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Delrin QB 78 valve stem question

April 13 2017 at 5:41 PM
Sophie948  (Login Sophie948)
AR&P

I bought a Delrin valve stem to put on my QB 78, bought Delrin because I've given to understand that it's easier to seat than Teflon. Stock valve stem works fine, I'm just into incessant tinkering.

So how do you seat a Delrin valve stem?

I was thinking probably put a bit of fine automotive valve grinding compound in the valve body where the stem seats, a bit on the Delrin where it hits the seat, then insert the stem and just kinda twirl the stem with my fingers while pulling on it to give a bit of pressure.

So how do you do this if you - unlike me - know what you're doing?


    
This message has been edited by Sophie948 on Apr 13, 2017 5:41 PM


 
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B3ntong
(Login Oragon2)
YF

Delrin valve stem

April 13 2017, 10:02 PM 

Got mine from MA and was told to check the seat of the valve make sure no nicks and uniform all around. Apply mild grinding compound then chuck the stem to a drill then slowly turn it while pulling on it (valve out the air tube of course) turn it until you get a uniform in print of the valve seat. I had to turn mine 2x to get it to seal forever.

 
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Sophie948
(Login Sophie948)
AR&P

Thanks! (n/t)

April 14 2017, 12:04 PM 


 
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Scot Laughlin
(Login classicalgas)
YF

I generaly use a piloted end mill ("counterbore cutter") to true and smooth the valve seat

April 14 2017, 8:03 PM 

in the valve body, then undercut the valve head in the seat area at about 3 degrees, to make a lip that conforms a little. I'll trim the valve head OD at the seat until there's very little overlap /overhang past the port edge(.020" or so) which makes the valve open easier, due to the smaller seal area.

Without that specialized tooling, the lapping process is your best bet, just remember that the compound will embed in the softer material, and cut the harder, so if the valve head seal area isn't square to the stem, you are screwed, and if the seat in the valve body is way out of square, you'll spend a lot of time lapping.

 
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Jason
(Login nervoustrigger)
YF12

Poppet material

April 15 2017, 8:35 AM 

I just wanted to clarify Delrin is harder to get to seal than Teflon (PTFE).

Amongst common seal materials, easiest to hardest...or in terms of most suited to low pressure or high pressure:

polyurethane
PTFE
Delrin
PEEK

 
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Sophie948
(Login Sophie948)
AR&P

Interesting stuff, folks...

April 15 2017, 12:06 PM 

Thanks for the info. (And yes, I'm a DIY guy so I don't have access to any specialized tools)

Anyone know what the stock popett material is in a QB?

 
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Jason
(Login nervoustrigger)
YF12

The stock QB poppet...

April 15 2017, 12:49 PM 

The seal is a polyurethane tube bedded between the hardened steel stem and the brass body. It's a pretty good, durable design and even has withstood long-term use in HPA conversions running ~1500psi.

 
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Scot Laughlin
(Login classicalgas)
YF

The factory poly seal is also quite forgiving of seats that are

April 15 2017, 1:09 PM 

not square to the stem or slightly damaged, unlike harder materials.

The downsides are... they used to fail early at anything over 1500 psi or so, and they don't open as easily as harder seals, so power is limited unless you stuff a stupid spring in there.

I think QC and materials in the QB's have improved a lot in the last decade, so you probably wont have trouble lapping in a hard seal, and you aren't likely to have troubles with the factory stem.

 
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John in FL.
(Login gratewhitehuntr)
AR&P

This thread is useful. Funny that Sophie defines "DIY" as a guy with no tools. nt

April 27 2017, 11:38 AM 


 
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Sophie948
(Login Sophie948)
AR&P

Hey, I'm a DIY guy with tools...

May 8 2017, 1:59 PM 

It's just that the one I use the most is a very large claw hammer. I just don't have specialized tools - like a ball-peen hammer.


    
This message has been edited by Sophie948 on May 8, 2017 2:01 PM


 
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