XTR M900 trigger rebuild - the complete HOWTO

[Thias]

Hi there!
The weather is cold and wet outside. Time to wrap yourself in warm clothes and stay inside. And also time to do all those time consuming cleaning tasks on your bike. There wont be time for that during the summer!
So here we go with new lube and less dirt in your M900 trigger. Scared already? :twisted:

First thing is to remove the lower cover. In my case its just 2 screws, no need for explanation. Except that there is a version with just one screw for the cover.

After that the main nut can be removed. Be careful, its a LEFTHANDED THREAD!

Please note, that in the picture above the plate holding the spring to retract the thumb lever is mounted the wrong way. Seems someone took that trigger apart before. :facepalm:
Also note, that the main shaft with its 6 grooves allows those plates to be slided on in two ways. And only one of those is right.

After that you can pull off the first lever, together with its spring and its (slide bushing) washer:


On the lever there is this small hook-like part that is supposed to move very easily. If you trigger had problems switching with the thumb lever slipping or not engaging this is the part that is causing it.
The hook is held by a small snap ring. Slide the snap ring off to the side, take out the hook and clean the little axle and the hole. Use a freeze resistant grease before putting the hook back in place. And don't forget the little spring.

Taking off the second plate you get to see the "trigger finger lever assembly" - one slide bushing washer, another spring and the lever.

Take them off. And also that thin brass washer plate


Underneath that is the ratchet mechanism.


This little bad boy is the most complicated to reassemble. So read carefully now, I warned you! :roll:
There are tree parts that need to be marked before taking apart or it will be complicated to find the right position later on. Those three parts, I don't know their correct designation, I will call them "outer ring", "inner ring" and "main shaft with base".
So. Be sure to mark the position of the inner ring to the main shaft. And the outer ring to the inner ring.
Then you can pull out the main shaft (spring: boing!)


and separate the inner from the outer ring and you're done disassembling.


Congratulations. :xmas-big-grin:

to be continued ...

 

[k-rod]

Re: XTR M900 trigger rebuild

Wow - this is priceless information, and so nicely and clearly presented! Thank you immensely.

 

[Thias]

Re:

Is anybody even interested in the reassembly? :roll: :mrgreen:

 

[k-rod]

Re: XTR M900 trigger rebuild

Jeepers dude, lay it all out ... inquiring minds want/need to know!!!

Seriously.

 

[Thias]

Re: XTR M900 trigger rebuild

OK. So here comes the rebuild.

After cleaning all the parts (and oiling them again if you were using degreaser ) take a closer look at the spring that came out between the base and the outer ring. It has one hook pointing sideways (that fits into the plastic part of the outer ring) and one pointing upwards (that goes into the little hole in the base of the main axle). If you look at that spring from above, the first mentioned hook pointing at 12:00 position, the second hook should be pointing at something between 8:00 and 9:00. Then the spring is ok. If it is pointing to anything "later" than 9, it has gone weak. One possible solution for that (besides take a new spring) is to shorten it a bit. Snip off 5mm and bend a new hook.

With the spring sorted, grease the base, the spring and the outer ring, fit the spring like so:

fiddle the other ends hook into that little hole like so:

and be sure not to use that second hole. It won't work because of the spring rubbing against that white plastic!

Slide the base all the way up into the outer ring, then use your markings to put the inner ring onto the axle. If you for whatever reason have no marking :roll: you may want to take a closer look at the following pic:

This is how it should look like before you proceed to the next step. That one (not springloaded) hook of the inner ring must be in line to the little cutout in the base.
Dont' slide the inner ring completely in yet. Just as far as shown above. Because now you have to put the right amount of preload on the spring. It would be no fault to first mount what you have now assembled into the trigger housing and fasten it to the brake lever so it wont come apart again when you now try to tension it.
Now turn the outer ring clockwise for about 130° so the spring supported hook pointing out of the inner ring fits into the cutout of the last gear in the outer ring (is that even correct language??) and slide the inner ring in all the way. I put a marking on the following photo so you get what I am talking about:


The next steps are as easy as putting everything together as you took it apart, but obviously in reverse order. That spring that retracts the "trigger finger lever assembly" might put up a little fight. But at least for the other spring, I have a last "top tip":

First hook one end of the spring onto the lever. Then fit that disk thingy while hooking the other end of the spring into position 1. Then refit the main nut (left handed thread...). Make sure there is just a tiny little bit of tension on that spring now, even in position 1. Else rebend it a bit. Also make sure all the parts are aligned correctly, nothing is tilted, none of the last springs coils is squished underneath the disk thingy. Then just pull the hook into position 2 to put tension on that lever.

The End



...

Oh, no. There is one more thing. As I mentioned in the first post, there is two different versions of m900 shifters. The differ from the outside, but also very much from the inside:






So what should be next on my list? The left hand side or the second version?

 

[k-rod]

that certainly doesn't look very easy (I have the XT version of those); and how on earth did you ever manage to figure all that out? Even more surprising, is that some engineer designed the whole assembly in the first place ...

thank you for the pics and steps!

 

[Thias]

I guess I have a knack for this kind of puzzle. And I took apart and put that dang thing together at least 20 times in a row. I think I am now able to do it blindfolded

 

[scottproonly]

I guess I have a knack for this kind of puzzle. And I took apart and put that dang thing together at least 20 times in a row. I think I am now able to to it blindfolded

Have you ever thought of becoming a watchmaker?

 

[syncrosfan]

Very Nice! I have a couple of M900 shifter combos that need attention. This will greatly help me (bookmarking this right now!).

 

[regan_ev]

That's an awesome bit of work and a very well-presented write-up.

I read a comment in another thread earlier where someone suggested RB gets a technical section. This thread is just the sort of thing that would make such a section worthwhile. So to make it complete, get both the left shifter AND the other version posted up.

(And how many people like me we unaware until reading this that there even were two different M900 units? I always thought that was the reason Shimano gave component revisions different numbers to properly differentiate them!)

Top job