Score - 760 Powermaster

[Nail Gun]

Yesterday I was surfing my Louisiana shooter's forum and a particular thread caught my eye.  The OP ran across an old BB gun from his childhood and was asking how to dispose of it without freaking the garbage men out.  He said it was an old Crosman pump that hadn't held air in 20 years.  I immediately sent him a PM asking him if he'd be willing to let me see if I could save her before sending her to the landfill.  He wrote back saying I could meet him today and to have my way with it. 

Today we met up, I immediately noticed she was an early 760.  He shared a few childhood stories about her including the fact his father gave her to him when he was 11.  I offered to return her if I could get her going and he declined.  I figure I'll give him another shot at re-claiming her if I'm successful.

In the mean time, she's very different from today's 760's.  The receiver is solid metal as is the piston arm.  The forearm is wood but the butt-stock is solid, dense plastic.  The front sight is a ramp-style and the barrel band is solid metal.  It looks like the barrel fits completely back into the receiver and the bolt is solid brass.  It also appears the rifle is a self-cocker similar to the 130 Pelgun. 

Overall the gun is very heavy and solid considering her size, especially compared to today's 760's. 

Are there any pitfalls or nasty surprises I need to be aware of before I tear into her?  Are seals still available?

[egillman]

Looks like you scored a 760 third variant, which was only made in 1974-75 and had the plastic stock and wood forearm.  I have restored about 8 of the first variant, which had the wood stock and forearm.  I'm not sure what the internal differences are, but you  can get the parts diagrams from the Crosman web site, and most all parts are available from JG Airguns.  These old 760's are a lot of fun to restore (and shoot!)

[mav72]

Dreamed of getting one of those but too many projects and not enough money to do em right..... LOL

[Nail Gun]

Currently I have her apart.  The valve is indeed a self-cocking mechanism.  If memory serves correctly it is exactly the same valve as in my .22cal 130 Pelgun pistol.  It is a smooth-bore which seems to be the only drawback I've found so far (drawback per my preferences that is).  It is fairly robustly built.  The buttsock is 100% solid plastic.  It's actually heavier than it would be if it were made of solid wood.  Overall it makes me realize there is a significant void between today's "box store" pumpers and the ultra high-end pumpable PCP's... 

I think I have the valve sufficiently rebuilt using bulk O-rings.  I've removed the surface rust, re-crowned the barrel, re-finished the forearm, gave the buttstock a good rubdown with lacquer thinner.  I also added a sharp taper to the bolt to allow air to flow more freely.  In addition I've polished the contact surfaces between the trigger and the hammer.  I anticipate she'll be shooting tomorrow. More to come...

It's funny, the guy I received it from was the original owner.  He share a few stories about his adventures (should I say misadventures) with the AG.  He didn't completely understand that Crosman still makes a "variant" of this rifle today.  Per egillman's dating, he received this rifle one year before it was produced.  It goes to show how memories fade, change and alter over time.  Heck, I honestly can't remember the age I was when I received my first rifle.  If I had to select an age and trace the rifle's age by its serial # I'm sure I'd be off by one or two years, that was in the 1990's! 

Thanks folks.  Hopefully I'll have a range report shortly.

[Nail Gun]

UPDATE - Although I'm not done with the rebuild I did assemble it enough to test the valve rebuild.  My rebuild took!  It's building and holding pressure.  Now I've got her to this point, I have one more question:  Since the valve is the same (or darn similar) to the 130, is this rifle's recommended pump volume also 6 pumps max?

I ask for two reasons:
1 - Due to the self-cocking valve the trigger pull increases dramatically above 5 pumps.
2 - Over 7 pumps, the valve does not immediately re-set.  I'm aware the valve cap is flung back with more force as air pressure increases.  Apparently when it's flung back with enough force it overcomes the reset spring.  Also, the inner O-ring I used to re-seal the inside of the cap I has a tighter fit with the valve stem than the gasket did.  I'm sure it requires more force of the spring to re-seat it due to the tighter o-ring. 

[egillman]

Hello again,
Before you put it all together, you might want to watch this video. 




