Crosman 760 Mod



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Offline nekksys

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Crosman 760 Mod
« on: October 21, 2013, 01:38:01 PM »
I'm creating this thread in case anyone new to the forums here wants to follow along as I mods my classic Crosman 760 Pumpmaster. As this project progresses, look for photos.

Last week, I ordered from Alliance Hobby a Benjamin Discovery .177cal 24inch barrel since it is the correct diameter to fit directly into the 760s barrel band and receiver. I also ordered a replacement 760 valve kit. The barrel and valve arrived today.

I'm having the end of the barrel turned down to match the original smooth bore barrel. This should remove most of the counter-bore on the transfer port hole. The barrel has the recessed transfer port typical of the Disco. I'll enlarge the transfer port later. Due to the Disco barrel being rifled and the difference in barrel lengths (second photo below), I expect an extremely significant gain in accuracy.

The replacement valve kit, which included a new pump cup, has proven to have some pleasant surprises. First off, the release valve plunger is Delrin and has no seals. (Fourth photo down, on the right.) This is precisely what I wanted to do with the original valve so I'm now money and time ahead. Second, there is no spacer in this valve which some models used to reduce valve volume. This valve has only  the Delrin check valve. (Same photo, white object on the left.) This valve appears to have an increase in overall volume. This will require a little more pumping to completely fill this volume however I don't expect this valve to fail under pressure. I may swap the new aluminum valve body for the original brass body. Jury is still out on that change... The new aluminum body will have to have the transfer port enlarged to match the barrel transfer port. Lastly, the kit included the transfer port seal which should improve air release since I believe my current port has a slight leak.

Keep watching... I'll post more photos after I get the barrel turned and blued and as I get everything else installed.
« Last Edit: October 21, 2013, 01:47:18 PM by nekksys »
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline Rex

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Re: Crosman 760 Mod
« Reply #1 on: October 21, 2013, 02:19:09 PM »
Looks fascinating.
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Offline ezman604

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Re: Crosman 760 Mod
« Reply #2 on: October 21, 2013, 02:25:56 PM »
Good deal. Bookmarking this one. Will watch and enjoy. Can't wait to read more progress and test firing results!!!
Thanks for sharing.
SAFE & Happy Shooting!!!!
Dave
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Revelation 760 PumpMaster (Vintage 1967)
Powerline 1000S .177 (semi-tuned by me & innards by kiwi Pete)
Benjamin Super Streak .177/.22 (converted to NP & w/GRT-III)
Beeman HW97K .177
Benjamin Trail NP XL1100 (home tuned by muwah with Tuna trigger)
Stoeger X-20 RAMmed by Gene and Tuna trigger
Winchester 1000SX .177 (semi-tuned by muwah)
Winchester 1000B RAMmed by Gene
Crosman 1377 "Goldfinger"
Disco .22 Multi-Shot Custom

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Offline nekksys

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Re: Crosman 760 Mod
« Reply #3 on: October 21, 2013, 02:37:33 PM »
One thing I forgot to include: Goal specs for this mod...

Goal FPS: 1000+ (May not be possible but I am gonna try to make it happen.)
Goal FPE: 10+ (Again, may not be possible. May have use a heavy pellet and sacrifice FPS.)
Goal Powerplant: Multi-pump or PCP (I haven't really settled the internal debate on which would be better. Maybe I'll make it field convertible...)
Goal Stock: All-wood, thumb hole. (If anyone wants or has someone who will make one, just let me know and how much you're asking for it.)
Goal Optics: 4-16x44mm w/ Mil-Dot
Goal Trigger pull: 2lbs max. (This will be a challenge since I will probably have to shave the sear to reduce engagement and polish the stuffings out of it.)
Goal Accessories: Green under-barrel laser sight, muzzle brake. (I may even go as far as modding the receiver to include a barrel "shroud." It won't be a true shroud but will add weight which will help to hold on target.)
« Last Edit: October 23, 2013, 04:15:24 PM by nekksys »
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline ezman604

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Re: Crosman 760 Mod
« Reply #4 on: October 21, 2013, 02:46:17 PM »
Hmmm...okay. May evolve into a project other than what I was hoping for. Since my first airgun was (and still is) a Revelation 760 Pumpmaster circa 1967, I was looking forward to seeing the mods for a pumper. Oh well, will continue to watch your project develop.
:)
Revelation 760 PumpMaster (Vintage 1967)
Powerline 1000S .177 (semi-tuned by me & innards by kiwi Pete)
Benjamin Super Streak .177/.22 (converted to NP & w/GRT-III)
Beeman HW97K .177
Benjamin Trail NP XL1100 (home tuned by muwah with Tuna trigger)
Stoeger X-20 RAMmed by Gene and Tuna trigger
Winchester 1000SX .177 (semi-tuned by muwah)
Winchester 1000B RAMmed by Gene
Crosman 1377 "Goldfinger"
Disco .22 Multi-Shot Custom

Ain't this a great place to be

Offline nekksys

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Re: Crosman 760 Mod
« Reply #5 on: October 21, 2013, 02:59:10 PM »
May evolve into a project other than what I was hoping for.

