Monday, November 26, 2012

Daisy 790 Loading Trough Repair

Here's some of the Daisy 790 parts gun I purchased almost two-and-a-half years ago.  It was in a cardboard box and priced to low to pass on.  At the time, I bought it so I'd have spares for a Smith & Wesson 78G purchased ten minutes earlier.  Keep in mind that the S&W 78G and 79G (.22 and .177 calibers respectively) designs were sold to Daisy in the very early 1980's.  Daisy renamed them 780 and 790.  Daisy just changed the "G" to a "0".  For the most part, the parts are all interchangeable, so simpletons that we are, we write about the guns as though they are identical.  If you're new to this blog and need repair information on the Daisy guns, check the blog index for "Smith and Wesson 78G" as Nick and I have cataloged the bulk of the work there.

After getting the Daisy home and starting assembly, I realized it was missing the trigger.  OK.  It's a parts gun.  Set the box aside and shot the S&W 78G.

Eventually, I took the time to work the phone and the internet and tracked down a trigger.  The I hit the next roadblock--and the likely reason the gun ended up in the box in the first place.

The real problem:  The loading trough isn't aligned with the breech.  It's noticeably offset to the right.

It's also far too wide and deep for a .177 cal pellet.  Actually, it's too large for even a .22 cal.   And why is it in the white when the rest of the gun is painted?   Did Daisy have an insert for the trough for the two different calibers?  And why wasn't it centered with the bore?  Very odd.

Thought about plugging the hole and re-drilling.  Didn't think that was a very good idea.


Here's a better pic of how much it's off.  The loading bolt gets pushed to the right and then drags and wedges in the breech before it locks closed.

I finally just decided to file out the left side of the trough by hand and make a sleeve.

In this case, it's not important that the hole in the receiver be perfectly round.  It's important that it's centered for the barrel.

Cut down a piece of brass tubing.  The OD of the tube is a pretty good match to the size of the trough.

Cleaned up the ends and filed it roughly to the length of the trough.  Exactly length not terribly important.

Slit the tube in half.

Deburred the edges with a small file.

The brass insert will take up the excess space in the trough.

Coated the trough with J-B Quick Weld epoxy.

The sleeve in position.

Pressing it into place allowed the epoxy to squeeze out and fill all the gaps around the tube.

Gave the loading bolt a thin layer of grease so excess epoxy wouldn't stick.

The barrel was installed and the bolt latched closed.  This held the sleeve in perfect alignment to the bore.  Gave it a couple hours to set up.

Oh, there's still one more problem with the gun--If you look closely, the top of the casting is damaged at the front sight cross pin.  The sight is only held to the gun with the elevation screw.  I'll probably add a dovetail for a red dot.

The reassembly was done with a .22 cal barrel made for my S&W 78G.  So, now, my Daisy 790 is technically a Daisy 780.

There's more epoxy on the right than the left.  Worked great as a filler.

Semi-finished--and the bolt works!  Add that dovetail next time I fire up the mill.

Quick S&W/Daisy tip:   Removing the cylindrical retaining sleeve is often a huge pain in the neck.  Nick made a really nice tool for this the last time he worked on a S&W 78G.  I took a different approach a while ago and forgot to show it.

Ran a 5/16"-18 tap into the end.  I've done this when the sleeve was stuck inside the gun.  Got enough thread bite on the tap and pulled the sleeve out.  Once out, clamped the sleeve in the vise (in soft jaws) and cut five or six full threads.  Deburred as necessary.

Important to make sure the threads face the muzzle when reassembled.  A standard 5/16-18 bolt can now be screwed in and used as a handle to pull it out next time it's stuck.

More soon.


go play in traffic said...

I have been doing a lot of research on line regarding co2 puncture valves. every search has brought your blog up as a result. I am curious if you are familiar with a puncture valve for 12g canisters that punctures the cylinder but does not let co2 out until you twist the canister counter clockwise. I have one in my possession but cannot find where to buy more. it's the most simple thing, and yet I can't find another. Please help

derrick38 said...

What's the application in airguns? Do those require a threaded CO2 cartridge neck? They're relatively common in cycling for roadside flat tire inflation, but typically use 16 gram threaded cartridges. The Planet Bike Red Zeppelin is a nice one.

derrick38 said...

Just read your blogpost. So, did you get a Foundry?

Sandy Gill said...

I had an almost mint 790 that I loaned to a neighbor for squirrel control. He jammed a pellet in the barrel, panicked, did not tell me, then tried to 'fix' the problem by firing more pellets (no, I could not even make this up!). I finally asked him to return the airgun when he admitted he the above and said was just going to throw the gun away!!

I have cleared the barrel, but discovered that he stripped the o-ring on the co2 screw cap, so all the co2 leaks out when you puncture. Does anyone know if this requires a 'special' type of o-ring (it is not the traditional black synthetic o-ring found in common applications) or if I can just substitute a regular o-ring. I have not tried to remove the original o-ring since I did not want to cause any more damage than has already occurred.

I had also given my neighbor the original box with the manuals. He threw all that away. Does anyone have the manuals that they could copy/scan and email to me?

Thanking you in advance,
Sandy (Outspoken1)

Felice Luftschein said...

If you look at the posts on the S&W 78 and 79 you'll find a wealth of info and teardowns. You need at least a hard viton o-ring and preferably a polyurethane o-ring. Regular o-rings will swell.