Friday, July 15, 2011

Simple Fix for a Leaking S&W 78G Piercing Cap

Two days ago, I gassed up the S&W 78G with a new 12g cartridge. Heard a faint gas leak from the cap after piercing. It wasn't a fast leak so I went about shooting my targets in the basement range. After about 30 shots, I set the gun aside assuming it'd need some attention. The next day, it was still holding pressure. Even so, I didn't remember replacing the piercing pin seal when I overhauled the gun a while back. Figured it was simply time.

Got another 8 or 10 shots off to empty the gun completely of CO2 then removed the cap.

The outermost o-ring seals the gas tube. It's not the one leaking.

The piercing needle is in the center of the cap and has a tiny o-ring that seals around it's OD. To get to it, I aligned the side hole and drove out the roll pin.

With the roll pin removed, the components of the cap are extremely straightforward.

Unscrewed the small bushing to reveal the o-ring.

Yeah, I probably could have done the entire fix without removing the roll pin, but I wanted to clean and inspect all the parts.

With a dental pick, the seal was out. It's lost it's elasticity. A few small cracks.

After all the years in the gun, it's difficult to accurately measure. A #006 o-ring looks like a good match.

Reassembly was simple.

Smeared the o-rings with a bit of Buzzy's Slick Honey. Gun works perfectly.

Note: I often get emails about where to find Slick Honey. Local bicycle shops can source it from the largest parts wholesaler, Quality Bicycle Products. QBP part numbers are: LU2004 1 oz--about $8, LU2005 2 oz--about $11. Super thin, seems to almost melt between your fingers. A little of this stuff goes a loooong way. Supremely tenacious, hard to displace. Will not harm o-rings. We use this stuff religiously in the shop on suspension fork bushings and seals to reduce stiction. QBP does not sell to private individuals.

More stuff soon.


Anonymous said...

I have a 78G and I also had a faint gas leak but it still held pressure. The gun though could not fire more than 10 ft on low power and the pellets would drop after 50 ft on high power...

I hear the leak though from the back underside of the breach area. Is this leak caused by the piercing o-ring cap in the grip or another internal leak that would require an overhaul inside the gun?

derrick38 said...

10 feet? Sounds like you have bigger problems than a slight piercing pin leak. We've covered a full tear down or two of the S&W 78/79G pistols here. Pretty easy to repair. Every o-ring should be probably replaced considering how old the gun is.

Anonymous said...

I was afraid of that. Thanks for the advice!

Ray said...

Where is the best place to I buy a #006 o-ring?... My 78g needs the same one replaced...

Thanks in advance!

Felice Luftschein said...

Bryan and Associates or Mac1?

Unknown said...

I just happenes upon one of these pistols and when I punched the c02 it all blew out this area. Think a simple oring is the answer?