GTA
Airguns by Make and Model => Vintage Air Gun Gate => Topic started by: TerryM on April 03, 2017, 11:00:13 PM
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A coworker of mine brought me his 1964 Benjamin 312 to work on today. He has had this gun since it was new, when he was a kid. Said it's not shooting like it used to. It still has the original seals in it and he said he has always used 3 in 1 Oil. I brought it home, took a few shots over the chrony and was surprised it still gets about 525 fps on eight pumps. Slow, but better than I expected. Inconsistent, sometimes the check valve sticks and it won't take air till it is pumped repeatedly. I'm sure it is very dirty inside. The heads of both front plug screws are broken off. I think I can get the plug out but I don't know about the screws. They are pretty small, anyone experienced with this problem? I may try to find a replacement plug. Ought to be an interesting job.
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try using a left handed drill bit. center punch the screw, then drill it with the lefty bit. MOST times, the screw will back out.
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Once the head is broken there usually is no tension on the rest of the screw.
Quite often you can use a sharp pick and turn them out. Being a screw that small you would need something fine. if you have a sharp dental pick, that might be just the ticket.
Being a steel screw in a brass end cap, a little bit of light heat may help as well. The brass will expand before the steel and open up a bit of clearance...
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Thanks for the tips, guys. I hand drilled a shallow .037" hole in each screw, turned them out with the tip of a utility knife. The pivot pin is bent, I'll have to find a replacement or make a new one.
Man, this old thing is nasty...
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replacement parts, and seal kits can be had from Precision Pellet. Rick Willnecker is a really nice guy to deal with. His prices are reasonable, and very good quality, as well.
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AGREED!
Mr Decker turned me onto him a while back and I am going to stick with using him.
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New pivot pin and front plug screws are on the way from Rick. Spent some time this morning cleaning up this gun, what a mess. There were clods of oily dirt in the pump tube. Had to hook the valve springs with a bit of wire to pull them out. I don't see how this gun was working at all!
I resealed my '62 312 a couple of months ago. It had a single port exhaust valve. This one is a '64 and it has the dual port, channeled exhaust valve. So, apparently, Benjamin made the switch in valve types sometime in that timeframe. I was glad to see it, this type is easier to deal with.
I made a nifty slide hammer to aid in pulling the valve, I'll post a picture of it later...
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I might soak something like that in diesel.
Not sure if it would hurt any seals or not though.
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Yea those single ports are a pain along with that notched lead ring. They do shoot a little higher FPS.
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Got my parts in the mail today and got the gun put back together. This gets easier each time I do it, I learn a new trick here and there on the forums. To late in the day to chrony but I fired a few shots into a block of soft pine. The gun barks with authority, I'm pretty sure it's shooting quite a bit harder. Can't wait to shoot it over the chronograph tomorrow and see how much better it is.
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Velocity testing was disappointing, didn't see much increase. Shoots much more consistently though.
Another problem has arisen. At some point in the past, someone hammered on the ends of the pivot pin in an apparent attempt to keep it in place. That's how it got bent. It also enlarged the holes in the tube that the pin fits in. With a new pin installed, the front plug wants to shift forward when the gun is pumped. One of the new plug screws has already sheared off. I see two possible fixes: Drill and thread the plug for the next bigger size screw or drill the plug and tube for a 5/32" roll pin. I'm leaning toward the roll pin as the surest cure.
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Let me try and get photobucket to work. I had the same issue with a Dan. Where someone used a bolt and nut instead of the roll pin for the pivot so it enlarged it.
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Ok got it to work. So it's still a 3/16 roll pin, the next size up was too big 5/32, would require drilling out. So a 3/16 roll pin with 1/8 solid pin hammered inside. I guess it all depends on how large the tube has become, but this has worked and hasn't budged at all. Even through the 10 shot chrony string at 8 pumps.
(http://i1030.photobucket.com/albums/y364/Rob112o/image_zpskkkgpils.jpeg) (http://s1030.photobucket.com/user/Rob112o/media/image_zpskkkgpils.jpeg.html)
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Good job on the Sheridan but I think I need to do something different on this 312. I think if I replace the tiny threaded screws with something more substantial, the pivot pin will be alright as it is. I have to be careful not to use to large of a pin, the pivot pin retaining screw still has to fit underneath it.
