Just taken apart an XC Pro thumbshifter (right side). I couldn't find anything to help me, so thought I'd put a little "How To" together.
When you're taking it apart, it's all pretty simple apart from the circlip (#14) and the tiny ball bearings that create the clicking in component #11. Be very careful not to drop them - there's nothing holding them in and they'll be impossible to find (or replace, I imagine).
Here are the component parts, with number 16 being the item that came off the shifter first:
To put this wonderful piece of machinery back together, take the pillar (#1), drop on the washer (#2) and place the lever on top (#3).
Next, drop in the washer (#4) into the lever housing. The next two washers (#5 and #6) provide the spring tension and have to be dropped in carefully, the first with the concavity upwards, the second with the concavity downwards. Next drop in washers #7 and #8.
#9 goes on next - note that the tabs are different sizes so there are only two ways it'll go onto the pillar. It's a symmetrical item, so if it goes on, it's right. Then the washer #10 goes on. #11 goes carefully on next ***DON'T DROP THE BALL BEARINGS*** - YOU WILL NEVER FIND THEM AGAIN
. Again, it's symmetrical and can go on two ways - if it fits, it's right. Then #12 goes on with the indentations facing down (they engage with the small ball bearings to ensure a firm click).
Next slip on the cap #13. If you look carefully, you'll see the knurling inside has one part cut out - that bit engages with the largest tab on the pillar as you drop it in. Now you'll need to put in the circlip - it can be troublesome so be patient. All you need to do now is engage the pillar (with all the rest of the items now mounted onto it) onto the shifter body. You'll see that the tabs on the bottom are irregular sizes so you have to make sure it's the right way around. Add the locking screw and you're done.
To check it's working right, make sure the collar isn't on "Friction" and then give it a shift - you should hear firm clicks all the way through the gears. Sweet.
You should end up with something like this:
Assume an epic posture with an expression of fearsome omniscience on your face - you are now a god.