I put the scanned in instructions for replacing just the 41A2817 gear at the very bottom of this web page.

Unplug the thing.
When you take the cover off the bottom of the garage door opener box, your gear may look like mine.  Lots of white plastic shavings from the stripped gear.  



You will see three sheet metal screws on top of the lifter case that hold the whole gear assembly in place.  Look on top, where the chain sprocket is.  They unscrew from inside (below).  But you can't pull the gear and shaft out of the top until you remove the little plastic snap on retainer on the end of the shaft.  Close up of old one, and new one going on below:





Here are the pics putting the new on, so you can figure out how the old comes off:









Once you get that clip off, and slide off the little white gear, you can lift the whole chain sprocket, and the shaft with the stripped white gear out the top.  
Again, here it is, pictures taken during the reassemble so the parts are all clean and new, but you can see how it lifts out:

In pic below, little gear on end of shaft is off, and you can then slide the whole assembly out the top :



Below, pulling assembly through the top:

This (below) is the 041C4220A. Also try looking for same part number without the leading zero, "41C4220A".    



So you could just put in the new assembly at this point, and re-use the old  worm gear (the long white gear on the motor shaft that is still in your opener at this point).  My worm gear didn't look too bad, but I replaced it anyway, since the kit includes the gear and everything you need to replace it.  But if it looks good, leave it, it gets a bit more complicated to replace it too.

Again, if you just need to replace that big white gear, the shaft and chain sprocket, you could just drop in the new one at this point, grease up the gears real good, and you are done.  Pretty easy!

You unclip the mechanism that controls the travel limits, it is this part. Little worm gears that cause copper contacts to close at each end of their travel.  



You have to unplug these wires that go to the "RPM Sensor" at some point:



Then it unsnaps from the plate it sits on, just press the tabs.  






Let it rest to the side.  Now you have just the motor held in by a few screws, and the wires can be unplugged.  They are all color coded, so you can easily tell,  you re-connect red to red, etc.  Next few pics:



You will unplug the motor wires here:



Once you lift out the motor, this will be what is left in the garage door opener case:



Now, to replace the worm gear, pull off the plastic end, notice how it rotates it's little splines through that electric eye (RPM sensor).  That is how your lifter knows it is rotating, and this must be put back on and connected, and the electric eye clean.  Anyway, pull off the black plastic end of the shaft the worm gear is on, then unscrew the allen screws and pull off the rest of the end bearing and cap etc.  




IMPORTANT POINT!  Reading other people's experiences, this is where you want to lay out the pieces as you remove them, in order, and line up the new parts in the same order.  It is the only way to know you put it back together right.  Below:
old parts lined up as they came off, new parts ready to slide on, after I put on the spankin new white worm gear.  It just slides on.

You may have trouble sliding the bushings and parts over the motor shaft, or find it hard to slide the old parts off.  I had to file a few rough spots on the motor shaft with a flat file.  Didn't take too long.  If you force these thin bushings they can break, so file the shaft if they won't slide off.  







Put all those pieces back on, grease the shiny bearing parts, and when you put the black cap on the end, clean the electric eye and make sure the tines pass through it: Below:

Cleaning the electric eye with a small paint brush:



And the black cap back on the end: (the splines MUST pass through the eye like you see here)


Tips:
Attaching the mounting flange of the new gear / sprocket shaft assembly to the top of the garage door closer can be difficult.  The pre-drilled holes in the flange of the gear / sprocket shaft assembly may only line up properly in one specific position, meaning you may have to rotate it and look through the little screw holes for the best alignment.  Also, the holes in the new flange are not threaded and the threads will be cut by the screws as you put them in.  This makes the screws turn really hard the first time, so try them in the holes and turn them in with a wrench before you are trying to do it over your head.  Put the screws in, then of course you will need to take them back out to mount the gear / sprocket shaft assembly from below, but at least now the screws will go in easier and straighter.  (this tip was suggested by a reader) 

One or tips you might want to add;

 After sliding the gear assembly in from the top, and before you insert and tighten the screws, make sure the sprocket cover slots on the top are aligned correctly. (I think he's referring to same as the first "tip" a few lines up, "slots" or screw holes the same I think.)

Grease the gears with the supplied grease.  

The worm gear replacement is the harder part, and in my case, not the gear that was really torn up.

The gear kit is readily found on the internet, shop around for the liftmaster part number 41C4220A:
  




Let me hear from you if you found this useful!  dwdlift@spamex.com


The written instructions that come with the kit!







For larger diagram below, right click, "view image", then click the little "+" sign, should get you full size.  







If the light on the back here is flashing a code, and you are having a problem making the opener work right, here is what the flashing codes mean:
       # of flashes

• 1 The Protector System wire open or totally misaligned
• 2 The Protector System wire shorted or black/white wire reversal
• 3 Door Control or Multi-Function control Panel wire shorted
• 4 IR sensor slightly misaligned (dim or flashing sensor LED)
• 5 Motor overheated/Possible RPM sensor failure -- unplug to reset
• 6 Motor circuit failure -- replace RX/LX board
• 7 Rapid Flashing - Chamberlain transmitter with the correct format code being received

OK, I don't want this to get confusing, but here are the written instructions if you want to ONLY replace the stripped, large gear, the 41A2817 and the worm gear. You have to get the gear on and off the old shaft and re-use it.  I did not do that, but I wanted to give the instructions to you below.  Again, so you can decide if this looks like something you want to do.  Personally, I don't think the cost savings is worth the hassle of re-using the shaft, chain sprocket, old bearings.  And those roll pins would not come out for me!
  Here is what you need if you want to pound out the roll pin (part "G" in the diagram below) that holds the gear in place (click here, it explains what tool you need, a 5/32" punch or similar).






chamberlain / liftmaster / sears / gear kit