Delrin QB 78 valve stem question

Delrin QB 78 valve stem question

Joined: May 22nd, 2016, 9:38 pm

April 13th, 2017, 9:41 pm #1

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?
Last edited by Sophie948 on April 13th, 2017, 9:41 pm, edited 1 time in total.
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Joined: February 25th, 2009, 1:05 pm

April 14th, 2017, 2:02 am #2

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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Joined: May 22nd, 2016, 9:38 pm

April 14th, 2017, 4:04 pm #3

Thanks! (n/t)
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Joined: March 28th, 2002, 6:54 pm

April 15th, 2017, 12:03 am #4

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?
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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Joined: June 4th, 2013, 4:42 am

April 15th, 2017, 12:35 pm #5

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?
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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Joined: May 22nd, 2016, 9:38 pm

April 15th, 2017, 4:06 pm #6

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?
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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Joined: June 4th, 2013, 4:42 am

April 15th, 2017, 4:49 pm #7

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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Joined: March 28th, 2002, 6:54 pm

April 15th, 2017, 5:09 pm #8

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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Joined: February 1st, 2014, 12:51 pm

April 27th, 2017, 3:38 pm #9

This thread is useful. Funny that Sophie defines "DIY" as a guy with no tools. nt
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Joined: May 22nd, 2016, 9:38 pm

May 8th, 2017, 5:59 pm #10

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.
Last edited by Sophie948 on May 8th, 2017, 6:01 pm, edited 1 time in total.
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