Derailleur Pulley Bolt
 

Sadly, on trying to replace a pulley, the bolt head is stripped. Any tips on how to get it off?

Also, I'll need to buy a new bolt but I'm unsure, while searching online, what will be compatible with my Shimano RD-M663 derailleur.

https://cimg8.ibsrv.net/gimg/bikefor...e0f8c7e262.jpg

File or dremel screwdriver slots into the bolt head.

+1 ^^^.
-Tapping a torx bit into the hole may have enough grip to turn it,
-Bolt/Screw extractors Screw Extractors - Ace Hardware
-Remove the other pully bolt and try to rotate the cage halves in opposite directions which may loosen the bolt a bit,
-Drill off the head of the bolt and you'll have a long enough stud left over to grab with pliers.

EV-RD-M663-3043A.xls (shimano.com) Gives you the part number (#10 or 11 in the diagram) to do a search for a replacement or go to your local bike shop.

These small bolts and such are often sourced from damaged or old units sitting in the LBS or local bike recycler's "not yet tossed out" boxes. Also know that many small bits are shared across several grades of ders. Sometimes the only difference is the finishing of the bolt (plated, oxide coated, galvanized or painted). Andy

A BIG downside to screw extractors/easy outs is that if you break one off you now have a piece of hardened steel to contend with, too.

Thanks folks - I've sent for the drill bits. Fingers crossed.

Mission accomplished, thanks to a combination of methods offered here and other forums:

The new pulley bolt arrived today. I removed the bottom pulley and with the chain on the smallest sprocket, I was able to rotate the derailleur's outer cage plate after having removed the limiter-screw. This allowed the bolt to turn anti-clockwise enough so that, when I inserted the allen-key and put downward pressure on the tool, I managed to get sufficient bite into the bolt to get it fully turned and removed. Hallelujah!

Thank goodness I didn't have to resort to drilling etc. Thanks also to the denizens of this thread for the inspiration, freely given, which spawned a solution to a tiny bolt with a big headache.