Drill the head off and use vice grips like was mentioned above. I had that happen to me once and was able to use one of those small dremel cutoff wheels to notch the head enough so I could back the bolt out with a flat head screwdriver. Using a hammer to tap a small chisel would do the same thing but I'd be hesitant to hit it too hard in fear of damaging the actual primary cover.

 

...exactly what I did when I did this exact same thing on my wife's old 883L. Mistakenly used the T-25 instead of 27. Was able to "tap" in the 27 to the chewed up screw and got it out. Changed 'em all over to a stainless allen head bolt.

 

Too bad studs and hex nuts with washers are not used.

 

I have used a left hand drill bit to with the drill in reverse. This some times puts enough torque on the screw to loosen it up.

 

i had a similar situation but not on a bike. AND the head of the phillips head broke off into the screw. No way to dril this supposed “hardened steel”. I used a dremel to cut into the screw head to make a slot for a screwdriver. Took some time but worked

 

Victory!!
After kicking myself for buying a Ryobi screw removal tool ( as suggested elsewhere ) but not before it breaking off, I attacked said screw with a couple of small dremel cut off wheels and finally removed enough pressure on the screw that it then easily backed out. I'm puzzled why this screw, out of the other 4 managed to wedge itself in nicely. I inspected the chain case cover and there was no damage; no filings dropped into the reservoir. No nuts appeared stripped. No stress fractures or any other apparent damage. So, my misadventure cost me about $80.00 for a stock derby cover and 5 nice new stainless screws. The original cover is damaged, but looks like it would contain fluid properly. I could hide the damaged hole under the passenger foot rest, but decided to use the new one and will keep the original as a reminder . . . for what I'm not sure! And for those who were curious, yes I have both a manual and two torque wrenches which I use religiously. I changed my own fluid last time, and used the torque wrench to tighten all the nuts to the specs given in the manual. Looking at the screws, I can only guess that perhaps it oxidation or electrolysis of some sort caused the nut to be stuck. Thank the almighty for dremel tools and cut off wheels . . . civilization would fall without them.
Thanks to everyone who posted . . . the help was greatly appreciated.

Mark!

 

This will take care of your problem. Ace hardware sell chrome head allen bolts that will solve your current problem forever. All the gaskets can be reused if handled properly during disassembly. Hope this helps.

 

I don't know about Harbor Freight but Northern Tool has a socket,and a set of sockets, with T27 bits installed. Might be a good investment for you.

 

Turns out I did have a T27 in one of those swappable screwdriver things sold by H-D . . . T25 there as well. I just missed that one . . . didn't even think to try and use a T27. I have nice sets of Hex and T from Craftsman and HF . . . just no T27's in those sets!! I did solve the problem, but I now know that T27 == Derby!

Thanks Again!

Mark

 

I haven't seen this mentioned yet so here's a little tidbit of advice for you. Next time you need to change the primary lube, remove the derby cover before you drain the fluid. That way if you have a stripped T27 you can still ride the bike until you get the stripped bolt out.