Yesterday, I started disassembling a bike I just bought.
Everything went well till the last part I had to take off the bike, the NDS crank arm.
For some reason, I had a bad feeling about this last bit of dismantling, everything just went too well.....
Sure enough, the Campy crank dustcap is well seized on to the crank arm, and previous removals had pretty much worn out the allen head hole to make any attempt of using it to take it off, uyseless.
After some well placed cuts with my Dremel, a copious amount of spray penetrant lube, a couple of twisted coins and bent screwdriver tips later, it slowly let go of it's grip on the crank threads and it finally came off....

I know it's an ugly way to win, but Yay! Victory! The crankarm survived the traumatic removal unscathed! On to the clean and re-build phase!
Funny thing was, While waiting for the bike to be delivered, I bought a pair of NOS Campy dustcaps (on sale for 20% less) after I misunderstood the seller calling out a dustcap missing from the bike on his ad. It turns out it was a pedal dustcap and not from the crankset. Coincidence?, Clairvoyance?, Just luck?....... I should really buy that winning Lotto ticket this week.....

Winning ain't always pretty!

Two small holes and a pin-spanner, next time maybe....

Thought of that too, but no way the holes and the pin spanner would have held up to the amount and type of force I had to exert to remove it. the holes wold have stretched and the pin spanner would have slipped. It needed a lot of impact force to break free, not just torque.....

Heat, and penetrating fluid first. Of course it's just a dust cap, no big deal anyway.

Hold a proper funeral service for it, that is about all to do at this point. Have a directeur sportif do the words over the corpse, say three "Hail Eddys", and a "Tullio Our Father". Then its off to the closest Italian bar for the wake, and several toasts.

Bill

everyone knows bikes without dustcaps are faster!

Yup, my brother and I always binned crankset dustcaps along with dork discs, chainring guards and CPSC reflectors the first thing after we get our new bikes home. For some weird reason, I only started putting the dustcaps on my crankset after I joined this forum
Little did I know that those chintzy chromed plastic dustcaps from Stronglight will be prized/priced so much these days......

Or tap in a 5.5mm allen.

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How will that help?

I've actually done this. The 5mm hex hole on an old campy cap was so badly stripped out that a 5.5mm hex tapped right in with a little judicious hammer work. The metal on these old campy dustcaps is soft enough to allow it. The 5.5 grabbed in the newly enlarged hole quite well and the cap is still in use.

Given the circumstances described by the OP, most likely the 5.5 Allen would have acted like a dull reamer, and swaged the hole out to 6.35mm diameter.

Still worth a shot. It's worked for me, and apparently for others here as well.

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Yeah, mine probably wasn't quite as stuck as the OP's. Also I'm guessing that pounding in the 5.5 also helped the PB Blaster I'd squirted in there work it's way into the crank threads as well. A little of this and a little of that.

No problem, just paint the crank bolt heads '70s style.

Lesson learned: The small Philips head screwdriver that finally worked to drive the cap around worked much better than the flat headed one that just cut through the material and hit the crank bolt below the cap. Having the cross pattern on the Philips also helped anchor the tip in position (It dug into the material, but did not punch through) and not slip off the slots I cut with the Dremel, as I pounded away with a hammer on it. I don't think a flat tipped awl would have worked as well (most like would have slipped off the slots before it can make the seized cap turn).

Craftsman's bit tips for screw heads that have been gouged/stripped out works for me. I have used them on some really stubborn applications, like your dust cap problem child. They work very well in the hex key situations, also.

I cannot for the life of me remember what they call them, and I am at work so I cannot look in the tool chest right now. Edit coming a bit later.

Bill

I had a similar problem. Solved it by taking an allen key that was larger than the partly stripped hex hole, in my case 7/32" for Campagnolo, and filed a taper until it got a perfect fit.