Achieving 100% Air Seal in Your Gear Box **Some Guns Already Have 100% Compression In Stock Form once you have lubed it (or even if you don't lube it). Please try your guns compression before doing the following** (View "Checking The Compression" below) Piston Head Put a new o-ring on your piston. Generally a #14 o-ring from a hardware store will do fine. Sometimes it will be to big/to small. To get a even better air seal, get a old gear box cylinder, stretch the o-ring around it, get a lighter, and slowly turn the cylinder while holding the lighter under the o-ring. **Doing it to much will cause the o-ring to weaken/break/stretch to much. Be sure to not do it for to long**. The part about heating the o-ring is not always 100% necessary, but it is good to follow. After you have installed a new o-ring onto your piston, be sure to put a small dab of lube and rub it around the o-ring. This will ensure a good seal. Do not use silicone spray (as it will wear off quickly) or lube with petroleum as it will wear down the o-ring. I would recommend using most kinds of silicone grease. More does not mean better. Use only a tiny bit. To much may cause bad performance. To little will also cause bad performance. You may also want to port the piston if it is not already. To port the piston head, drill more holes/bigger holes in the holes that are already in the piston head. Just make sure not to go so deep it goes through the second part of the piston head. This is not necessary and you may be able to get a full air seal without it. If you want, try it without this first. I generally do not port my piston head and I get a full air seal. Cylinder Apply a very THIN amount of grease inside the cylinder. You don't want it to be bone dry, but you don't want there to be so much grease it plugs up the cylinder head nozzle. Cylinder Head Put teflon tape around the o-rings on the cylinder head for better seal. Not always necessary. Most cylinder heads on guns I have worked on do not need this step done. Put a small amount of grease around the o-rings on the cylinder head. Not to much. Checking The Compression Once you have completed everything above, install the cylinder head and piston into the cylinder and install it into the gear box (but do not close the gear box and leave out any other components such as the gears). Put your finger on the end of the cylinder head (do not put on the air nozzle yet). After you have done this, keeping your finger on the cylinder head nozzle, push the piston forward. Once the piston has gone past the ports in your cylinder (assuming there are some), you should not be able to push the piston forward at all. If you can, you might have messed up in the above steps, bad quality internals, or you heated the o-ring to much. Air Nozzle Do the above step again except for put the air nozzle on with the tappet plate and tappet plate spring attached inside the gear box. If you can push the piston forward after the ports in your cylinder, you have bad compression (obviously). Try placing a small o-ring (sorry, don't know the size) at the bottom of the air nozzle and gluing it on (make sure to not block the air flow) then put it onto the cylinder head. That should help. if you still have a leak, you either did not use a small enough o-ring or you have a crappy air nozzle. Try buying a new one that has a built in o-ring inside of it, aswell put a o-ring on the bottom of the new one as it will help. No need to lube the air nozzle. Wrote this all at one time so it might have some errors. If there is anything to make it better, something I wrote wrong, or if you have any questions, please suggest it/ask it. Thanks.