Step 1: On the left side of the Sig 552 gear box, there is a small Phillips head screw on the safety switch gear on the mech box.  Loosen and remove it.

Step 2: The entire remaining gear shaft will come out from the right side.  Set all those pieces aside where you won't lose it.

Step 3: On the right side, the wires for the motor run under neither a plastic gear guide.  Pull them out of the way.

Step 4: Now you must remove the plastic gear guide.  It must be 'swung' out of position.  Slightly pull up on it.. NOT a lot about 1mm.

Step 5: Underneath the plastic gear guide is a metal lever.  Pay attention to how it comes out and remove it and this is what you should have.

Step 6: At the very front of the mech box is a Phillips head or Torx bit head screw, remove it.

Step 7: Now that the screw is out of the way, you can slide the rail clip off of the top of the mech box.  This can be done by hand but we'll show you the easy way.

Step 8: The clip is held in place by a raised part of metal.

Step 9: Take a flat head screw driver or something similar and carefully push forward on the back of the clip, be sure not to slip and stab your hand or anything.

Step 10: Once it has gotten over the raised metal, it can be easily slid out of place.

Step 11: Under the front of the mech box there is another torx head/Phillips head screw.  This screw also secures plastic tab.  Honestly I don't remember what it is for, but anyway remember which way it comes off.

Step 12: At the bottom rear of the mech box where the motor goes, are two more Phillips head screws that hold on a plate as well.  Loosen and remove them and remove the metal plate.

Step 13: Now you only have two more Phillips/Torx head screws to remove.  BE SURE TO HOLD THE MECH BOX after you have removed the last screw.  This is so it doesn't 'magically' pop open and have all of the internals fly away. It most likely won't but better safe than sorry.

Step 14: Carefully begin to pry the mech box open from the back.

Step 15: When the mech box is open about 5mm take a small shank screw driver and insert it into the back of the spring guide.  You may have to 'angle' it into place.

Step 16: With the screw driver in place holding the spring guide, hold the screw driver firmly so that the spring doesn't 'pop' and completely separate the two halves of the mech box.

Step 17: As you get the top of the mech box open, with the hand that is holding the screw driver from step 16, move it into place to hold the spring in place.  This is so that it doesn't 'bend' and pop out of place.

Step 18: Firmly push in on the spring and remove the spring guide from it's 'seat'.  This is what you should have.

Step 19: Now take some time to notice if you have any shims that are still attached to the top cover of the mech box.  I recommend that you place those back onto the gears themselves.  BE SURE to put them back on the corresponding gears.  Remember, it is a MIRROR IMAGE.

Step 20: Probably the hardest part about reassembly is the trigger mechanism.  It is a two part lever system.  I can't really tell you how it goes together but just pay attention to that when you reassemble the mech box.

Step 21: Pull out the stock spring.  THE STOCK PISTON IS ATTACHED to the stock spring, so remove the entire assembly.

Step 22: Grab the piston head with a firm grip and twist it off counter clockwise.  If you use a paper towel, it gives you a bit better grip.  Just remember to reapply any grease that you wipe off.

Step 23: Now that the piston head is off, the spring will come out easily from the piston.

Step 24: Notice that the spring is wrapped around a 'thing'... I don't know what it's called.  Remove the spring from it.

Step 25: Notice that the 'thing' has a certain orientation in the piston.  This is the piece that the piston head actually screws into.  YOU NEED IT.

Step 26: Place the 'thing' into it place and screw the piston head back into place.

Step 27: Check your spring.  Notice that the M100 spring is longer than the stock spring. 

Step 28: Place the piston back into the cylinder with the gear teeth facing down towards the gears.  BE SURE TO REPLACE THE GREASE you may have wiped off in step 22.

Step 29: Make sure that the piston is in its 'groove'.

Step 30: Place the new upgraded spring into the back of the piston.

Step 31: Check to make sure that the tappet plate is not pressing against the tab on the sector gear, otherwise you may damage the tappet plate during reassembly.  We place the tab near the corner of the tappet as shown in the picture.

Step 32: Place your spring guide into the back of the new spring.  Using the screw driver, carefully place the spring guide back into place.

Step 33: Reassemble in the reverse order.

Step 34: Remember the trigger assembly?  Use a screw driver to help 'guide' the levers into place.

Step 35: You are done.  Enjoy.