Tune/Mod Questions for Bob (CDT) and Gene

[CO_AirGunner]

I'm working on doing an Air-Venturi gas ram mod to my Gamo Whisper.  Upon disassembling my gun for the mod last week, I found that the factory seal was pretty beat up as installed by Gamo, so I elected to replace the seal while I was at it.

The seal that came out of the gun measured 0.996" (with wear), and the gun's bore measures 0.985".  I was unsure as to which seal to buy from JM, so I ordered the 2-for-1 pack which included a small Apex seal and a small Tesla seal.  I received those yesterday and immediately measured them.  The Tesla seal measures 0.998" and the small Apex seal measures 1.022".

My first question is which seal would I be best to use?  The Apex seal does fit into the bore without too much trouble (it is only 0.037" oversized after all), but would I be better to use the small Tesla seal?

My second question is about honing the compression chamber.  The inside bore in my gun is not very coarse.  In fact, it is surprisingly smooth from the factory.  I think I would actually make things worse with anything less than 400 grit paper.  Is this normal, and is it still worth doing the work on the chamber?  I have some nice sandpaper flapper-wheels that are just under an inch and would work nicely prior to honing, but again, I think they would actually make the surface rougher than it already is.

Thanks in advance guys!!!

[tjk]

I had been been contemplating honing the comp tube on my G1 as well. After inspection, the only marks/scratches in the tube are around the rear of the action. The actual compression area looks very smooth and clean. I think that you would hone the CC if you find any irregularities in the ID of the tube. Since I don't have the correct measuring tools to check for this, I doubt I will, nor see any nessesary reason to follow this proceedure. "if it ain't broke, don't fix it" comes to mind!!!!  The G1 was shooting in the 877 fps range in its pure stock form which is plenty fast for me. I've placed an order with ARH, and included some buttons for the piston. I hope this works to keep things in line better. The piston had some scuff marks and there was a little play in it. As for the seals, I gave Mr. Maccari, the seal dimensions and asked him to pick out the one he thought would work best. Just my thoughts,....I'm sure Gene and Bob will chime in with a far better answer than mine!!! LOL's, tjk
PS What ever you do, you'll want to debur all the slots/cutouts in the action.

[CO_AirGunner]

Thanks.  Deburr has already been performed.

[Gene_SC]

I would use only a 400 brake hone for the compression end of tube.

[CharlieDaTuna]

I use a Brake Cylinder Hone with either a 220 then 400 grit stones or just the 400 grit depending on the condition of the surface. Do not use anything above 400 grit stones as you do not want a shiny highly polished surface in this area.  I use a soapy water solution as a lubricant, honing the compression area in an up and down manner to create a cross hatch pattern at a low speed. It is a lot easier to control the hone at a lower speed.

 This cross hatch pattern is important as it provides for a better uniform sealing surface and even lubrication for the seal and piston while at the same time providing the even minute particles of lube to get past the piston needed for the combustion to provide the additional power. You do need to be carefull not to back the hone out to far when it is turning, because the hone will catch in the long slot and will undoubtedly break the stones.

Regarding the choice of seals. I would choose the one that fits the compression chamber the best.

[CO_AirGunner]

Thanks for the info guys!  I was a bit surprised by the factory condition of the inside bore.  I expected it to be rougher.  I'll go ahead and hone it with the 400 grit as planned.

As far as seal fit - how does one gage if it is too tight?  I expect they'll both fit fine with the small Apex seal just being "squished" more.  Given that they're tapered, this will just result in more sealing surface.  As long as it is able to slide back and forth relatively easy, should I just go with the larger one?

[CharlieDaTuna]

As long as it pushes into the compression chamber with a just a little effort it should be fine. It should not fit real tight though.

[CO_AirGunner]

Thanks Mr. Tuna!!!

[gosciu555]

How much should you hone? Do you measure in passes or just by "eye?" How do you determine if the surface is right? Also, do you think going 50% speed on a drill is too fast? When I go slower, I don't feel like anything is really "happening." Thanks in advance.