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 Post subject: Hammer bounce qb78d
PostPosted: Wed Sep 14, 2016 1:29 pm 
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Joined: Sat Dec 27, 2014 12:27 am
Posts: 10
I know I have asked this before but I will ask it again. How do you stop the hammer from bouncing back and hitting the valve releasing CO2 after the shot is taken. I know there are some HDD's out there from Steve Archer (who won't ship to Canada) and some home copies that have been made but do they really work , do they last or will they fail after a while ? From the what I have heard these are band aids that don't last long. I have been thinking about a heavier hammer spring and a lighter hammer ( I do have my pal) to reduce the bounce and maybe a heavier valve spring. I am not looking to increase the power just to reduce the wastage of CO2 and improve accuracy. Have any of you tried something like this? I have done most of the other mods to increase gas flow opened the valve and removed the screen and the filter both where damaged. So any thoughts on this would be great. Thanks


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 Post subject: Re: Hammer bounce qb78d
PostPosted: Wed Sep 14, 2016 2:13 pm 
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Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
I made one of the little steel double-wedge horseshoe type things for mine, and improved efficiency a bit. No more burp on firing. But it gradually became less effective if not entirely wearing out. Steel-on-steel when no lubrication is present is a bit predictable for eventually wearing out. Lube would have prevented it from working. So I'm using an SSG now and am very happy with the efficiency. My QB78D is an HPA conversion, but an SSG will help just as well with CO2. Since I like it as stock with cocking on closing the bolt my SSG had to be entirely internal. That meant a fair bit of trial and error before getting the rod length right then spring length and strength. Lightening the hammer a lot helped to further refine the efficiency at my desired power level of 20fpe. If you're running yours at stock power then I'd recommend an ultralight hammer, such as delrin perhaps, with a hardened striker bolt threaded in to hit the valve stem.


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 Post subject: Re: Hammer bounce qb78d
PostPosted: Wed Sep 14, 2016 3:29 pm 
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Joined: Sat Dec 27, 2014 12:27 am
Posts: 10
Thanks for the info Gerard have to look around for the delrin. I have heard of Delrin before but really don't know what it is or where to find it. My qb is not pal rated but I thought that a heaver hammer spring would compress less and reduce the amount of bounce on the lighter hammer sort of like a shock absorber effect and if needed put a shim under the spring in the valve if there still a bit of a bounce from the hammer. This Hammer bounce became a noticeable problem after resealing and opening up the path of gas flow by enlarging the holes to probe and the holes in the probe itself almost forgot I did recrown the barrel. So this maybe the last mod on this rifle. As it is now it's accuracy is a lot better and louder so I hope by trying this it will get quieter and reduce the gas it uses. Still getting 30 shots or so from the CO2.


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 Post subject: Re: Hammer bounce qb78d
PostPosted: Wed Sep 14, 2016 3:47 pm 
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Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
There's a custom builder on GTA ('Shorty' I think?) who persists with short, hard springs as his solution to the hammer bounce problem, and apparently a couple of his airguns are showing efficiency on par with well tuned SSG setups. In my initial work trying to get a 22xx carbine working well with 3,000psi from a HiPAC I found that a short, very hard spring was working well in concert with a lightened hammer to reduce bounce, gaining me a few shots per fill while also enabling opening the valve at the full 3,000psi pressure. It was very hard on the dainty little Crosman cocking knobs, so I had eventually to drill out the steel breech and make a much bigger custom steel bolt with a heavy duty bolt to pull on. No trouble after that (and it's now back to a much weaker spring with an SSG).

But with the hard spring I had to back off my RVA after every shot or two maintain a relatively stable shot string, otherwise velocity would rise dramatically (100fps and more) by 2,000psi and waste a LOT more air by then. Not useful at all. So I tested a lot with the Chrony F1 and established patterns of RVA adjustment, even used one of those pocket clickers to keep track of my shot count and match the RVA position to that schedule. Annoying. So I bought a Robert Lane regulator instead and custom fitted it inside the HiPAC, reducing the regulator outside diameter a little. Problem resolved, output pressure always the same between 3,000psi and about 850psi, so I could just focus on tuning the hammer and spring. Then I went to an SSG on that one when I passed it along to my son and the thing is a tack driver, delivering very consistent velocities.

Delrin (trade name, actually known generically as 'acetal' rod) is available from Sabic by phone order:
http://www.sabicpolymershapes.com/polys ... blend.html
That's one blend, they have a few I think. I just ordered black delrin rod (as I also use it for externally showing parts such as pipe stock adapters and muzzle brakes) and it works great for machining all sorts of things. You may be able to order the exact diameter to fit as a hammer, but you'd have to be sure of the exact measurement and I think it comes in 1/8" increments. Of course it wouldn't work as a striker itself, far too fragile. And you'd need to make a steel insert for the sear to engage as that would not work directly on delrin either, at least not for very long I think. It is a very easy material to machine, just need to keep the dimensions fairly thick as in thin sections it becomes very brittle.


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