QB78, how light to go on the striker? How to fix leaky cap?

QB78, how light to go on the striker? How to fix leaky cap?

Joined: December 18th, 2009, 3:35 pm

January 27th, 2011, 6:23 pm #1

I am thinking about lightening the striker on my sons qb78 to make it more efficient. How light should I go? Also planning a HDD.

This is the configuration. I am thinking seriously about doing the barrel porting to breach mod alsohttp://www.network54.com/Forum/113813/m ... ny+results...

I opened the barrel port up a little and checked the rest of the tune out. Turns out it is the cap that is leaking, I fixed the valve, fixed the oring tube leak and fixed the breech seal well there was one more lol. I put a new epdm #113 80 spec on the cap and it slowed the leak alot but not totally fixed it.

I shot this string at about 15-30 seconds between shots basically load aim shoot record number and repeat. Temperature was 70F (got the fire on) I did not shim the hammer at all.

Shoting 14.3 CP (Box)
err
677
675
677
675
673
671
668
665
666
665
err 672
670
667
666
665

So a HDD and lighter hammer.... how will this work?

Bruce
Last edited by bruce.itterman on January 28th, 2011, 12:38 am, edited 1 time in total.
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Joined: January 30th, 2005, 11:00 pm

January 28th, 2011, 2:59 pm #2

Bruce, I think you should replace the O-ring on the cap again. After you remove the sharp edges from the end of the tube, and inside of tube, use some 400 grit wet or dry sandpaper to smooth the inside where the o-ring seats.
I am a big believer in the light striker, it will extend your shots, and make the rifle more efficient. I remove about 1/4 of the striker weight.
Bill...
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Joined: December 18th, 2009, 3:35 pm

January 28th, 2011, 3:44 pm #3

the tube was originally did not look so great (the seam in the pipe was not so smooth also), so honed the tube 320, 400 and 600, cleaned up the little hole (It originality had a burr on it) and cleaned up the end of the tub/ threads with the 600. The only thing I did no clean up was the cap before putting on the new o-ring.

I guess I will do that next and also another 0-ring. I purchased 65 EPDM #113 80 duro o-rings so I have lots of them

Thanks so much on the info on the striker. Any pics?

I was reading some people and is it possible I am screwing the cap on to far before backing it off? Its 1 bubble per sec out were the screw/hole in the outer cap is that I see the leak.

Thanks,
Bruce
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Joined: December 18th, 2009, 3:35 pm

January 29th, 2011, 12:58 pm #4

I think the leak fixed itself. It was still charged last night.

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

January 29th, 2011, 7:07 pm #5

the tube was originally did not look so great (the seam in the pipe was not so smooth also), so honed the tube 320, 400 and 600, cleaned up the little hole (It originality had a burr on it) and cleaned up the end of the tub/ threads with the 600. The only thing I did no clean up was the cap before putting on the new o-ring.

I guess I will do that next and also another 0-ring. I purchased 65 EPDM #113 80 duro o-rings so I have lots of them

Thanks so much on the info on the striker. Any pics?

I was reading some people and is it possible I am screwing the cap on to far before backing it off? Its 1 bubble per sec out were the screw/hole in the outer cap is that I see the leak.

Thanks,
Bruce
valve body, pedestal spring, and delrin head valve. 800-830 fps on co2 with 14.3's.

IMO, if max power isn't your goal, you can't make a modded factory striker too light..lighter is better.

It might be possible to make a plastic or aluminum striker(with steel only where needed) too light to work well, but maybe not.

There's a pic of one of mine just a few posts down.
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Joined: December 18th, 2009, 3:35 pm

January 29th, 2011, 9:13 pm #6

Yeah I was looking ahttp://www.network54.com/Forum/113813/m ... per+70%27s

Thats a very highly modified striker and valve in the pic. You use a stationary sanding belt and disk to make it?

830 FPS is amazing!!!!!!!! but I am happy with about 700-720 FPS and not wasting so much co2. I am basically there now but it wastes co2... A lot of wasted co2 actually.

The qb78d has the following:

-Barrel Ported (could have gone more but did not want to risk a pellet blocking the port) It is not opened up much at the top only lengthened a little in the front and back but it is tapered from the breech port to the barrel nice and smooth and perfectly matched there.
-Bolt has the end drilled to 5/32" also the small hole in the bottom of the bolt was enlarged (I will not prob this gun even though I know probing it would give more FPS and allow me to open the barrel port more towards the from of the gun). I basically just port matched everything to make it flow smoothly.
-Tube was polished to 600 wet dry
-Striker was cleaned up and polished lightly
-1/4 Poly port as breach seal
-Valve work
--ported the exit port on the valve (Could have gone more but better too little then too much)
--tapered the end of the valve to 5/16
--Piercing head cut to match pointed end of valve
--Washer has 4 rounds ground out of it.
--Sharp end of the piercing pin has the corner taken off to allow better flow of co2 into the valve
--Drilled 4 7/64" holes in the end of the valve.
--The filter in the valve is gone.
-Whole gun was lightly lubed and then wiped off so most the oil is gone but hopefully enough to keep from wearing too much yet not get too much dirt.
-I have not touched the trigger yet but it is surprisingly good.. about 2 1/2 lbs maybe a bit lower.

My brother-in-law wants help to mod his, I will most likely help him. He might go for the probe and washers to give more wallup. We are both looking at a HDD very seriously and I want to lighten the striker as much as I can without loosing velocity. Finding a happy medium is important to me and give me something to play with in the winter months when I can't shoot due to the -22 F and colder cold spells we get.

Bruce
Last edited by bruce.itterman on January 29th, 2011, 9:17 pm, edited 1 time in total.
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Joined: March 28th, 2002, 6:54 pm

January 30th, 2011, 3:02 am #7

to rough it in, then the disc to clean up. The curved cuts were done with a carbide cutter in the dremmel ,then finished with a sanding drum, also in the dremmel. The only cut done in the lath was the cup in the back.
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Joined: December 18th, 2009, 3:35 pm

January 30th, 2011, 11:31 pm #8

I think the leak fixed itself. It was still charged last night.

Bruce
Still holding!!! nt
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Joined: December 18th, 2009, 3:35 pm

February 1st, 2011, 2:01 am #9

Still Holding tonight!!! nt
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Joined: December 18th, 2009, 3:35 pm

February 2nd, 2011, 1:25 pm #10

Still Holding last night nt...
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