Saturday, March 21, 2009

Notes on Cleaning and Resealing the Benjamin 310, Part 2

Almost done...

I measured the depth of the tube to the valve face.

And the distance from the end to the pivot center.

And the length of the pump assembly from pivot center to plunger face.

And then I adjusted the length...and reassembled. All that measuring got me close, but I still had to take it apart about 3 more times before I had the pump headspace set to my liking. Anyone have a better way of doing it?

Then I went to work making a replacement aperture disc. I took some rough scaled up measurements from the Benjamin book...the threads seem to be 3/16"-40. All I had was a #10-40 die but there's only about .0025" difference between those two so I just adjusted the die to cut oversize.


Turning the shank...

Here's the adjustable die.

It fits!

Drilling a .081" aperture hole (#46 drill). I went big because I wasn't sure what would suit my friend best.

Boring out the recess in the front. I intentionally ran it fast so it would chatter at the bottom, hopefully diffusing the reflection.

Yes, it's screwed in backwards for this shot. Oxpho Blue is awesome stuff.

Mounted on the rifle.

Another shot.

The 310 is a smoothbore and the bolt is bored to just hold ball ammo. Kind of neat.

Well it's held pressure overnight, several days in a row. I got velocities of 590 fps using the Gamo round balls (8.2 grain) and 620 fps with CPL (7.9 grain) pellets, both at 6 pumps, which is the maximum recommended. Better to just pump it up 3 times for plinking. I'll see if it holds air for a week straight and plink with it off and on before giving it back to my friend.

Wednesday, March 18, 2009

Notes on Cleaning and Resealing the Benjamin 310, Part 1

Now to get the 310 shooting again...

The valve components were covered with oxides.

I cleaned them up with steel wool and a fine wire wheel.

The inlet seal was extruded and hard...I'll replace it with a teflon seal since they seem to work well.

The exhaust valve seal was also hard and extruded.

So I dug it out and de-crimped the cup.

Last time the de-crimping made the cup look rough so I decided to make a die to smooth out the cup a bit and restore it to roundness. It has an OD of the ID of the cup and a hole for the stem to fit through.

The outer die is basically a tube.

I pressed the cup though on the arbor press.

It restored the roundness.

I cleaned it up with a file on the lathe.

I used the same crimping die that I showed in the previous Benjamin resealing post.

The new seals. The right lead seal is a bit tweaked.

Two seals? Last time I resealed I only used one? That's because I missed one that seals the valve body (337 Outlet Valve Seat) against the air chamber. It was luck that it stayed in place and worked.

This shows the two lead seals needed.

I hate cleaning out the tube and air chamber. This is two Irwin bit extensions with a holder for some ultra-fine scotchbrite pad. I wrapped masking tape around it so it wouldn't scratch up the tube.

Used with some light oil it scours out the deposits without messing up the surface of the tube. I then clean everything out with paper towel wads held with a parts grabber.

For the air chamber I use oil and some paper towel held in a slotted dowel. Just keep cleaning until the paper towel comes out clean. It takes forever.

Tuesday, March 17, 2009

Partial Resoldering of a Benjamin Model 310 Air Rifle Barrel

An old friend of mine asked whether I could fix his Benjamin air rifle, "bought with my own money", new, when he was 12. The main complaint was that it lacked power (more on that in another post) but there was also the issue of the front section of the barrel being loose.
Being a paranoid sort of guy, I reasoned that if I tried to fix his barrel without any experience I would end up destroying the gun. So I asked around on the forums to see if anyone had a beater gun that I could practice on, with the understanding that I would probably ruin it. A kind gentleman sent me one, and I did a marginal job. The front section of the barrel was resoldered, but I ended up globbing a large amount of solder on and the finish was completely ruined. I'm not even going to post pictures of the results, it was that humiliating. It'll look ok if he polishes it up though.
Oh, ok, here are two pictures of the loaner rifle.

Here you can see that there are some large solder blobs (the gun was pretty chewed up and I think had already be resoldered partially so at least the front sight isn't my fault), and that I've had to buff the metal down to the bare brass.

After cleanup it wasn't so bad, but you can see gaps in the solder line. This is entirely down to poor preparation.
In any case, I had some data about what not to do, but still felt that I didn't have enough of a plan. I decided to peruse some gunsmithing books and read everything I could on resoldering shotgun ribs, which is an analogous problem. So I read what I could and felt much more confident. At least soldering gave Dunlap some fits: "I either spoil the bluing and get a good soldering job or am so careful about the finish I get a lousy sweat joint." (Gunsmithing by Roy Dunlap)
The last part of my puzzle had to do with getting over my fear of desoldering the rest of the gun. I bought some "Tix" solder from Brownells. It has a very low melting point of 275 deg. F so I could in theory would not need to heat the gun up too much.

Here you can see the problem.

It's separated by about 4" from the front.

I used some sandpaper to clean the join area on the tube as far back as I could go.

And lightly holding the barrel against the paper sanded the other surface.

This is my setup. A moist towel around the tube, with it clamped in a vise. If the towel starts steaming I would be getting it way too hot. A kant-twist clamp holds the barrel to the tube. That front sight is soldered to the barrel and could become desoldered easily.

I fluxed with Tix flux. Next time (Oh, how I hope there isn't a next time) I will be even more careful about fluxing. You really only want flux on the joint.

I carefully heated the barrel and tube for just a few moments with a torch and touched the solder to the join on both sides. Here you can see that it did blob up a bit but it also shows a nice shiny line along the join. I may get a non-contact thermometer if I do this again so I can better gauge the temperatures.

Remember what I said about fluxing? The areas on the tube that had flux on them ended up having solder flow...but again you can see an unbroken solder line where the barrel and tube join.

This is how I wish it all looked. That small blob just picked free.

I used a graver, riffler files and a scraper to gently scrap the excess solder. Then I polished with steel wool. The original finish is pretty tough so I didn't expose the brass (much).

The graver.

A poor picture, I'll take some others when I'm done with the rifle. The blobs are far less noticeable now though.