by Tom Gaylord
Writing as B.B. Pelletier
My S&W 78G pistol.
A history of airguns
Today’s report is written by reader 45Bravo. This is his report to us on resealing the S&W 78G and 79G.
If you’d like to write a guest post for this blog, please email me.
And now, over to you, 45Bravo.
Resealing the Smith & Wesson 78G
This report covers:
- Where to start?
- The reseal
- Get started
- Wrapping it all up
So, all of the gas leaks out of your Smith & Wesson pistol as soon as you pierce the CO2 cartridge. It’s probably the piercing pin seal. Since these pistols are now over 40 years old, they need TLC. (And I’m not talking about the TV channel.)
Where to start?
The factory seals didn’t fare well with the lubricants used by most shooters at the time these guns were made, and the seals disintegrated over time. If you are going to change seals, I suggest changing them all at the same time.
Since this is a CO2 gun, you want to use o-rings that are not affected by carbon dioxide. If you use a standard hardware store o-ring, the CO2 gets absorbed into the o-ring material, causing it to swell to several times its normal size. The wrong type of o-ring on the outer rim of the piercing cap will continue to hold the cap tight, even when your cartridge is empty. It will be quite awhile for the o-ring to shrink back to its original size so you can open the cap and change the cartridge.
There are a couple of places in the gun where you probably could use regular hardware store o-rings, like in places where they are captive, but I would not suggest that unless you want to go back inside the pistol again in a year or so. Urethane o-rings are the best. With proper care, you should not have to reseal the gun for decades.
Normally I use a complete kit from Mac1 Airguns that includes a new redesigned poppet (Ed. — valve), and a complete set of 90-durometer Mil-Spec urethane o-rings.
But not everyone wants to or even can spend $35 on their project, so I decided to source a set of urethane seals from eBay without the valve as a test to see how well they work compared to the others. There are several sellers on ebay selling different seal kits, I chose the one that offered urethane seals with free shipping, and a good price.
A single set that works for both the .177 and .22 costs $5.50, but 2 sets were $7.15, shipped from a U.S. seller. I ordered them on Monday, they arrived on Saturday.
The seller also includes an exploded view, and an o-ring size reference as to where they go in the gun.
Ebay seal kit comes with seals and a diagram.
Normally, the seal inside the poppet is trashed like the rest of the seals, but the one in the gun I am resealing looks new. Since this is going to be a budget reseal test, I decided not to change it. I just replaced the o-rings, and will see how long it runs.
As we go I will offer some tips to make things go easier and faster.
The largest o-ring goes on the co2 cap.
The next smaller one goes on the valve body plug.
Then there are 4 smaller o-rings that are the same size. They are used on the .22 bolt probe, the cartridge connector, and inside the piercing cap.
The single smallest o-ring is for the .177 bolt probe.
Tip: I suggest using a small zip tie, or velcro cable management tie like you use to hold laptop power cables to hold the trigger rearward while you do the reseal.
Using the cable tie is not necessary, but the trigger spring can be a little fiddly to get back in position, and you need an 11th finger to hold the trigger to the rear during assembly if you don’t use something to hold it.
The seal that fails most often, and the easiest one to change, is the one in the piercing cap. You do not have to disassemble the entire cap to change the o-ring, just have the right tool for the job.
Tip: You can use a Dremel tool, or a file to modify a screwdriver tip as shown to make a tool that clears the piercing pin, but lets you unscrew the seal cover.
The right tool for the job. Sometimes you have to make it!
Use a dental pick or similar tool to carefully remove the old seal. Clean the inside carefully, lightly lube the replacement o-ring with your choice of lube then center it over the piercing pin, and then screw the seal cover back in place.
Since this is a budget build, I am using Crosman Pellgunoil as a lubricant, because I am sure every CO2 shooter has some on hand.
Then remove the large old cap seal o-ring, clean the groove and put the new largest o-ring on the outer part of the piercing cap.
All of the seals/o-rings in this pistol were in this condition. This one seals the piercing pin inside the cap.
Remove the grip screws and grips, and then the rear screw under the right grip as shown below.
The rear screw on the right side of the grip is the takedown screw.
Then remove the power adjuster outer screw below the barrel.
Remove the outer power adjuster screw at the muzzle.
Tip: You can insert the eraser end of a pencil into the hole to remove the outer sleeve the power adjuster rides in.
The slide now comes off the frame. Be careful of the pin that holds the bolt in place.
Remove the bolt, and replace the old o-ring on the bolt probe with one of the 4 o-rings that are the same size.
The bolt or probe comes out of the pistol and the o-ring needs to be replaced.
Now, remove the 3 screws that hold the valve in the frame.
Tip: For pry tools on guns I use bamboo chopsticks from the Chinese restaurants. They don’t mar the finish, and they are free…
Remove the valve cartridge, it may take some gentle persuasion as there is a short tube that goes from the valve into the frame. That tube passes gas from the CO2 cartridge into the valve. It is called the cartridge connector.
In the frame, under the valve cartridge connecting tube is a very small brass screen, be careful to not lose or damage it, as it protects your valve from debris that might get into the system.
This tiny screen under the cartridge connector (gas transfer port) keeps dirt from entering the valve.