How do I remove these crank bolt plugs I have an old 10-speed bike (presumably 80's vintage). I'm trying to remove the bottom bracket and cranks but have struck a problem.

The crank bolts have a plastic cap over the top of them and I'm not sure how to remove it. They look like they should unscrew based on what I could imagine it would look like before they were trashed by the previous owner. I tried to unscrew them gently but they didn't move and as you can see from the photo below they're very fragile.



On the other side the cap has been completely broken.


I can see a nut, but with the plastic cap extending into the cavity I can't get a socket in there. I need to remove the plastic somehow.

Anyone have any ideas how to remove the cap and it's remnants?
Normally, the cap just unscrews. Obviously, that's not the case here. Use something like the tip of a knife, a small screwdriver or a chisel to break it up and pry it out. But try not to mess up the threads too bad, you'll need them for the next step.

I think what you have is a square-taper crank design; the bottom bracket spindle has a square-shaped tip, the crank is wedged onto that tip and secured in place with the nut. Once the nut is off, you'll need a crank puller. It's a cheap tool, you can get one at any bike shop. The puller has a sleeve that threads into the crank (into the same threads that the cap attaches to); then you screw down a bolt inside the sleeve that pushes against the spindle, and hopefully pops the crank off. This looks like an old bike, so it might be stuck on there pretty good.

When reassembling, just put the crank back on and tighten the nut. Make sure you put plenty of grease on the mating surfaces.