Monday, April 19, 2010

Quick Fix--The Smith & Wesson 78G from the Findlay Show

Fifteen minutes after getting back from the Findlay, OH airgun show, the S&W 78G sprang a leak. The seller had just gassed it up last Thursday and it was still holding two days later.

I've been looking for a clean 78G on and off for about a year. Mostly off. Friday night before the show, I put this gun at the top of my list. There were probably half a dozen S&W 78/79Gs and Daisy 780/790s there. This one looked well cared for and the price was very reasonable. I assumed when buying it that I'd need to replace all the seals anyway--that's just par for the course when buying a vintage CO2 gun.

Nick has brilliantly covered 78/79G rebuilds before here. Sorry for the redundancy, but maybe a few new pics will help someone--besides, the grip project I'm working on isn't quite ready for prime time.

Unscrew the threaded end plug below the barrel--S&W calls this the "sleeve retaining screw"and remove the "power adjustment extension" rod.

Removed the casting assembly screw.

Had some difficulty getting the extension sleeve out. A #2 pencil--eraser end first--pulled it right out.

The orientation.

The extension sleeve looked slightly peened on the ends. Perhaps this caused the difficulty in removal. With the lathe, I took a few thou off each end.

pulled out the bolt stop pin. It was a very slightly bent. I gently straightened, then polished it.

While the bolt was out, I replaced the bolt o-ring. Forgot to write it down. I think I used a #6 viton.

Removed the valve body screws. One on the left side,

Another pic.

Two on the right.



Brass valve assembly pulls out. This one was a bit stuck. Decaying o-rings sticking to the die castings.



S&W calls this the "cartridge connector". Uh, OK.

When I pulled out the valve body plug, the source of the leak was obvious.

The o-ring had the consistency of a gummy bear.

Took a bit of time to scrape it all out of the groove.

Another pic. Used a #12 viton o-ring as a replacement.

Might as well replace these, too. Used #6 viton o-rings on the "cartridge connector".

The valve stem seal looked fine. A bit of Crosman pellgun oil on all the o-rings and it was back in business. Refer to Nick's original posting series for reassembly. There are no surprises or tricks. It's a very straightforward design.

Soon: match grips for a Daisy 7X7.

12 comments:

windwalker said...

Model 78G leak

I would like to know how the Co2 chamber seals to the feedthrough hole to the gas cartrage connector (11213). - the one with the two 1/18 x 1/4" orings.

I have gas leaking out of the hex nut (11222)that is mounted into the frame. The pierce assembly , threads and seals are like new and no leak there. The gas is leaking out between the hex nut and the 11201 frame!. Looks like they used some kind of expoxy to seal the nut then used split pins to hold the nut to the frame.

Felice Luftschein said...

That's a problem I haven't run across. It does look like the two pins hold teh nut in, I never gave any thought to how it was sealed. Sounds like whatever was sealing it isn't anymore. Best to ask on one of the forums, Mac1 should know.

derrick38 said...

The gas tube is sealed above the hex nut by the main o-ring seal (11226) on the piercing body assembly. The o-ring (11041) on the piercing pin completes the seal. The hex nut shouldn't need any epoxy to form a seal. The main o-ring above it and the piercing pin seal do the job.

Up top, the o-rings (11213) that you referenced seal the connector to the valve and the gas tybe. The valve is then sealed at the rear by the valve body plug (11207 and it's o-ring (11047). The front of the valve is sealed by the valve seal (11210) on the spring loaded valve stem(11212.)

Unknown said...

My sleeve retaining screw is impossible to remove - HELP! What kind of screw driver can I use?
My &(G is in pristine condition otherwise - just need to change all the o-rings. I bought it in 1979 and kept it inside a drawer and forgotten for 35 years! Even the box is in original condition! It was a great pleasure to find it ( actually it was my daughter's boyfriend who found it for me when he was helping us tidying up my basement ;)) Any way please HELP! Thank you ! Louis

Unknown said...

My sleeve retaining screw is impossible to remove - HELP! What kind of screw driver can I use?
My 79G is in pristine condition otherwise - just need to change all the o-rings. I bought it in 1979 and kept it inside a drawer and forgotten for 35 years! Even the box is in original condition! It was a great pleasure to find it ( actually it was my daughter's boyfriend who found it for me when he was helping us tidying up my basement ;)) Any way please HELP! Thank you ! Louis

Dow Kennedy said...

Just finished replacing all the seals. The challange for me was getting the retaining screw out. But here is how i did it. First I got 1/2 flat screw driver Felo 0715732362 1/2-Inch x 8-Inch Slotted Screwdriver with Hex Bolster, 550 Series

then I applied direct flame heat from a butane torch and then cooled and then repeated several times. Success!

Dow Kennedy said...

use butane torch and 1/2 screw driver

Anonymous said...

Bought a 78G fr $7:00 of a facebook for sale site Jan 5th. Had to replace all o rings in order to get it operational. Noticed while doing so that it is missing the extension sleeve. Any ideas on how to replace this? Gun still shoots great but would like to have this part. Any help with this would be great.

Anonymous said...

hard to come by parts can be gotten from jgairguns. the extension sleeve goes for about 5 bucks.

Raymond said...

Hi my name is Ray and I got a 78g.22cal air gun my father bought it a long time ago and now my brother had it and it started to leak on him so one ay I went over his house and he told me it leaks like crazy and gave it to me for I'm a fix it all guy I can fix anything take it a part and put it back together well I got some o rings at home looked on line in how to fix it and now its working great not leaking any more and now I have another one to add to my collection awesome gun

Anonymous said...

I am having trouble reassembling the valve body with the new o-ring. I couldn't even press it in with a press. The o-rings came in a kit from Kung/blueline studios. Tried rws oil, the dielecteic grease, no change. Any suggestions?

Dennis Cotten II said...

Looking for a replacement piercing assembly for a 79G. Not sure exactly what it is called but the entire piece that screws into the handle to seal the CO2 and pierce the cartridge. Any help would be appreciated. Thanks