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I take back everything I ever said about roll pins. Spring pins, whatever you want to call them.
The first 312 I worked on, it was a pleasure disassembling the pump. So much easier with the screws and solid pivot pin. Then I got ahold of this abused one with it's bunged up pin and tube. Trying to decide what to drill out and how much. Then, I was digging around in my parts drawer and came across the front plug from my 342 parts gun. Bells went off. The holes line up perfectly with the 312 tube, just had to slightly ream the ones where the retainer screws went. I would bet Benjamin repaired lots of 312s this way in years past. I have to wonder if that is how the front plug assembly for the 342 came about. I'm so glad I didn't get crazy drilling before I found my 342 plug! 342 plug on the left, 312 on the right:
(http://i1076.photobucket.com/albums/w446/sotol11/IMG_1174%20-%20Copy_zps4s0qctxl.jpg) (http://s1076.photobucket.com/user/sotol11/media/IMG_1174%20-%20Copy_zps4s0qctxl.jpg.html)
I tried some mods on my spare valve and put it in this gun. I wasn't sure if it would do any good but didn't see any harm in trying. I turned a forward portion of the stem down to .125" from .155 and drilled an extra pair of ports in the exhaust valve body. It did, indeed, increase velocity a little. The gun will now shoot over 550 fps with 9 pumps, a little more than it was getting with 10 pumps before. Stock valve in front, modified in rear:
(http://i1076.photobucket.com/albums/w446/sotol11/IMG_1180%20-%20Copy_zpsyuiap29e.jpg) (http://s1076.photobucket.com/user/sotol11/media/IMG_1180%20-%20Copy_zpsyuiap29e.jpg.html)
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Velocity testing was disappointing, didn't see much increase. Shoots much more consistently though.
Another problem has arisen. At some point in the past, someone hammered on the ends of the pivot pin in an apparent attempt to keep it in place. That's how it got bent. It also enlarged the holes in the tube that the pin fits in. With a new pin installed, the front plug wants to shift forward when the gun is pumped. One of the new plug screws has already sheared off. I see two possible fixes: Drill and thread the plug for the next bigger size screw or drill the plug and tube for a 5/32" roll pin. I'm leaning toward the roll pin as the surest cure.
I have had success shrinking wallowed out holes in the pump tube with the rounded end of a ball peen hammer. Put the end of the hammer on the pin hole and hit it with another hammer to push the metal back into the hole and in effect "shrink" it. This only works if the hole hasn't been drilled oversize, if the hole isn't too big, and if the material is flared outwards or inwards so that you can push it back into place. This has worked for me on 2 Sheridans that had oversize/loose pivot pin holes.
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Not too many people know how to use the "other" end of a ball-peen.
Good tip!
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Put the end of the hammer on the pin hole and hit it with another hammer to push the metal back into the hole and in effect "shrink" it. T
You should never hit a hammer with another hammer. That can cause it to chip etc and hit yourself. Thats why they make punches etc in a softer alloy, way less likely to chip/fail and they just mushroom. Think about it, you never see a steel hammer mushroomed...
They make tools to dimple steel/ roll a hole inward etc.
Just an FYI
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Moeb74,
You went way too far the safety direction with that response. Hitting the ball peen hammer with another hammer is more of a tapping force. The ball peen makes a cheap stand-in for a hole-shrinking tool that uses a hardened steel ball to do the same thing (roll the material on the top edge of a hole into the hole).
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You could always use a soft faced hammer to hit the ball pein hammer.
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Here is the slide hammer I put together to pull the exhaust valve. Works great.
(http://i1076.photobucket.com/albums/w446/sotol11/IMG_1239%20-%20Copy_zpsiudyg2vz.jpg) (http://s1076.photobucket.com/user/sotol11/media/IMG_1239%20-%20Copy_zpsiudyg2vz.jpg.html)