I installed a steroid exhaust valve in my 392

[jon.k]

Here's the last mod I plan on doing to my 392s: the steroid poppet. I did one in my first 392, and maybe it was beginner's luck, but it never leaked and performs flawlessly. Unfortunately I didn't take pictures of that process, so when I retrofitted one to my second 392 I documented it as best I could with a phone.

you can read how I polished the leade of my 392 here:
http://www.gatewaytoairguns.org/GTA/index.php?topic=97694.0
and how I installed a billet lever and adjustable piston here:
http://www.gatewaytoairguns.org/GTA/index.php?topic=97520.0

Here's the gun I'm working on, I mentioned that this one is a pleasure to shoot and groups very well (in my opinion). It's a 2004 gun with the phase II stock but a plastic safety button:



To start I looked at the Mac1 poppet and compared it to the stock poppet. The stem on the stock one is longer by about 0.07", and this is reflected in the projection from the bottom of the valve. My goal will be to have the steroid stem extend exactly as far as the stock stem. Stock above, steroid below, before any work is done:



And here they are side-by-side:



Last time I did this I used a nice sharp drill bit in my drill press, and I got a ton of brass chatter. I read up on it and learned that a much more obtuse cutting angle should be used, so I nipped the edge off this dull 1/2" bit with my dremel so that it was about 60 degrees relative to the cut surface:



And then I secured my valve bottom onto the drill press table:



set the depth stop to 1.5mm:



And drilled away. Still some chatter; I reworked the drill bit again for an even more obtuse angle (more like 90 degrees) and managed to get rid of the chatter later, but we'll get to that.
Here's the before drilling picture:



Here's the after picture:



Now I've got to polish that surface. I use a stick, because this is a woodshop and sticks are my tool of choice. My stick is a dowel almost 1/2" in diameter, with a blunt hemispherical nose on one end.



And my abrasives are coarse valve-grinding compound from the auto parts store, followed by fine stainless buffing compound:



I grind by chucking the stick in the drill press and holding the valve body free-hand against the table:



Here's what it looks like after the coarse grind:



And here's what it looks like after it is more cleaned up with a water rinse and a paper-towel sponge off, before the fine polishing paste:



With the new poppet fitted:



At this point I was hoping the poppet would seal with just light pressure, and  I should not be able to blow through the bottom half of the valve. I guess the first one was truly beginners luck -- I could blow right past this new valve. I thought maybe securing the valve to the drill press was a bad idea -- maybe the hole was now oval instead of circular? Maybe the polish was too coarse? Maybe the concave surface was too concave?

I redrilled, repolished, and reshaped the concave surface about two more times, testing the theories one at a time. To ensure the hole was uniform I rotated the valve body during the drilling and polishing phases, but it still leaked. I decided the 135 degree cone that my 1/2" drill bit makes was too steep; in a test the valve sealed immediately on a flat surface, so the bottom of my sealing surface should be almost flat. I reshaped the blunt nose of my polishing stick to be more blunt and reground the valve body, and it nearly sealed during the breath test. I reassembled, put in three pumps, and left it overnight. The next day it had leaked down to zero pressure, so I took it apart, reground one more time, finished with the fine paste, and it now holds air indefinitely.

So the critical aspect is to have the poppet seal not against a steep 135 degree cone such as from your drill bit, but a nearly-but-not-quite-flat hemispherical surface.

Measuring final stem extension, it looks like I could have taken another 0.025" off, and actually I had to relax the hammer spring a tad to be able to cock the gun reliably (slipped the spring on a screwdriver shaft and compress it to coil-bind a dozen times):



And lastly, here's a cutaway drawing I made to illustrate the stock exhaust profile (left) and the exhaust profile after all of this grinding (right), with a little angling of the transfer port using my dremel and a small round burr:

[redlined_b16a]

Very nice write up.

Extremely informative drawings too