Although I don't believe all of the claims he makes, I do agree that by increasing the volume of the valve you can increase the fps of the gun.  I have done this mod on several 760's (by removing the aluminum spacer) and you do get more punch, although it takes more pumps to get there. 
Erik

[ezman604]

I have a first variant I bought in 1967-68. From what I remember it has a 10 pump max. I have resealed mine from parts bought at JG.

[Nail Gun]

That's an interesting link.  I opted to keep the valve as-is.  I re-assembled her and she's shooting very well with one small exception. 

When I shoot the bolt flies open allowing a lot of air to escape.  The spring-loaded tab is in tact and functional too.  I tried tapering the end of the bolt to allow the air to flow past the end more freely.  It sure didn't help...  Anyone have a good fox for that?

[egillman]

I have the same problem on my 760.  The bolts on newer Crosmans (like the 2240) have small o-rings that seal the breech behind the pellet.  I have thought about modifying a bolt to see if it would help.  I might also try wrapping a thin layer of electrical tape around the bolt to provide more friction between the bolt and the breech.

[Nail Gun]

I figured out how to keep the bolt to stay shut. 

1 - Remove the bolt knob
2 - Using an Exacto knife cut the rim off a SPENT .22lr shell
3 - Slide the shell casing over the bolt arm
4 - Re-attach the bolt knob and badda-bing! 

You're in business 

I'm still getting a puff of air in the face but it's obvious much more air is being used on the projectile!  If I can locate an 0-ring small enough I'll use a jeweler's file and make a groove in the bolt to help seal her off more. 

I ran a few shots through her.  BB's are providing shotgun like groups at 5 paces.  But, she seems to do ok with Crosman 7.9g pointed pellets.  Over the chronograph with 10 pumps she sent the first at 550fps.  By the sixth shot she was delivering 460fps.  Each shot was weaker than the previous.  What could be the cause of that?

[egillman]

As for the decrease in fps with each shot, either 1) you are not getting as much pressure when you pump, indicating the rubber "pump cup" may be worn or else 2) some air is leaking out of the valve before you pull the trigger.  I would pump it up, fire a shot, then pump it up the same, wait a few minutes, and fire a shot.  That should isolate the problem.

You have inspired me to make an "airtight" 760 bolt.  I got an old bolt, chucked it in my drill like a lathe and used a small file to make a groove around the bolt about 1/2" behind the tip.  I actually went a little too far and got all the way down to the magnet, but a little super glue fixed that.  Now, I am trying to find the right size o-ring to use.  The smallest ones I could find at my local hardware store are a little too big to fit into the breech end of the barrel.  Any ideas where I could get an o-ring that is about 0.177" diameter?

[egillman]

The air-tight bolt works!  First, I filed a groove all the way around the bolt at 3/8" back from the tip.  I also tapered the end of the bolt from the tip back to the groove.  I bought some #003 buna-n o-rings (which have a 3/16" OD)  and slipped them into the groove.  I put on a drop of Pellgun oil and slid it into the breech.  It was a tight fit, but it worked and there was no air escaping backwards when I pumped it up and shot it.  I am concerned about o-ring wear, so maybe a harder rubber or a different o-ring material would wear better.  Any suggestions?

[silent_airman]

I have a first variant I bought in 1967-68. From what I remember it has a 10 pump max. I have resealed mine from parts bought at JG.

Sounds like the one I had as a kid. Does it have the vertical ribs on the fore arm and a solid wood stock? I'd love to have my pal back. Maybe I'll get lucky as some yard sale this summer. You never know!

[egillman]

You can find these on American Airguns or Gunbroker classifieds.  The "first variant" is the one with the wood stock and wood ribbed forestock.  I have bought 6 of these, and restored and sold them in the last 6 months.

[Nail Gun]

I was out of town for the past week so I'm just re-joining the conversation.  I tested her by pumping her up 10 times and leaving her for a few hours.  She ended up sending the pellet at 500+ FPS after that.  That tells me she isn't leaking one bit.  I speculate the drop in velocity is tied to heat as she is pumped.  The hotter she gets, the less velocity she produces.  That has me scratching my head a bit...

I plan on re-introducing her to her original owner this week and giving him the opportunity to reclaim her.  If that's the case, the velocity issues will be a non-issue.  If he decides to let me keep her, she'll get a new rifled barrel and be relegated to a pellet-gun-only once I'm done.