I'm not completely sold on turning this wonderful little ANTIQUE into a PCP type. That kind of mod has it's advantages and disadvantages. The primary disadvantage is the risk of overpressure and detonation. If I can figure out how to make a simple, quick PCP changeover module, I'll make one. That kind of module will have to include a valve which opens only when in contact with the existing release valve. Any other kind of valve and you end up with a small, air-powered rocket in your pack. The module will also have to maintain pressure when stored so several can be carried at one time. (I know... I have a habit of setting the bar pretty high...) And... This module can NOT place undue pressure on the pump tube.

As you can see, staying with the multi-pump powerplant will probably be the way for me to go. For me, I really want this thing to turn into a good, fast shooting hunting rifle for squirrels, rabbits and such. I also wants good looks and some extra weight to keep the muzzle on target.
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline nekksys

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Re: Crosman 760 Mod
« Reply #6 on: October 21, 2013, 10:38:25 PM »
Managed to get the barrel turned down. If anyone else does this mod on a similar vintage 760, you will have to shave approx. .030in. - .045in. from the breech end of the barrel to get it to align properly with the transfer port. The existing transfer port in the Disco barrel is almost the same size as the drill (9/64") I used to enlarge the original release valve's transfer port. I only needed run the same drill into the port to clean up the fit. I used the butt end of the drill as a drift to hold the ports in alignment when I tightened the set screw. (In the second photo down, you'll see there is no gap between the breech block and the barrel.) If you do not shave the breech end, the bolt will leak! I don't have a photo to show the proper headspacing. However if you test fit the bolt into the barrel while holding both parts in a receiver half, you can look into the transfer port. You should see a slight portion of the plastic bolt IF you chose to enlarge the transfer port. If not, you may need to shine a light down the barrel to see the face the magnet stick out of. For the enlarged port, if you do not see the plastic, the bolt doesn't have sufficient depth to seal properly. I may have a new brass bolt made to improve the seal.

The final photo shows the whole thing reassembled. I did NOT enlarge the transfer port on the new release valve. After I get the first 500 rounds down the pipe, I'll enlarge the valve transfer port and see what kind of increase (or decrease) I get. After seeing my smoothbore barrel on this beauty for so many years, this barrel looks ridiculously long. Pretty sure I'll get used to it. The scope is a Tasco 3-7x20 I picked up at Walmart. Cheap scope, sits well, looks OK and holds settings fairly well.

First test shot after assembly was done at 15 pumps at a target 30yds away. I didn't feel any blowback and it sounded like there was still room for another 5 pumps in that valve. Barrel seemed to fall in line with scope as it had been set with the smoothbore barrel: nailed the 8pt ring on a 3" Shoot-N-C target using a Gamo Magnum 7.87gr. pointed pellet. No chrono readings as yet. Look for those to follow.
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline rangerfredbob

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Re: Crosman 760 Mod
« Reply #7 on: October 22, 2013, 12:44:35 AM »
To help things look a little more in proportion, go HERE, find the part number for the tube and pump linkage and call the phone number between 8am and 4:30pm EST with a credit card handy, Crosman charges very little for their parts and only $4 shipping on parts orders... I imagine those two parts will be ~$20 shipped, but you could order as many parts as you want for that same $4 shipping. Anyway, the Powermaster 66 tube is a little longer than a 760 tube, and with the linkage you will get a little more stroke, top that off with a flat top piston and valve and you have a nice little combo... I converted my Recruit (basically a powermaster 66 with a fancier stock and smooth bore) to have a rifled barrel, also put a flat top piston and valve together for it and it shoots very nicely and hard (no chrony, I'll try to round one up sometime) and holds ~11 pumps before holding air.

Lightening the trigger pull is easy, and I believe my Recruit has under 2lb pull with just bending the spring a little, in the trigger group there is the torsion spring all the way back against the metal thing that keeps you from pulling the trigger with the bolt back, bend that a little bit with pliers until you have a better trigger pull.

Pellets don't like to travel at over ~850fps before they become unstable..
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Offline Kraaash

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Re: Crosman 760 Mod
« Reply #8 on: October 22, 2013, 01:45:20 AM »
lets see some groups =)

Offline nekksys

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Re: Crosman 760 Mod
« Reply #9 on: October 22, 2013, 09:39:01 AM »
Anyway, the Powermaster 66 tube is a little longer than a 760 tube, and with the linkage you will get a little more stroke

I'm actually thinking of using the 2100 tube and linkage. Same diameter tube but longer than the 66's. Working on figuring out an adapter to push the 2100 tube toward the muzzle so the transfer port and everything else lines up right.

