Rebuild - Smith & Wesson 78G



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Offline almyjmoore

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Rebuild - Smith & Wesson 78G
« on: July 30, 2017, 04:30:56 PM »
I just finished attempting to rebuild my Smith & Wesson 78G. The first time I tried to shoot it, it shot twice then nothing. It still had a full CO2 but it wouldn't shoot. I did another take apart to clean and check everything over carefully. Everything looked good but when I recharged, Co2 came straight out of the end of the barrel. I bought the gun a few weeks ago and it shot good and held CO2 overnight. When I put in the next cylinder, it wouldn't hold and I found the O rings bad throughout the gun. I used a cheap O ring set from Ebay. It's easy to say that is the problem but is there anything else that I could have done wrong that would cause this problem? I really don't want to spend $40 or $50 on an expensive set of seals and end up with the same problem. Thanks, Al
  • mi, edmore

Offline the fuse

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Re: Rebuild - Smith & Wesson 78G
« Reply #1 on: July 30, 2017, 09:32:16 PM »
Hi, A 78G is at least 35 years old and probably needs a new or rebuilt valve stem if your replaced all the orings and it leaks out the barrel. That means it is leaking through the valve. Take it down and check out the seal on the valve stem. I bet it looks rotten. the material that S&W used is prone to breaking down. Dennis Baker or JG airguns can hook you up with one......others as well. BTW urethane orings are the best to use for the piercing cap if you want to shoot a second Co2 cart right after one runs out. Good luck. It's one of my favorite pistols.
  • Carmel, NY
I've got a touch of hangover, bureaucrat. Don't push me.
Rifles
Crosman 160, 180, Model 1
Daisy 822, 922
QB57
Pistols
Smith & wesson 78G
Crosman 38T, Mark I, SA6
Daisy 722, 717, 1200, 118

Offline jmars

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« Last Edit: July 30, 2017, 11:40:05 PM by jmars »
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Offline PaulT58

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Re: Rebuild - Smith & Wesson 78G
« Reply #3 on: July 30, 2017, 10:06:29 PM »
I recently rebuilt a 79G.  First time was with the cheap eBay o-rings, and had the exact same thing happen.  It shot fine the day I rebuilt it and then the next day, nothing!.  I bought the MAC1 rebuild kit, and have not had a problem since.  It comes with a hotter valve and better o-rings.  It shoots must faster than what the factory specs were when new.  Here's the kit I got:

http://www.mac1airgunshop.com/smith-wesson-rebuild-kit-for-co2-pistols-p/sw78reki.htm
  • Navarre, FL
Rifles:
AirMax: Plinkster & Varmint
Beeman: Silver Kodiak; QB Chief
Benjamin: Marauder; Maximus X2; Trail NP2
Crosman: 1077
Gamo: Accu177; Swarm Maxxim; Whisper Fusion
Hatsan:  MOD 95; Torpedo 100X
Umarex: Gauntlet (pre-ordered)

Pistols:
AirMax: CP1-M
Crosman: 1377; 1322; 2240 X2; 2300S; 2400KT
Hatsan: Model 25 Supercharger; Tac-Boss 250XT
Remington: M1911RAC
S&W: Model 79G

For Sale:
Hatsan MOD95 .22 - $125.00
Gamo Swarm .22 - $150.00

Offline almyjmoore

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Re: Rebuild - Smith & Wesson 78G
« Reply #4 on: July 31, 2017, 09:01:07 AM »
Thanks Guys, I will Order from Mac  as soon as he is able to restock the rebuild kit. I'm attaching a few pictures to see if you can see anything wrong with the part in question. I removed a greenish blue salt-like substance from that hollowed out area. It looks like an Oring or nylon gasket should fill that hollow but nothing is shown in any diagrams that I have seen. Do you see anything obviously wrong with this part? Thanks, Al
  • mi, edmore

Offline the fuse

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Re: Rebuild - Smith & Wesson 78G
« Reply #5 on: July 31, 2017, 11:20:29 AM »
Looks fine to me.....except that all the gunk that used to be a synthetic seal is gone. I buy my valve stems from Baker and the last time I think it was $12 plus shipping. I buy all my orings from an online supplier of quality orings. I'm just too cheap to buy reseal kits. I can do a complete reseal of a 78G for $17.   
  • Carmel, NY
I've got a touch of hangover, bureaucrat. Don't push me.
Rifles
Crosman 160, 180, Model 1
Daisy 822, 922
QB57
Pistols
Smith & wesson 78G
Crosman 38T, Mark I, SA6
Daisy 722, 717, 1200, 118

