Loose crank, missing piece

Well I realized that my crank is really loose today, so I tightened it up all the way and it was still really loose, so I looked at it and there is a black piece that goes into the crank that is missing.
Can I buy something to replace it or does anyone know what I can do to fix it?

Bad crank

Good crank

Thats nothing, its just something to keep dirt out of the cranks. If you are still having trouble with the crank that dust cap definitely isn’t your problem.

Any bike shop that deals with BMXs or mountain bikes should be able to give you a replacement cap, they may make you buy a whole ISIS bolt to get it though.

It looks like you may be missing your crank spacers though - the black pieces that go between the crank and bearing.

Cranks shouldn’t have spacers between the crank and the bearing. Either the spacer will be loose and useless, or it will be snug and keeping the crank from seating properly.

The piece that’s lost may be a self-extraction nut, but that’s not really important. If the crank is still loose after you’ve tightened it all the way, you may have damaged the crank tapers, the axle tapers, or both. The crank bolt itself looks partially stripped, which might indicate it was installed improperly to begin with (cross-threaded, perhaps, and thus requiring too much force to tighten).

Take the crank all the way off and examine the splines; if they’re significantly mashed, you may be replacing a crank. You could try (just for the sake of testing) switching the cranks to opposite sides; if the same crank is still loose on the other side, it’s probably toast.