Hi folks!

I don't often work on airguns though I have a couple. A friend needed repairs on a vintage gun, a Benjamin 347, same system as several other Benjamin models.

I ordered one of the repair kits off the internet from Airgun Warehouse that comes with the seal removal tool and also replacement pump piston cup. The gun uses the rubber cup that pops over a stud on piston rod, the new seal was the other [later?] type that's captured around its circumference.

Didn't want to wait to get the right cup, so I looked at it and decided to experiment. I used my hole punch, good quality Made in England, multiple punch spindles, to punch a hole dead center in the rubber pump cup.

After cleaning the mounting area & stud I placed pump rod in vice with other end firmly against table, used a drop of Palmolive liquid as lubricant because that won't harm rubber or gun parts, but a guy could use vegetable oil too I guess. Forced new cup down over stud with a little help from a screwdriver. Popped right in place, holds very securely and no chance of it coming off.

The particular punch spindle I used is .250" or 6.5mm, perfect fit, slightly smaller than pump rod stud diameter. Took quite a bit of clamping force on pliers-type hole punch tool, had to rotate cup on spindle to punch evenly all around.

Works great, excellent pressure. Please feel free to copy this to any relevant airgun website so it may help the next person?
Airfierce

Benjamin 340, 342, 347, 397 air rifle pump repair & replace seals

Hi again, folks!

The Benjamin 340-397 series are fine little air rifles, very accurate. As mentioned in prior post, there was a change in the P/N 3452 pump rod piston cup in later models. By using the later rubber cup in the earlier guns you will have a rifle that shoots far harder than ever before, even much harder than when brand new!

The reason is that old style pump cups fit over a pin on end of pump rod, and are hollowed out throughout interior diameter, with only a slightly heavier section at walls of cup which seal against air chamber, and at heavier center nib where pump rod pin fits through. Other than those two areas the cup is hollow, soft & rather flimsy.

Newer cup is an entirely different design as mentioned, a solid rubber puck, resulting in a much sturdier design which doesn't flex and collapse like the earlier one. Due to heavier design it seals much better against walls of air chamber and simply pumps more air in.

With early cup it's easy to get 10 pumps of air in, even a dozen. By doing the upgrade to the later cup, by punching the .250 hole through center to fit pump rod pin, it's quite difficult to get more than 3 pumps in! I could only manage 3, but my friend who is 6'4" and 245 lbs managed 4, but that was it!

With the new cup seal, 2 pumps or 3 at the most are easily equivalent to 10 or more with old style cup in respect of pellet power/velocity. This was testified to by the original owner of gun, an FFL holder and gun expert, who said it had never fired at anything even close to how hard it fires now!

A word of CAUTION!

When dis-assembling gun Benjamin left a nasty little surprise for the unwary. The bolt is retained in gun by bolt guide screw #3468 near its forward end, on left side & just behind pellet loading slot on top of receiver. In that area just behind P/N 3468 screw there is also a plate P/N 3469 called the bolt lock cam, held in place by 2 small screws, on side of receiver.

Hidden from sight and just aft of P/N 3468 bolt guide screw there's another smaller hole in bolt, with a tiny bolt friction spring P/N 3496, topped by a crazy tiny ball bearing P/N 3497. Tiny? Crazy tiny?? Ummm, yes...how about an external measurement on ball bearing of .0940" !! It's approximately the diameter of a cocktail toothpick at the pick's widest center part!

So, one removes the cam plate's 2 screws and lift plate off, then remove the bolt guide screw that protrudes from bolt. Place a wad of tee shirting or some such at rear of receiver so that as you slowly extract the bolt out rear of receiver the ball bearing is captured instead of flying out and disappearing into the vast far reaches of eternity!

When re-assembling those parts a dab of Vaseline helps hold things in place, but you may still lose your mind even if you don't lose the bearing. Extreme care is strongly recommended, this one ain't for sissies! A magnetic screwdriver or magnetic pickup tool helps pick up bearing when necessary. Holding it on top of spring with magnet then pressing it down and shoving bolt forward does the trick. Only push bolt forward far enough to expose the forward hole for P/N 3468 bolt guide screw and install it. If bolt goes too far forward the bearing will fly out the hole in front!

Clean out air chamber with carb cleaner or brake cleaner but of course be extra careful of eyes, respiratory, ventilation & skin exposures. A long rod or dowel with a bit of tee shirting on end helps clean air chamber, and you can then shove a small wire through air bleed hole at receiver end to clear it, with another flush of carb cleaner. Lube/saturate the felts on pump rod with a good gun oil, and lube chamber before inserting pump assembly.

Please feel free to copy these posts if it will help others, but I ask that you duly credit this site for location & URL.

Thanks folks, happy air gunning!
Airfierce

I too have one of these fine air rifles, an inherited one. The internal valving needed repair so I installed the kit which fixed the problem but didn't need the piston until a year later. I stumbled upon this site and modified the new cup. I ran a 0.2500" diameter chucking reamer through the hole to remove any burrs. It installed easily, I adjusted the rod length to compensate for the longer length.

PERFECT!!! It now pumps like new! Thanks for the great tip!