Offline almyjmoore

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Re: Rebuild - Smith & Wesson 78G
« Reply #6 on: July 31, 2017, 07:04:00 PM »
Fuse, What do you suppose that missing seal was made out of. Would it have been an Oring or more like a nylon or rubber washer and would it have extended just outside of the cup? While I'm waiting for new parts I can experiment with a couple of different types of homemade seals. I just need to know sort of what the seal should look like or what they are made out of. I wonder why the seal kits that I have seen for sale didn't include a valve stem seal. Thanks, Al
  • mi, edmore

Offline oldair

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Re: Rebuild - Smith & Wesson 78G
« Reply #7 on: July 31, 2017, 08:00:33 PM »
The valve seal would be a fairly stiff square-section 'washer' fitting tightly to both the OD and ID of its mating metal pieces, approximately level with the top of the brass cup.  Some have used 90 durometer urethane, others have had success with Delrin (more rigid thus more tricky to get sealed, however the entire head can be made of the plastic allowing you to eliminate the brass cup altogether).

The reason eBay sellers don't include the exhaust valve nor even a seal for it, is that it's extremely cheap to gather up the O-rings needed (likely under 50 cents each) but the exhaust seal, or better yet the whole valve stem, needs to actually be made.  The seal alone doesn't lend itself to casual repairers, as the steel stem needs to be pressed out of the brass cup in order to install the seal 'washer' - best done with a couple fixtures to not damage the parts.

Don R.

Offline almyjmoore

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Re: Rebuild - Smith & Wesson 78G
« Reply #8 on: July 31, 2017, 10:51:45 PM »
Thanks Oldair, I think I will just order the part and wait until it gets here. It sounds like producing a working seal for the valve stem is sort of complicated. Al
  • mi, edmore

Offline the fuse

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Re: Rebuild - Smith & Wesson 78G
« Reply #9 on: August 01, 2017, 01:56:52 PM »
It is harder to rebuild a valve stem but it can be done with the right tools. Time is money to me. I just buy them.
  • Carmel, NY
I've got a touch of hangover, bureaucrat. Don't push me.
Rifles
Crosman 160, 180, Model 1
Daisy 822, 922
QB57
Pistols
Smith & wesson 78G
Crosman 38T, Mark I, SA6
Daisy 722, 717, 1200, 118

Offline almyjmoore

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Re: Rebuild - Smith & Wesson 78G
« Reply #10 on: August 02, 2017, 10:21:58 AM »
I tried to order kit with valve part from Mac but the parts are out of stock. I tried Baker a few days ago but he hasn't responded. Who else sells the new valve stems? Thanks, Al
  • mi, edmore

Offline the fuse

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Re: Rebuild - Smith & Wesson 78G
« Reply #11 on: August 02, 2017, 11:22:36 AM »
Try Allen at AB airguns and John at JG airguns. I've heard talk about the vintage airgun hobby dying. These guys can't keep up. That's why Baker didn't get back to you and Mac is out of stock. I'll look around because I may have a rebuilt 78G stem here somewhere.
  • Carmel, NY
I've got a touch of hangover, bureaucrat. Don't push me.
Rifles
Crosman 160, 180, Model 1
Daisy 822, 922
QB57
Pistols
Smith & wesson 78G
Crosman 38T, Mark I, SA6
Daisy 722, 717, 1200, 118

Offline Robert Skelton

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Re: Rebuild - Smith & Wesson 78G
« Reply #12 on: August 02, 2017, 12:24:11 PM »
It is harder to rebuild a valve stem but it can be done with the right tools. Time is money to me. I just buy them.

I make my seals out of Delrin as I have a lathe. Once you get the hang of it, it's easy. When you get the material turned and drilled it's only a couple of minutes to make and fit each one.

