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PostPosted: Tue Nov 08, 2022 3:05 pm 
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Joined: Tue Oct 18, 2011 12:21 pm
Posts: 606
Location: Ontario
My SW 78G has been sitting unused in my cabinet for some time so it was not really surprising that I found a slow leak when it was pressurized.

It seemed that I had another candidate for a valve job.

I don’t have a service manual for the 78G but I found enough info on the web to be able to proceed.

Again I did the minimum disassembly that would allow me access to the valve.

Before I started, since it was suggested that the trigger spring was a pain to put back, I placed a rubber band to keep the trigger in position

Basically removing the threaded plug under the barrel gave access to the adjusting rod and the sleeve to slide them out, then removing the small screw at the back of the frame unlocked the Top Slide from the Frame. Once separated, a few parts like the cocking lever, spring and plate can be placed aside.

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The Valve Assembly can be taken out by removing three screws, it is a good idea to pay attention to the tiny screen found in the hole of the frame so it isn’t lost.

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For some reason, the shaft of the stem goes through the brass cup for the seal. It seems that the short end serves as the spring guide. When I made my replacement valve stem from Delrin, to ensure that it would be leak free, I used a shorter pin sitting in a blind hole. For the spring guide I just turned down a smaller diameter on the Delrin head.

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By now I was getting familiar with the procedure, a bit of heating followed by a smacking of the valve stem on the seat then some lubrication (including some Pellgun oil) and reassembly.

The Smith & Wesson 78G is now working as it should.

R-Gun Pete


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PostPosted: Tue Nov 08, 2022 7:57 pm 
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Joined: Sat Sep 03, 2005 9:25 am
Posts: 4590
Location: Ontario, Canada
A new style stem for a Crosman 1322/77 (part 760-145) works fine for the S&W 78/79 pistols and Daisy 780/790. The valve return spring is somewhat guided by the hole it sits in so i didn't bother with the tiny nub on the stem. The stem length needs to be trimmed a bit and it works great.


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PostPosted: Tue Nov 08, 2022 8:22 pm 
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Joined: Tue Oct 18, 2011 12:21 pm
Posts: 606
Location: Ontario
Hi TCooper.

Thanks for the information.

I didn't realized that they were so cheap because when I had to order one for my QB Chief it cost me $30 once the shipping was calculated.

I can see that part 760-145 is available from Canada Shooting Supply for $3.99 and GravelAgency has it for 2.84. It practically doesn't worth it to machine it.

Anyway it is good to know.

R-Gun Pete


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