Before you do a mini conversion on a marker there are a few things you need to consider. First, if your cocker body has a serial number, you will lose it. If that bothers you, I would advise against doing a mini conversion. Second, on most cockers the front of the body sits in a recess in the back of the front block. As a result, most cocker bodies are squared in the front. When you remove this portion you may be left with a cocker body that does not fit snugly into the recess in the back of the front block. The cocker will function as normal, but the front block will be able to twist from side to side if you try to move it. This isnít really a big deal but I still wanted to mention it. Finally, you can do a half block conversion with a rotary grinder or other tools and end up with a nice final product. Mini conversions shouldnĎt be done this way. The front face, where the front block sits, needs to be as close to perpendicular to the axis of the lower tube as possible. If itís not, your front block may not seal and your pneumatics or pump kit will point to the left or right. Your ram canít work efficiently if it is not properly inline with the pump arm. With some pump kits, your pump handle will rub the barrel if itĎs not on straight. You really do need a milling machine, or something that can firmly hold the cocker perpendicular to the travel of the cutting tool. Iíd recommend indicating in your vise just to be safe.
Now that thatís out of the wayÖÖ.
DO NOT CUT ANYTHING YET!!
When you cut off the front of the marker you will also be cutting off the threading for the banjo bolt. You could cut it off, then re-thread it when you are finished but that would be the hard way. If you donít start the tap straight you will pretty much ruin your cocker body. Itís much easier to go ahead and run a tap down into the lower tube. You will need a 9/16-24 tap for this. Screw the tap into the lower tube as you would the banjo bolt. It will stop where the threads end. Go ahead and tap the lower tube all the way to the bottom. It doesnít need to be that deep, but itís easier to just go ahead and tap it all the way than it is to measure and see how deep the threads need to go. Plus, the extra threading will only add volume to the lower chamber.
Now you are ready to cut. Roughing this part with a saw will save you a lot of time when you get to the actual milling. Using a band saw go ahead and cut the front of the body off. Leave the lower tube at least ľĒ longer than the top tube. It doesnít have to be pretty. You will be milling all of this off later. I always lay the body on a piece of paper to make sure it doesnít get scratched.
Now, you are ready to do the milling. If your milling machine has a large spindle you will need a long end mill to make sure that you donít accidentally bump the feedneck with the spindle. Line the vise jaws with paper to make sure you donít scratch the body on the hardened steel jaws of the vise. Leave as little of the body sticking out of the vise as you can. This will help reduce chatter. Since you are doing a peripheral cut over such a large area chatter very well could be a problem. If chatter is a problem for you try either lowering your RPM, taking a smaller cut, feeding faster, or using some sort of cutting fluid. WD-40 works great for aluminum. You really donít have to measure until you completely remove the air holes where the ASA attaches. You can see what remains of the last hole in the picture below. Once you completely remove that last hole, itís time to slow down and start taking smaller cuts.
Keep removing metal little by little until you have approximately .100Ē of the lower tube remaining. This isnít an exact number. Some mini cockers Iíve seen have a good bit more than that left, some have less. Iíve seen a variance of as much as .125Ē from one cocker to the next. .100Ē is a nice median.
Now you are ready to put your mini-cocker front block on it. You can use either an integrated front block/ASA or you can use a mini-cocker ASA that screws into the bottom of a standard front block. You will also need a shortened pump arm. If you canít find a mini-cocker pump arm you can always cut and re-thread yours. Itís pretty easy to do. Just lay your pump arm on top of your body. Line the front of the pump arm up with the front of the lower tube. Now, mark where your back block will need to be. Add about ľĒ for threads. Now itís as easy as cutting it off with a band saw and running a die over it.