In my experience (read: opinion) the Shimano M900 gruppo is one of those defining mountain bike component gruppos. They looks amazing on nearly ever bike (92-95 of course) and most high end projects would not be the same without them. Of course I am not the only one who thinks this way and consequently these parts are very hard to find and more often then not come at a high premium for mint to NOS parts.
Used and abused parts are not hard to find. Probably the most abused are the cranks and rear derailleurs. I currently find myself replete with about 10-11 M900 gruppos, ranging from beat to Mint/NOS and among all those parts I noticed that the cranks are one component I can probably restore.
As best as I can tell the cranks come from the factory with a complicated finish process. First they appear to be forged and then the critical features were machined for precision. Then they are clear anodized to prevent the Auminum from oxidizing. The last step is a satin finish clear coat to achieve that unique bluish/gunmetal finish M900 is known for. The Shimano M900 is either ink stamped or applied via transfer decal just prior to clear coating.
So, here are some early pics of my first attempt at refinishing M900 cranks. I've only done some basic surface prep in the tumbler. still a lot to do to achieve the polished finish to get it ready to anodzing.
Above - Top crank is a scratched up example with clear coat stripped / Bottom crank after 2-3 hours in tumbler (was in same condition before)
Below - Close up pics of both cranks for comparison.
So, the tumbler removed a lot of the surface imperfections. Now I need to do some manual clean up on the end of the crank arm to remove some deeper gouges and then back into the tumbler for a higher polish finish.