Pellets don't like to travel at over ~850fps before they become unstable..

I know a pellet loses velocity more rapidly while it's supersonic and the supersonic-subsonic transition can cause them to tumble. This isn't a consideration for short range shots of <30yds. Longer ranges will need a heavier pellet so they stay subsonic throughout flight.
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline nekksys

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Re: Crosman 760 Mod
« Reply #10 on: October 22, 2013, 10:59:08 AM »
After the following bit of info from AirgunAndy,

The tube for the Powermaster 66 (and the Recruit for that matter) is identical to the 760 tube in all ways except length ( about 1.5 inches). [...] It's a direct bolt-in replacement.

I've decided to go ahead with the PM66 pump tube, linkage and fore grip mod. In fact, I'll probably go with the Recruit stock as well since it's also a direct bolt-in replacement.
« Last Edit: October 22, 2013, 11:19:15 AM by nekksys »
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline rangerfredbob

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Re: Crosman 760 Mod
« Reply #11 on: October 22, 2013, 04:48:15 PM »
The recruit stock is pretty nice, the rear is kinda wobbly but not bad, on my reworked Recruit I can hit my 1.5" spinner every shot offhand at 35 yards as long as I don't pull one direction or the other... Here's my Recruit, they have a 20" barrel so yours will be about 4" longer than mine...

  • Oregon, Salem
Two Beeman RS2's one in .22 and one in .177, Marksman 2004 (Beeman P17)
Cummins B3 .177, Industry B3-1 .22 and similar pistol
Crosman 1322, 1377 times 2 (.22 new, .177 from mid '90's second .177 is a project), and 2289G
Crosman 66 Powermaster, 2240, 1077, and 357
Daisy Powerline 990 and 880
Benjamin Marauder .22

Offline nekksys

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Re: Crosman 760 Mod
« Reply #12 on: October 22, 2013, 08:06:13 PM »
Nice Recruit, FredBob! Thanks for the tip on using the Recruit pump tube / linkage as well. Since 2 folks voted that way I figured I could at least try it out. I know the Recruit stock can be wobbly but I think I have a solution for that issue: shims. Slip one between the pin and the bore opposite the lock and see if that doesn't tighten it up: 0.005" should do it. Maybe thinner if needed.

And now for the first chrony readings!!! Remember, I'm using the stock pump tube, a stock replacement (unmodified, possibly a Recruit?) valve and a Discovery barrel with an enlarged transfer port. Pellet used is a Gamo Magnum 7.87gr. Calculation method was an average of 3 shots at each power level. Here they are:

1) 5 pumps - 404fps, 2.9fpe
2) 10 pumps - 498fps, 4.4fpe
3) 15 pumps - 573fps, 5.8fpe
4) 20 pumps - 606fps, 6.5fpe

A little surprising! With the original barrel, Crosman claims 625fps with BBs, lower with PBA and lead pellets. It should be obvious the extra 5" of barrel causes a significant slow down. What I found even more surprising was how accurate this gun is compared to my Ruger Air Hawk. After sighting in my Air Hawk and using a pellet it liked (Crosman Premier Hollow Point), it was difficult to put pellets into the same hole on every shot from 25ft. With this gun and the Gamo pellet, I was 2 for 3 in each shot string into the same hole! And I have not sighted in the scope for this barrel yet!!! I did have a flyer in each group but I could tell which pellets weren't going to be accurate by how they fit into the breech: sloppy fit = flyer. Maybe a pellet swage would help correct this... All I can say is I got lucky when I mounted this barrel. I am very reluctant to take this gun apart again.

I'll swap valve guts (Delrin release plunger and check valve) over to the original brass, now modified, valve this weekend and test again. I expect higher velocities due to the more sudden increase in breech pressure. Although I didn't feel any blow-by from the breech and bolt, I'll be ordering a new bolt to insure a tighter fit.
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline nekksys

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Re: Crosman 760 Mod
« Reply #13 on: October 31, 2013, 04:38:58 PM »
I picked up a Torrent to snag the pump tube and stock from. Gun looks great with the Torrent/Recruit stock and tube but this mod hasn't given me the results I've wanted...

Even with the tube mod, transfer port valve mod and extra pumping, I'm only getting low 800s out of this gun with an 8gr pellet. I feel I'm now in the range of diminishing returns. I have become very reticent about sinking more money into it and keeping it as a pumper. I'm beginning to believe my only recourse to get the velocity I want will be to build my PCP/CO2 design. It's a deceptively simple design that doesn't require cutting the existing pump tube; leaving the gun able to return to factory configuration. With minor changes, this mod should also be usable in other Crosman guns.

