Need Help Getting Delrin Poppet to Seal - QB78/79

[sr1sws]

Though mis-adventure (OK, I'm occasionally stupid), my stock QB79 poppet got damaged.  I was able to resurrect it and it's working, but for how long.

At the suggestion of another forum member, I ordered a Delrin poppet from Mountain Air.  Nice looking little part.  His instructions suggested using a drill and fine abrasive (e.g. chrome polish) to seat the two components.

I initially tried Mother's Mag Polish - no joy.  Could see faint ring where the the poppet and valve seat meet.

Tried some valve grinding compound, followed by rubbing compound (car) followed by polishing compound (car) followed by Mother's - no joy.

The configuration currently 'hisses' air when assembled with my Ninja SHP V2 Pro tank charged to 1500psi.

Suggestions, recommendations?  I'm sure this is something I'm not doing correctly - I don't see any issue with the poppet from Mountain Air and this is NOT a complaint about them.

Thanks!
Steve

[Gippeto]

I'm constantly surprised by the amount of trouble folks have getting delrin to seal.

I use a piece of suitably sized rod (delrin because it's cheap) faced off on the lathe, with various grits of peel and stick sand paper attached. A couple licks with this simple tool fixes most valve seats quickly and easily and IMO should be included with a valve stem. If the stem is made with any kind of care, it seals right up.

Could flatten a piece of dowel spinning it in a drill press and pressing lightly against sandpaper the iron table to make the tool

Light pressure, let the paper do the work, focus on keeping the rod square to the seat...should ideally "just" slide into valve body. If the seat is ugly, change the sand paper on the tool frequently. Using 320 grit to start if things are really ugly, 600 if not bad. Finish at 1000grit or finer.

HTH

Al

[Bruce]

The problem with qb valves is the machining is not ideal. The seating surface is not perfectly aligned withe hole the valve head goes through.

To seat the part I use an old valve first with wet dry sandpaper on the valve part that mates with the seating surface. I put the .125" part of the valve in my electric drill. I also use lubricant, oil water whatever you want.. if you use oil you must de-grease the valve before you use it. I use the wet dry till I can see a flat polished surface for the valve to mate to... after this is done I then change to the delrin valve and valve grinding compound the finest grit I have. Between truing up the seat and then mating the delrin popit it normally seats in a few tries.

One thing to think about is cleaning afer the sanding and mating of the surfaces. Any spec of dust will make the valve leak. So make sure everything is really clean well before the valve is put back together and installed.

B

[fivestar45]

Without a lathe, I'd try a variation on Al's idea. Get a pc. of 800 wet/dry paper cut a disk about the size of the stem head with a 1/8" hole punched in the center and stick it to the stem using some rubber type glue that you can remove later. Install the stem in the valve body,  give it a few turns and check the seat for a suitable face.

[rsterne]

Once you have the valve seat squared off and smooth, then you can use a fine abrasive such as Solvol Autosol, or even toothpaste, to seat the poppet to the valve seat.... Put a small amount on the seat, drop in the poppet, chuck the back of the stem in a portable drill, and run it at low speed while pulling gently on the stem.... Reverse the rotation and repeat a few times.... Disassemble and clean thoroughly.... I have never had a valve I couldn't get to seal this way....

One thing, make sure your hammer spring isn't holding the valve open until you get enough pressure in the tube to overcome it.... Cock the gun for the initial fill if necessary....

Bob

[sr1sws]

Great, thanks everyone!

So, it seems the basic problem (in at least some cases) is that the seat and the hole for the stem are not machined square to each other.

I *think* this is probably my root problem, as I noted the Delrin poppet did seem to want to rock somewhat.  I will attempt to square things up.

Thank you all for the advice!

Steve

[sr1sws]

Well, no luck.  For now I've reverted to the original poppet.

Based on some suggestions, I put a small disk of sandpaper on the sealing face of the poppet (grit out).  To check progress, I put magic marker on the brass valve seat, inserted the poppet/sandpaper assembly and rotated in mini-lathe with 'moderate' pressure.  Brass seat looked good with a uniform shiny surface.  Removed sandpaper and repeated with auto polishing compound (white). 

While doing the above, noted that it seems hole for the poppet stem is not concentric with the brass valve body.  Not sure how big a deal this might be.

Tried a "mouth blow test" - blew through valve while pulling on the stem to simulate spring pressure - leaks   I understand that a large part of the sealing force comes from the HPA - but I figure it should pass this simple low pressure test

Recut the sealing face of the delrin poppet and again used the auto polish compound to seat the poppet/valve.  Retried mouth test - leaks

Tried mouth test with original poppet - passes.

Right now I'm back to using the slightly damaged OEM poppet and have given up on the delrin poppet for the time being.

I'm out of ideas - anyone with new/different suggestions?

Steve

[Rocko]

If you can't get yours to work, I have an extra complete valve assembly if you need it.

[sr1sws]

If you can't get yours to work, I have an extra complete valve assembly if you need it.

Sounds good... will send PM.

Steve

[fivestar45]

I could be off on this, BUT.. I'd think that checking the sealing a Delrin stem head would need more pressure than can be generated by blowing into it. I've had examples that wouldn't seal initially with co2 pressures. However if I warmed up the tube to boost the pressure they would seal, and seat and would hold every time
after that initial first seal.

[sr1sws]

I could be off on this, BUT.. I'd think that checking the sealing a Delrin stem head would need more pressure than can be generated by blowing into it. I've had examples that wouldn't seal initially with co2 pressures. However if I warmed up the tube to boost the pressure they would seal, and seat and would hold every time
after that initial first seal.

Easy enough to re-test, just a gun tear down required.  I also read that the delrin seat can be cut at a 3* angle to improve sealing versus "flat".

Steve

[sr1sws]

Success!

I obtained a new valve from from a fellow GTA member and cut the Delrin poppet seat at a shallow angle.  Took the new brass valve seat and reassembled with my cut down valve "shell" and the Delrin poppet.  It's been holding air for a day - much better. 

Maybe tomorrow I'll be able to chrony the new assembly to see if the new poppet made any velocity change - or any other change.

Steve