QB78/79 valve stem
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QB78/79 valve stem
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Topic: QB78/79 valve stem (Read 1384 times))
GuyFromTheWoods
Shooter
Posts: 13
yes
Real Name: Derek
QB78/79 valve stem
«
on:
July 21, 2015, 12:11:27 AM »
The valve stem on my qb went out due to seal failure. I tried replacingit but that didn't work out. I ended up making a teflon valve stem and lapped both the valve and valve stem faces. It seals very well. While making it, however, I realized I had lost the old one so I couldn't take measurements on length. I made it into a ball park range, but I am shooting on average 625 instead of 690 like I was before. No matter how much I turn up my power screw, it won't go any faster.
The OEM stem allowed me to get to past 700, but it started gulping co2 at that point. It has to be something with the stem. I did some work to my transfer port, but it made it less restrictive so I should have a spreed increase. Any body have the spec on the oem stem? Maybe I'm to short.
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Macon, MO
HYspd
Expert
Posts: 1463
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Real Name: Chuck, also answers to Grumpy and Hey YOU!
Re: QB78/79 valve stem
«
Reply #1 on:
July 21, 2015, 02:02:46 AM »
Teflon is too squishy it will take a lot of the hammer "drive" to get the valve to open..
you're probably .080-.100 short on the stem..
search for a thread by rsterne titled $200 pcp there's a ton of info on the QB series valve in that thread
ok search didn't even work for me so I went digging
http://www.gatewaytoairguns.org/GTA/index.php?topic=49943.0
«
Last Edit: July 21, 2015, 02:08:57 AM by HYspd
»
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Stuarts Draft VA
The Black Knight....a severely recrowned, chamfered, lapped deburred, honed, Krytox lubed and resealed Titan GP NP .22 with brass washers, a GRT-III trigger, a CP 3-9X40 and a BNM LCD soft bedded into a Fury II stock that is stuffed with 1¼ pounds of duct seal making for a 9 pound, solid shooting, 19fpe (with Crosman Destroyers of all things, at 770fps) 30 yard bottle cap assassination machine...
MODDER FOREVER FOREVER MODDING
fivestar45
Sharp Shooter
Posts: 607
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Re: QB78/79 valve stem
«
Reply #2 on:
July 21, 2015, 05:43:56 PM »
I'd think that your stem guestimation is probably a little bit on the long side, thus shortening the striker stroke a bit. I just took a measurement from a new QB78 valve stem..from the seal under the head to the end of the stem it was 1.440".
HtH
Anthony
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GuyFromTheWoods
Shooter
Posts: 13
yes
Real Name: Derek
Re: QB78/79 valve stem
«
Reply #3 on:
July 21, 2015, 10:18:35 PM »
Alright. Thank you very much for the dimensions and source.
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Macon, MO
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QB78/79 valve stem