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Designed New Bolt For Crosman 760
https://www.airgunforum.ca/forums/topic89310.html
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Author:  MyCrosman [ Mon Aug 23, 2021 10:47 pm ]
Post subject:  Designed New Bolt For Crosman 760

The stock bolt probe of the 760 is all plastic...with no o-ring to provide a good seal....just hard plastic pressing against the barrel. I sawed off the plastic bolt at the base where it joins the block and drilled a hole in the block to fit the new steel probe....which is actually a nail of the same diameter as the original bolt....The nose has been extended to push the pellet skirt to very edge of the port for a clear path of air....The bolt was fashioned so when the probe is fully closed and locked in...the seal is forced to expand slightly to form a very tight seal in the barrel. I did this be making the bolt with a very slightly wider circumference at the rear of the seal. Works very well as you can see by the numbers below, No valve work (except for plugging the hole) or porting has been done. It now shoots fast enough...reworked bolt probe added 40fps over just plugging the valve hole.

Stock numbers .............. 7.48 gr. wadcutter
3 pumps 262.8fps....witn new probe...422.6
5 pumps 327.4fps.................................519.2
10 pump 414.8fps.................................621.8

My gun now shoots with 3 pumps the same velocity of a stock 1377 with 10 pumps

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Author:  notec [ Mon Aug 23, 2021 10:51 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

Excellent work!!

Author:  MyCrosman [ Tue Aug 24, 2021 9:24 am ]
Post subject:  Re: Designed New Bolt For Crosman 760

notec wrote:
Excellent work!!


Thank You..it was harder than I thought to get the right angles on the bolt to mount the o-ring....And also how far up or down the bolt to place the o-ring......that was my 3rd attempt..3 hours of sizing..cutting..grinding and polishing to get the perfect fit.

This guy from the U.S. gets 577 fps with ten pumps AND he's using a 6.64 grain pellet so it looks like the new bolt on its own actually accounts for a lot more than 40fps increase...Lot of work but it was worth it..
https://youtu.be/64_OisMzrEY

Author:  jckstrthmghty [ Tue Aug 24, 2021 9:37 am ]
Post subject:  Re: Designed New Bolt For Crosman 760

Nice work. A retail part like this would probably be equivienent in price to the entire cost of the 760.

Author:  killercrow [ Tue Aug 24, 2021 1:34 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

Nice custom work.
What did you use to cut the groove and profile?

Author:  MyCrosman [ Tue Aug 24, 2021 4:01 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

killercrow wrote:
Nice custom work.
What did you use to cut the groove and profile?


For the short simple answer I used a power drill in a holder...I think they call it a poor mans lathe..a flat file....very small square file...a small triangle file and a small half moon file plus various grits of sandpaper and a micrometer to occasionally check that metal was actually being shaped/removed during the sandpaper phase.

Author:  Dukemeister [ Tue Aug 24, 2021 5:34 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

MyCrosman wrote:
For the short simple answer I used a power drill in a holder...I think they call it a poor mans lathe.

That there shows awesome ingenuity and skill! I have used my drill press occasionally as a poor man's lathe as well, but mainly for woodworking, although I have done a few jobs on brass and aluminum. Well done on this one, :drinkers:

Author:  notec [ Tue Aug 24, 2021 5:37 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

MyCrosman wrote:
killercrow wrote:
Nice custom work.
What did you use to cut the groove and profile?


For the short simple answer I used a power drill in a holder...I think they call it a poor mans lathe..a flat file....very small square file...a small triangle file and a small half moon file plus various grits of sandpaper and a micrometer to occasionally check that metal was actually being shaped/removed during the sandpaper phase.



Hey a drill press and a 4 way vise and you can do quite a bit of modding. Good on you for getting creative!!!

Author:  jckstrthmghty [ Tue Aug 24, 2021 8:09 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

notec wrote:
Hey a drill press and a 4 way vise and you can do quite a bit of modding. Good on you for getting creative!!!


That's essentially what a mill is, with better axial bearings of course.

Author:  notec [ Tue Aug 24, 2021 8:10 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

jckstrthmghty wrote:
notec wrote:
Hey a drill press and a 4 way vise and you can do quite a bit of modding. Good on you for getting creative!!!


That's essentially what a mill is, with better axial bearings of course.

That and the chuck wont drop out of a mill :wink:

Author:  killercrow [ Tue Aug 24, 2021 10:35 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

Right on! I got by doing things with a drill press and files for years.
Got me started anyway. :wink:

Author:  ricksplace [ Wed Aug 25, 2021 1:40 pm ]
Post subject:  Re: Designed New Bolt For Crosman 760

I started with a table top drill press and a cross-sliding vise (over 30 years ago). Now I have two lathes, a milling attachment and a large floor stand drill press. It's worse than my airgun addiction. :)

Author:  MyCrosman [ Thu Aug 26, 2021 10:08 am ]
Post subject:  Re: Designed New Bolt For Crosman 760

Another good thing about the mods was that I only have to pump it twice for a 358fps ...7.48gr. wadcutter....great for indoor ranges to 30 feet..I think my 1377 is going to be gathering a bit of dust for awhile :D

Author:  jckstrthmghty [ Thu Aug 26, 2021 10:58 am ]
Post subject:  Re: Designed New Bolt For Crosman 760

Pump twice? Ug... yeah I know. I'm so lazy.

Seriously though, great work.

Author:  notec [ Thu Aug 26, 2021 11:02 am ]
Post subject:  Re: Designed New Bolt For Crosman 760

Well im sure this post will inspire our other members. Thats what it is all about. :mrgreen:

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