Sunday, May 11, 2008

Shimano XTR RD-M950-GS overhaul

After almost 12 years of faithful service, the 1997 Shimano XTR RD-M950-GS rear derailleur (apologies to Sheldon Brown), procured in November 1996 — spec'd on Ivy, a 1997 Specialized Stumpjumper M2 FS — finally threw a fit and demanded an overhaul in no unequivocal terms.

Oh, she's throwing a fit, all right. No negotiations this time. Time to break out the tools.

SteveUK compiled an excellent write up of the steps here (Thanks, Steve!), so I shall not belabor a duplicate effort. Membership (it's free) is required to view images on the MTBR forum, but you can always cross-reference the text to the composite image above.

An exploded diagram of a generic rear derailleur. Parts count (and names) may vary but you get the general idea of how things come together.

Shimano XTR RD-M950 rear derailleur partially disassembled.

Full-strength Finish Line Citrus Degreaser may be used for the metal parts, but regular dishwashing detergent is recommended for O-rings, seals, and plastic components.

Note:  the 1.6 mm (a little under 63-thousandths of an inch) ball-bearings WILL slip into the drain. Degrease these 16 tiny spheres separately in a container. E.g. your colleague's or mother-in-law's coffee cup / cereal bowl.

Lay out the parts on a tray and use a fan to speed up drying.


Tension Pulley sub-assembly. The seals can be removed with a seal puller (or, very carefully, with a large sewing pin). More information (and pictures) found at Pulley TLC II.

Centurion (Thanks, NicIz2HardKore) Centeron Guide Pulley. The seals can also be removed in the manner described above. More information (and pictures) also found here.

Tension pulley (left) and Centeron pulley (right). Rear derailleur cage inner plate (bottom). The titanium pulley bolts (top) are from HBC.

Derailleur pivot bolt sub-assembly. The larger O-ring sits under the head of the pivot bolt. The smaller O-ring sits on the middle notch. Together, both keep crud out and the grease in.

Body arm sub-assembly. The cage stop bolt (far right) prevents the cage arm from unwinding all the way around from the spring tension. One end of the spring has a longer prong. The longer prong fits into the derailleur body. Re-assembly would be impossible the other way round.

The notch hole closer to the Centeron pulley bolt hole is the factory setting for the spring tension. Using the other setting increases tension in the lower part of the chain, thereby reducing the amount of chain bounce on rough terrain — but at the expense of greater drivetrain drag. Some riders prefer this set up, as a higher chain tension means a lower incidence of dropped chains.

Bolts and other parts.

Top row, left to right: hollow cable tension bolt, spring, detented cable tension bolt plastic sleeve, cable fixing bolt, cable fixing washer.

Bottom row: upper limit screw, lower limit screw, cage stop bolt, B-tension adjustor bolt.

Only the hollow cable tension bolt and the cage stop bolts are stock; the other bolts are titanium upgrades from HBC.

RD-M950 rear derailleur disassembled. There are 56 removable components (if you include the next step). Some individuals remove the return spring in the derailleur parallelogram with a pair of needle-nose pliers but, IMHO, that's an unnecessary risk. A small toothbrush under a running tap cleans everything in there just as well.

This step is not covered in SteveUK's walk-through. The cage arm (also called "outer plate") itself is another sub-assembly. The stainless steel washer sits under the head of the bolt, while the O-ring sits at the first notch, keeping a miniscule amount of grease in. The thick washer at the bottom slides over the threaded portion of the bolt after the bolt + stainless steel washer + O-ring is re-inserted into the front derailleur cage shaft.

This image should make things clearer.

A drop of Loctite™ on the last thread of the shaft helps the thick washer seat. A small drop will do. Too much and it may migrate into the shaft / bolt area and prevent the cage arm from rotating.

After the O-rings are washed (with dishwashing detergent, not degreaser!) and dried, silicone spray rejuvenates them. You can get the Trident Silicone Spray (Part # LP27) or equivalent from any SCUBA diving shop. Divers use this to maintain their masks, mouthpieces, fins, wet suit zips, et cetera. It is also good for plastic parts. Do not use Autosol™ as it draws out the plasticizers from your plastic components, giving a shine but rendering them brittle, reducing their life.

Supplies for lubrication and re-assembly. You don't need both Loctite™ 242 and 243. 243 is dearer but it is oil-resistant and slightly stronger. The anti-seize lubricant is required as the RD-M950 pivot bolt comes stock in titanium. As for grease, you are spoilt for choice, but avoid lithium grease as it washes off easily. Personally, I like Finish Line's Premium Grease as it doesn't wash off. The Finish Line Stanchion Lube is incredibly slippery and penetrates well. I like to put a drop of it on the pivots; working it around, and then dripping a drop of Finish Line Dry Lube over it.

A 1 ml syringe facilitates this operation with a minimum of fuss. Be careful; if you inject yourself with the Teflon™ liquid, you might wind up running for office.

Shimano RD-M950 rear derailleur overhauled, re-lubed, and reassembled. Don't forget that your B-tension, lower limit, and upper limit screws are now all out of adjustment.

Derailleur reinstalled and tuned.

Some Shimano rear derailleurs do not have a cage bolt, but utilize a 2 mm retaining bolt instead. Mac Attack II has more information on this here and here (MTBR membership required to view images).

Another one from Mac Attack II, this time for a Shimano Sora road rear derailleur. To remove the access cover, pry the access cover equally from both sides. More info here (MTBR membership required to view images).


cowbot said...

THANK YOU for posting this!!!
I have same bike i want to keep running. And i NEED to overhaul soon! Thanks!

My big problem is finding a replacement fork. Can you possibly give suggestions? I wouldn't mind a slightly higher height/travel 80-100mm.

-ben said...

Hi cowbot,

You're welcome! Nice to meet a fellow stumpy owner :-D

With regards to a fork, I really like the Fox F80 / F100 RLT forks. Price is a little on the steep side, and annual maintenance is a an additional factor, but they are stiff (and, as the Munda Biddi Trail Stage 1 proved, robust and reliable, even with a Bob Trailer loaded to the max). Hope that helps!

Ride on!