have been replaced and lubricated.
Not sure if there is an answer OP.
I gave up on it and changed to another brand which works perfect

 

[stepfen]

This morning I went ahead and disassembled the valve. While I was draining the tank, I went ahead and unscrewed the rubber knob. Once the knob was removed (together with the screw, spring and washer) the stem/seat could be rotated just with my fingers (!). The problem is that by that time the tank was empty too - hence I am not sure what solved the problem. Either there was something wrong with the knob assembly or something was wrong with the internals (eg lack of lube) and even a pressure of 30bars was enough to tighten things that much. Or maybe a combination of the two.

To be sure (I didn't have access to a compressor to try things out on the spot), I totally disassembled the valve, cleaned and inspected everything. I didn't notice anything out of the ordinary - all parts were in good condition. Hence I lubed them (this time I was a bit more generous) and put them back together.

Now (still with the tank empty) the knob turns quite easily - not as nice as my in my SoS valve but still ok.

Real test will be under pressure once I fill the tank in. Let's see.

 

[stepfen]

Today I filled in the tank. The knob feels fine now.
Maybe I was just too stingy with the tribolube during last service.
Thanks a lot for all the suggestions.