For all you mechanics out there I am sure you hate these things. Dust caps are the bane of my existence. They are cheap and bend easily. I want to get a better sturdier one for a bike I am working on. What do I want to look for? Is it just the sizing of the crank that matters or are there other factors?
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Assuming you mean the bottom bracket bolt cover (per wikipedia) there are two horrid types, and no good ones I'm aware of.
The expensive ones are metal and exactly match the thread of the crank so they shield it and keep it clean. They're also fragile and easy to damage as they're not strong enough to take much force but they have a large thread contact area so become quite solidly stuck over time. They're also expensive (did I say that already?).
The cheap plastic ones don't look as nice, but are lighter and easy to replace. They're also easy to damage when removing them and often the little plastic tabs break off even if you're very careful. The flip side is that you can buy a packet of 10 for about $5. But not online, by the look of it. eBay has single caps at about $1 each, which is IMO unreasonable, and no-one's selling larger quantities. I think you'll have to pay whatever your LBS charges, unfortunately, or buy a box of 1000 from China. Classic small part problem - you're paying $1 for the LBS to handle the product which is most of the cost.
I used to have an older road-bike with Shimano 600 parts. Rather than dust-caps, the Shimano 600's from the mid-eighties had self-extracting crank-bolts. What a great idea. If you can find a set, I recommend them.