Once I get this mod built, I'll post back with photos and chrony results.
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

Offline Millipede

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Re: Crosman 760 Mod
« Reply #14 on: November 04, 2013, 03:53:23 PM »
my advice, coming from my own experience and directed to nobody in particular, is: if you're NEW to the hobby, hold onto the brass valve for later until you're confident about doing modifications to your nice and shiny brass valve, I ruined mine and they don't make brass ones anymore  :(  sometimes they show up on ebay though 

Offline Millipede

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Re: Crosman 760 Mod
« Reply #15 on: November 04, 2013, 04:03:13 PM »
BTW, 800 is pretty good i'd say..  how many pumps was that at though? did you put in a flat piston? I was able to get over 900 fps from a 1377 with ft piston, 24 inch barrel and some other little changes, at "just" 20 pumps   8)

Offline K.O.

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Re: Crosman 760 Mod
« Reply #16 on: November 04, 2013, 05:43:26 PM »
there are many tricks such as thinning exaust valve stem for  flow and cutting valve threads to increase valve volume.

these share 1377 valves ?

 I read their volume is  .1 cubic inch well a 24 inch barrel has a volume of

.02459 x 24  = .59016

add .1 to that and you have an ending total volume of about   .69   cubic inches

that is 6.9 times as large as beginning(valve)  volume

so,

Boyles law(close enough for our purpose) says

if starting psi is about 1000 to 1200 psi

then at 24" psi is  about 145-174

okay so if willing to pump more...

and can take valve volume  to .15 cubic inches

 psi @ 24" is about  203-243

check valve can be bored also  (which will lighten it also, open bit quicker maybe bit longer) of course dont over do

I will be modding my 1377 next spring  to a 16" or so barrel and meant to shoot heavies which will mean wasted energy with lites.

right now stock and seating  the pellet just past the port with 7.4 pointed it tops out about 535 fps @ 10,12 and 15 pumps no gains

With 10.5 Ultra mags @ 10 pumps it is down to  about 430 and climbs back up to about 475 @ 15 pumps

I have till spring to figure out where the balance of valve volume, ability to flow and barrel length are balanced about right...

I have not seen a lot of materiel about all this though and am sort of new to modding   but it does seem there are significant gains to be had.


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PC77 18.7" barrel, valve .12ci  sleeved intake, stuffed piston, .045 cup shim, 700 fps @ 15 & 755 @ 20 pumps with 7.4g Crosman points

1322 steel breech, accurized 14.5" barrel, valve  .13ci, .160 porting, .052 pump cup shim, 640-650 fps @ 22 pumps w/14.3g CPHP

1322 MLT steel breech, 682 fps @ 22 pumps w/14.3g CPHP 13xx/66 hybrid

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Offline nekksys

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Re: Crosman 760 Mod
« Reply #17 on: November 05, 2013, 12:47:31 AM »
BTW, 800 is pretty good i'd say..  how many pumps was that at though? did you put in a flat piston? I was able to get over 900 fps from a 1377 with ft piston, 24 inch barrel and some other little changes, at "just" 20 pumps   8)

I still have my brass valve. I'm holding on to it for nostalgia... In fact, I'm hoping someone has a CAD file of the stock metal receiver sides so I can have some milled up out of 7075 AL (with appropriate mods of course...) so I can retire my originals.

I was at 20 pumps and had a best speed of 810fps. The rest hovered just below 800 with the lowest at 792. I've put a PM66 tube, a modified Torrent valve and added a bolt seal between the barrel and breech block in this thing so far. At 10 pumps, I've gained 200fps over stock. Taking it to 20 pumps only gains me another 100fps. Going the extra distance to do superfine tweaks just to eek out another 10-20fps isn't worth it to me. I'll stick to the 720fps I'm getting at 10 pumps.

I've just finished rough modeling of my PCP mod. It's designed to handle 1000psi AirSource cartridges and put ZERO pressure on the pump tube itself. This one is sized for the Torrent / PM66 pump tube. Only the reservoir and pressure tube sections need resized to fit a 760 tube. I tend to over-engineer for added safety so the reservoir walls are fairly thick. I'll be modeling the cartridge cover tomorrow. For now, here's the CAD rendering of the current iteration... (Yes, I know there are open holes visible. I'm tired and don't feel like modeling the screws that will plug the holes!)
« Last Edit: November 05, 2013, 12:54:17 AM by nekksys »
Ruger Air Hawk .177
Crosman 760 .177, modified
Crosman Torrent .177, modified
Slavic .177 springer

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