PCP Valve Pin / Seat Design

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mtnGhost

mtnGhost

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Dec 15, 2018
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Defected from the PNW, United States
I was going to post this in another thread, but wanted to get more eyeballs on this for broader feedback. I need to make some valve pins / seats on my lathe. Having never done this before (and not having any old valves that I can dismantle / reverse engineer of this type), this is the idea that I have. For those that have successfully made valve pins before, does this barbed design look plausible?

Notes -

  • the barbs would actually be smaller (none of this is to scale nor proportionate)
  • the composite / polymer seat would be press fit in place
  • I might add a brass collar pressed in from the left side to hold it in place further 

1578350852_14830645835e13b904cd0778.56745409_8545500B-1109-4ABC-8EF9-15847C65ADA6.jpeg

thanks 🙏 
 
I don’t use a sealant as the thread doesn’t protrude all the way through so there is no need, if however the taipan has the thread going through then I would use a silicone sealer and a threaded nut ether side of the delrin.



I use 3mm silver steel rather than ti,. After threading, I heat it until straw colour then quench quickly in saline solution to harden it, Ti is soft and bends easily.... doesn’t thread well with a M3 die box, expensive too.



Bb
 
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It really depends on how large of a poppet you need to make. For .350 throats and less I've been making peek poppets with 2mm stems made from drill rod. I have some of these in service for several years and many thousands of rounds. No sealant is used, no threading nor barbs, etc. A very tight interference fit is all. I drill a 1/16" hole in the 3/8 peek poppet then slightly round the 2mm drill rod's tip and drive it into the 1/16 hole until bottomed out. Once this is done I used the 2mm stem as an arbor and complete my "machining" in a dremel.

I'm not saying this is the best method for all applications, but it has worked well in the half dozen applications I've used it in. They range all the way up to literally thousands of shots in an 85fpe .30 Flex.
 
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It really depends on how large of a poppet you need to make. For .350 throats and less I've been making peek poppets with 2mm stems made from drill rod. I have some of these in service for several years and many thousands of rounds. No sealant is used, no threading nor barbs, etc. A very tight interference fit is all. I drill a 1/16" hole in the 3/8 peek poppet then slightly round the 2mm drill rod's tip and drive it into the 1/16 hole until bottomed out. Once this is done I used the 2mm stem as an arbor and complete my "machining" in a dremel.

I'm not saying this is the best method for all applications, but it has worked well in the half dozen applications I've used it in. They range all the way up to literally thousands of shots in an 85fpe .30 Flex.
Click to expand...

Sounds practical and works well. I like that combo. 
 
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Been making Custom poppets, seats & generally been modifying valves for years !!

Typically .125" ground shafting pressed into a PEEK head, or Acetal works just fine. No barbs or other tricks required, Loctite 640/680 an option tho not required.

Hole drilled at .120" so the shafts a press fit in until it bottoms out. * Done in a mill vise.
 
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Looking at your drawing again ... the barbs are facing the wrong direction. I've made a number of them over the years and had no failures without the barbs... or threads. I've used Superglue a couple of times but no failures without it.

A couple of things that I've seen. One, if you undercut (think umbrella) the head slightly , it will seal easier. Two, I made a valve for my Steyr that was an inverted cone, sealing on the angled part of the head, rather than the flat part. It created its own sealing flange that was very minimal after just a couple of firing cycles but has never leaked and gave me a 100 fps jump with no other changes. I used the same design in a Walther LPM with similar effect. I've not tried it with any high power rifle though.

I've made all my poppets from Delrin in the past but recently purchased some PEEK, as I've seen good references for it. Seems like it might be tougher and longer aging.

Bob


 
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Been making Custom poppets, seats & generally been modifying valves for years !!

Typically .125" ground shafting pressed into a PEEK head, or Acetal works just fine. No barbs or other tricks required, Loctite 640/680 an option tho not required.

Hole drilled at .120" so the shafts a press fit in until it bottoms out. * Done in a mill vise.
Click to expand...

Perfect! I have a tendency to over analyze points of failure and resilience, so bare please with me here lol .. When I start working through the physics involved between the pin and sealing head - the bond would undergo stress from the driving forces from the hammer striking the pin and the collision of the head with the transfer port material as the valve shuts closed (primarily driven by the forces of the valve spring). 

In any case, I'm starting to gravitate more towards an all press fit design. I think the head material would have better integrity without being gouged up, although using some 680 would be a wonderful choice of a bonding agent. I love that stuff! It hadn't crossed my mind to use it (or 640) in this sort of application, so thank you! 😎

The other thing that I'm overthinking: what are the preferred characteristics of the material used for the sealing surface? The POM-C sealing heads on my valves appear to deform waaaay too quickly. PEEK seems much better (kind of a pain to turn on my lathe). POM-H - I want to test some and see how it holds up over time. That being said, I have a few different varieties stockpiled. Some is waxy, some real slick, and one is was harder than the rest in the materials specifications sheet. I'm thinking the harder black one would be the more appropriate material to use out of the POM-H:

1578427531_4842111835e14e48b4f8ca3.03003925_616D7F21-C595-4478-8440-9914FA2B7E09.jpeg
1578427532_13674437145e14e48c123480.58598847_FAEB1854-DF1E-40E3-A1AA-5C7FAE44C540.jpeg
 

Hard to capture the scratch and impact marks in the photos, but thr taupe colored material is softer and more waxy. I suppose my question then becomes (in simple form): is it better to use a harder or softer material for the sealing head in a PCP valve system, where the regulated pressure exceeds 150bar (full time, compare with say - a 12FPE rig)? Would I be correct in choosing the harder material, given the concerns for deformation?

I appreciate the replies and I really enjoy this sort of discussion 😎
 
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Harder material is going to be easier to knock open. that is assuming the hard material has the right mix of properties. I went straight to peek for 2 reasons--I saw most of the more experienced tuners referencing it and 2) it was readily available in the US via McmasterCarr, Amazon, etc. Posters in other countries have expressed challenges in procuring peek in some countries. 

I'm not a machinist nor do I have machinist equipment. But after a few failed attempts I found a series of operations that allow me to make up a custom peek poppet in about an hour. I "machine" peek using my dremel and files. I run it at max speed with a light touch. Coarse material removal is done with a hack saw blade. A good sharp file cleanly cuts the peek, leaving next to no fuzz. Once shaped, I finish with fine wet/dry sand paper. Final finish on the sealing surface is with 1200. A light touch and high RPM are the key to success in this operation. Assuming the valve seat isn't damaged I rarely have challenges getting it to stay sealed down to 800-1000psi....If the valve seat is damaged I just tape a piece of 600 or so grit wet/dry paper to the poppet, insert in the valve, chuck it in a drill, and with light pressure refinish the seat. The point I'm trying to make is I haven't found peek to be as intimidating/challenging as I expected it to be



I just realized something I haven't previously thought of---I typically charge from a tank so I've never had to hand pump against a poppet leaking at low pressure. My tunes are all 100 bar or greater so I don't have any low pressure experience---the poppets I make using these operations may not work for sub 1kpsi applications....
 
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