At almost 71 I have the rest of my life to make things. You will find that once you retire you will be looking for projects. You will start reading the obituary's to see how many years you might have left. Thinking will I have enough time to do all my projects.
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Offline cbinfl

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Re: Rebuild - Smith & Wesson 78G
« Reply #13 on: August 02, 2017, 12:26:58 PM »
FYI, Mac 1 has SW kits in stock.  I ordered one last night.


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Offline almyjmoore

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Re: Rebuild - Smith & Wesson 78G
« Reply #14 on: August 02, 2017, 01:33:35 PM »
I took the old valve stem apart last night, cleaned it up and made a new seal out of some sort synthetic rubber. It' fairly hard but I can dent it with my fingernail. Since I've never seen a valve stem with a good seal in it, I was making it look like I thought it should look. When I put things back together, there were no leaks. Also the gun wouldn't shoot. Now I'll have to carefully unscrew the CO2 chamber to release the pressure to see if I can figure why my seal doesn't work. I'm going to get a new valve stem as soon as I can so I feel comfortable experimenting with the old one.
  • mi, edmore

Offline the fuse

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Re: Rebuild - Smith & Wesson 78G
« Reply #15 on: August 02, 2017, 02:10:11 PM »
No worries. You just put it back together with something in the wrong place. The valve stem would not make the gun not shoot. Perhaps the hammer is in upside down. Are you sure the Co2 cart is pierced? It is so simple in there just think about where the gas has to go and you should be able to reason it out.
  • Carmel, NY
I've got a touch of hangover, bureaucrat. Don't push me.
Rifles
Crosman 160, 180, Model 1
Daisy 822, 922
QB57
Pistols
Smith & wesson 78G
Crosman 38T, Mark I, SA6
Daisy 722, 717, 1200, 118

Offline oldair

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Re: Rebuild - Smith & Wesson 78G
« Reply #16 on: August 02, 2017, 05:51:07 PM »
From your description, it sounds like the gun is now pressurized.  If so, you should realize that forcibly undoing the piercing cap will cause it to blast away from the gun like a rocket.
If you go ahead, please set up some kind of SERIOUS restraining scheme to prevent damage to you or your work area.  Many years ago, I thought a Crosman 600 piercing cap on a flea-market find was just rusted stuck.  When the vise-grips finally got it free, the result was a big dent in the floor joist above my basement workbench, a ricocheting cap, and nearly a lost eyeball!

Offline PaulT58

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Re: Rebuild - Smith & Wesson 78G
« Reply #17 on: August 02, 2017, 08:21:13 PM »
FYI, Mac 1 has SW kits in stock.  I ordered one last night.

That's the one I got, it worked wonders!
  • Navarre, FL
Rifles:
AirMax: Plinkster & Varmint
Beeman: Silver Kodiak; QB Chief
Benjamin: Marauder; Maximus X2; Trail NP2
Crosman: 1077
Gamo: Accu177; Swarm Maxxim; Whisper Fusion
Hatsan:  MOD 95; Torpedo 100X
Umarex: Gauntlet (pre-ordered)

Pistols:
AirMax: CP1-M
Crosman: 1377; 1322; 2240 X2; 2300S; 2400KT
Hatsan: Model 25 Supercharger; Tac-Boss 250XT
Remington: M1911RAC
S&W: Model 79G

For Sale:
Hatsan MOD95 .22 - $125.00
Gamo Swarm .22 - $150.00

Offline almyjmoore

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Re: Rebuild - Smith & Wesson 78G
« Reply #18 on: August 02, 2017, 10:51:14 PM »
I'm sure the gun is pressurized, I can't unscrew the cap without a tool. I've been in this position before and I will put a rag over the cap and turn slowly until the CO2 barely starts releasing. It takes 10 or 15 seconds to release the pressure. I'll be gone for a few days but will be excited to get back and try out the suggestions to repair this gun. Thanks, Al
  • mi, edmore

Offline almyjmoore

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Re: Rebuild - Smith & Wesson 78G
« Reply #19 on: August 05, 2017, 09:41:58 PM »
   I got back home today and put a towel over the end cap and slowly unscrewed the end cap until the co2 was escaping. When some of the pressure was relieved, I cocked and shot. It shoots good at half co2 pressure. Does anyone have any idea what might be wrong before I disassemble it again? Thanks, Al
  • mi, edmore

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