Shimano DX RD-M650 SGS overhaul

This derailleur came with Michelle, my 1991 Bridgestone MB-3. Apart from the replacement / upgrading of its jockey wheels to Control Tech sealed cartridge bearing pulleys, the unit has never been serviced.

A picture of the derailleur in its stock form here.





After an external scrub down.





Somehow, the B-tension screw bent over the years. I replaced it with a generic stainless steel 4 mm x 12 mm allen-head screw.





The steps for disassembling the derailleur are similar to those posted for the Shimano XTR RD-M950 GS overhaul. Exceptions are noted in this post.

The cage arm is secured by a single Philips head screw. Removing it allows the sub-assembly to slide out.





Contrary to the RD-M950, the longer prong of the RD-M650 cage arm spring fits into the cage arm. The factory position (tension setting) is the hole closest to the end of the arm.





The o-rings compressed over the last 17 years, allowing ingression of dirt and water.





Derailleur body.





Fully disassembled and cleaned.
41 parts.





All the o-rings were replaced.





For a little more reliability, I replaced the stock Philips head limit screws with stainless steel variants (4 mm x 16 mm set screw w/ 2 mm allen head). HBC's titanium versions will work too, but are overkill for this application.





Shimano RD-M650 rear derailleur overhauled, re-lubed, and reassembled. Don't forget that the B-tension, lower limit, and upper limit screws are now all out of adjustment.


Specifications
Total capacity:  38T
Minimum gear:  12T
Maximum gear:  32T (although I gotten away with 34T with some fiddling)
Weight:  320 grams


At some point in future, the derailleur cable clamp nut (which is beginning to corrode) might be replaced with a corrosion-resistant HBC titanium version.