Old Style Benjamin Stems from New Sheridans

Precision Airgun owner, Chuck Trepes, asked me to modify some new style Benjamin/Sheridan valve stems to work with vintage Benjamin pump guns. Seems the new Sheridans and Benjamins use a fairly normal coil spring inside the check valve, while the older Benjamins used a conical shaped volute spring. The new stems have a post that the spring fits over, but it's too large in diameter for the old conical spring.

What is the #99-079 number on the envelope? I'm going to guess that these are actually Crosman part #397-038. The one on the left has the smaller diameter post that Chuck needs.

As long as I roughly match the diameter, the spring will stay in location.



Really a perfect job for a small lathe like this. The model Chuck provided didn't have any semblance of a truly round post. It varied by several hundredths in diameter. I settled on turning these down to the average diameter.

As long as the volute spring fits over the end, it's a go. This was such a non-precision job, I just used a comparison caliper rather than measure each piece. I was still within about two or three thousandths of an inch over the batch.

I can't find my notes as I write this. Believe I took the diameter to 0.248". I'll find it and edit if necessary. The important part is the spring fit, not the actual number.

In about 20 minutes, I had six done.

Gecado Diana Model 5 Pistol, Fixing The Piston Head

Now to deal with that messed up piston head.

I chucked it in the 3 jaw. You can see the folded over burrs that are keeping the loose head in.

I turned away a couple of thou' at a time. I wanted to remove the minimum.

Then when I had a nice clean ring on the face I started working on the inside rim.

At this point I was able to pop the head out. Those burrs were thick.

The head...

The back side. As you can see it's a rivet of some sort that retained the original seal.

Dialing in the piston in the 4 jaw chuck.

Boring the recess true and clean.

Ready for a head.

The insert roughed out.

Using a form cutter to make the dovetail button. Notice the chatter marks.

Final cutting, chatter removed at very low speed.

You can see the tool I ground. The Diana pistons seem to have a 70 degree dovetail as best I can measure.

Cutting the insert off.

The insert.

Fits!

I soldered it in with soft solder. The new acetylene tank did not catch on fire like the old one did last time. A nice even ring of solder holding the insert in. I cleaned the face up with a very fine pass across the face.

You can see the solder flowed inside.

Using my extremely handy Diana seal tool.

Popping the seal on. Notice I cleaned up the OD near the head as it was a bit scored.

The seal in place. Looks good.

Another shot. I bought the seal from Air Rifle Headquarters. He gets a lot of my money these days.

Now to derust, deburr, clean, reblue the pistol, make breech seal (o-ring) shims, lube and reassemble...then test. Then if it works... fix the cap that screws on the end...all the ones I have are beat up and I should make a threaded arbor so I can reknurl/cut new knurls after truing. fix the stock... fix the windage screw... I'll never run out of projects.

Gecado Diana Model 5 Pistol Disassembly, Part 2

On I go.

Pushed this pin out.

Came out easily.

Removing the pivot bolt lock screw.

Barrel and breech removed.

Lack of a breech seal...need to make one.

That rivet looks like it won't come out, being flared out at the ends.

But it pushed out easily.

Trigger & spring.

Cocking transfer bar slides forward, up and out.

Trigger pull screw on the right, lock screw on the left.

Mist be missing the plunger/ball on the end?

Disassembled.

Lock screw removed.

The end cap lock screw was missing.

Cap unscrewed.

Just like the other Diana 5.

The piston was stuck so I resorted to this insane solution, my home made slide hammer vise grips holding a bent hook tool...

Which gripped the inside of the piston slot.

Braced in the vise with a piece of brass rod through the pivot holes.

The piston was pulled out.

Unlike any other Diana piston head.

And the head is loose...If anyone has taken apart a similar Diana and has a picture of a good condition piston head like this I'd love to see it. As is I'll probably whip up a replacement dovetailed head that matches the later 5 and solder it in.

You can see the step inside the piston that the head butts up against.

Now to get to work fixing the problems and getting it to shoot.