M1A1 Thompson Upgrade If you are mechanically inclined at all. This should be no problem. I'll start off with a list of things you will need. Thompson M1A1 Whisky Cigarettes Tools: Flat screwdriver No. 1 Phillips screwdriver No. 2 Phillips screwdriver 1,5 hex driver (fancy name for a very small Allen key) Lithium grease Silicone grease Parts Spring (I suggest a Prometheus MS110 Should put you around 350-360fps) Metal Bushings (These are a must have) Shim set Spring Guide (optional) 20Amp fuse change your fuse from 15 to 20 Amp or your fuse might blow after the upgrade. I like to put down a white towel on the surface I'm working on. This helps prevent those tiny little pieces getting lost. Uh...the mallet is there for when you get pissed off because things got lost anyway and you want to smash something. First thing to do is remove the stock. With a flat screwdriver, take out the 2 screws connecting it to the receiver.
Pull the stock off and disconnect the wiring harness. Set the stock and screws aside. Make a note of which screw goes where. Push the exposed connector up into the body of the gun. To separate the upper and lower receivers, push the little button at the back of the receiver.. Slide the upper receiver forward and the pistol grip back.
The gun comes apart in two nice neat pieces. Set the upper receiver and barrel aside. You won't need it until you're ready to put the gun back together. Next step is to remove the pistol grip. Pretty simple. Take out the screw with a flat screwdriver and just slide it off. Next you have to take off the selector switches. With a small hex driver loosen them until they can be easily slid of their posts. You don't have to take the all the way out. That's how small pieces are lost and the afore mentioned mallet gets involved.
THIS IS IMPORTANT. If you have read any reviews about upgrading Thompson. People always say they lose their semi auto. This is because they lose the very tiny pin that sits into the selector switch. DO NOT LOSE THESE! Next remove the 2 selector posts. Use a No. 1 Phillips to unscrew them and put them beside their respective switch.
Next, disconnect the motor wires. They just pull off. Try not to bend the connectors. They will break. Then you will most likely need to use the mallet again. Now you can disconnect the motor. Use a No. 2 Phillips to take out the 2 screws. The motor should then easily pull away. Just set it aside with the 2 screws.
Position the mechbox so that the nozzle is facing to your right (See picture below). Slowly pull the mechbox from the lower receiver. Do it slowly and carefully because there are a couple very important pieces you may lose. Remove the two black selector discs and put them with their respective switches.
Remove the selector plate and put it with the switches. It just pulls off. Next step is to remove the trigger housing. It's just the very front and the back screws. Both are a No. 10 Torx.
Next remove the top... thing...?... I don't know if it has an actual name. Just use a small flat screwdriver and slide it off.
OK. Now you are down to the bare mechbox. Go and have a smoke and some Whisky. Remove the 3 remaining screws. Set them aside in the order you took them out. Remember where the spacer is located.
Slide a small screwdriver into the back of the mechbox and push down to take up the spring tension. Now pry off the upper shell of the mechbox. Pull up on the screwdriver and disengage the spring and spring guide. Be careful it doesn't shoot across the room. (I've seen it happen) The spring and the piston will be attached to each other. Set that aside for now. Lift out the cylinder and tappet plate. Do it slowly so the tappet plate spring doesn't go flying off to a corner of the room. Set them aside.
Now you can remove the gears. Just put them aside for now. This is your anti reversal latch. Make a note of how it sits and remove it. You can now pop out the stock nylon bushings and replace them with your new steel ones. I used Systema Version 6. Try to put them in straight. They should seat nearly flush with the inside of the mechbox. DO NOT hammer them in. It takes some force with your thumb but you shouldn't need to pound on them. The next step is the tricky part. Shimming. Sometimes it's a piece of cake. Sometimes... not so much. I just copied a post that HoJo put up. 1. Shim one gear at a time (spur, then sector, then bevel is easiest, but anything will do) 2. Aim for gears that are one level away from being tight. They should be able to move freely, but should be one shim away from being tight (i.e. minimize sideways movement) 3. After shimming one gear, shim the next gear so that it is 1 shim away from touching the other gear (flat surface to flat surface), so that it's just a little higher than the first gear. You want maximum tooth connection, but without any other part but the teeth of the gears touching each other. 4. Remember that the bevel gear is pushed upwards (if the mechbox is open and facing you) by the motor, so make sure there are some shims on top to hold it down, should it be raised too high, and lose connection (or make poor connection) with the spur gear. 5. Get used to opening your mechbox a dozen times. Every time you change a shim, you should seal your mechbox with the 3 screws around the gears, so that you'll get an idea of how tight things are when the mechbox is sealed. 6. When in doubt, or having problems, shim looser. I've seen guns where the teeth are just 50% touching, and they still worked fine, but won't last as long, so you can live with loose shimming just fine. 7. Think about what you're doing, and what you're aiming for... the physics behind shimming are at about a grade 1 level, so there's no mystery to why it works or not. 8. Make sure your metal bushings are in flush with the mechbox, poorly installed bushings might be causing problem you may attribute incorrectly to the shims. 9. Shim in haste, repent in leisure? I could spend a couple of hours telling you step by step what to do...but I won't. Your best bet is to do a few quick searches and read all the information for yourself.
If you've gotten this far, you should indulge in some more of this. Once you think you have shimmed nicely. Grease your gears with a small amount of lithium grease. Don't go crazy with the stuff. You really don't need that much. Set the mechbox aside. Now it's time to get the stock spring out. There are 2 ways to do this. The easiest way is to put a cloth over the piston head and just unscrew it. Sometimes that doesn't always work though. So you have to take a sharp knife a cut away some plastic on the front of the piston head to expose the screw. Use a Phillips no. 2 to remove this. The head will come off and the stock spring and metal piston headlock will slide out of the piston. Just set the old spring aside. Place the metal piston headlock back into the piston the same way it came out. A little pin on it should seat into the piston. Now you can screw the piston head back on. Now you can start putting the gun back together. Clean then regrease the piston head O-ring. Slide it back into the cylinder in one quick motion. Put the tappet plate and nozzle on the cylinder.
Seat the whole thing back into the mechbox. Make sure the tappet plate and tappet plate spring are seated correctly. You should be able to push the nozzle in and have it spring back. Your mechbox should now look something like this: There seems to be some conflict among people as to where the post on the spur gear is to be located. I put it at the 1 o'clock position. Why? I just do. Now you can put in the new spring. The end of the spring with the tighter coils is where the spring guide sits. Put the spring guide in then slide the spring into the cylinder. Push it all the way in until you can get the spring guide into its slot. Put a small screwdriver in it to hold it there.
Place the top half of the mechbox on. It should click in place easily. You may have to fiddle with the anti reversal latch to get it to seat properly. Replace the 3 screws that hold the mechbox together and slide the...whatever it's called back on the top. Reconnect the trigger housing. Put the selector plate and the 2 selector switches back onto the mechbox.
Slide the mechbox back into the lower receiver. Install the selector switches and motor
Slide the pistol grip back on. If the wires are in the right place it should just slip on easily. If it doesn't. Relocate the wires until it does. Rejoice with some more Whisky. Assemble the rest of the gun the same way you took it apart.