Installing a Mac1 steroid exhaust valve in a 392PA--how I did it

[jon.k]

I was going to ask if anyone had done this, and solicit advice, but I decided to grow a pair and just do it. Unfortunately I didn't take pictures.
I bought the steroid poppet a few months ago and didn't have time to install it until late last night. The Mac1 site says the concave delrin face seals against a concave hole to ensure the edge makes contact, and he mills out the valve body to make it all fit. I measured extension of the crosman valve post from the valve body at 8.8mm, and the Mac1 stem extension was 7.3mm, so I planned to remove about 1.5mm of material around the exhaust valve mating surface.

No mill in my shed, just a drill press. I decided to drill with a 1/2" bit and use the resulting 135 degree conical hole in lieu of a concave surface. My 1/2" bit needs to be sharpened so I used the next size down in my bit index, 31/64". I hoped the bit wouldn't chatter too much and I could just use the hole as drilled-- but it is a long, sharp bit that tried to dig and grab at the brass as soon as it touched down, and the resulting hole was a chowdery chattered mess. Here I could have taken a picture of how not to do this.

Plan B had been to do some light polishing with valve grinding compound if the hole was not perfect. I was now on plan C, heavy polishing now that the hole looked like a my-first-drilling-project example. I cut a 1/2" wood dowel to about 5" long, carved a rough 135degree taper in one end, and chucked it into the drill press. I trued it and sanded the cone on the end smooth, applied abrasive, and spent the next 1.5 hours polishing, checking, adjusting the taper, and polishing more. When I was done it looked like a perfect concave surface, so I felt like this might work. The next finest abrasive I have is a gray polishing stick for stainless steel, but I decided the valve grinding compound was fine enough. I washed it, reassembled the valve, and measured the post extension at 9.1mm. I had to polish off a lot of material!

The fiirst pump was not encouraging. I could hear air leaking slowly through the exhause valve. At 3 pumps the valve sealed, so I fired it to clear out any residual grit that might damage the delrin face. The valve held the next pump perfectly. I pumped and fired a few more times to test to continue to clear the valve. It sounded great, with a nice sharp crack!

The valve held 3 pumps overnight, and I shot a few pellets at a can this morning. No chrony so I can't say if it made a difference in performance, but it works well and should last longer than the stock rubber valve. 8 pumps knocked an upside-down can off the top of a stick at 25yds, then made it jump around on the ground in later shots. It _feels_ and _sounds_ great, so it must be working great! The main pivot on the this 1997 pump arm has a tiny bit of wear in it from years of the previous owner not oiling, so the next buy will be a billet lever and adjustable piston kit from the Mac1 shop, and after that a genII brake. When I'm done the gun will have all the steroid parts, but the trigger will still be so-so and the valve volume is unchanged, so it'll be far from a genuine steroid tune.

If I do this to my other 392 I'll take a few pics and revisit this post. Probably will be a while, since I spent much longer than I anticipated polishing the chatter away. I read up on drilling brass and I guess since the 1/2" bit needs to be sharpened I could have adjusted the angle of the cutting edges to optimize for the material. Maybe I'll do that for the next one.