I have a Specialized HardRock from the '90s. It's seen better days, but I got it for cheap. I had a bike shop look it over, but for some reason the guys missed the rounded off teeth on the front gears. Whenever I ride it, the chain slips every few rotations. I'd like to get a new crankset, but i'm not quite sure what will work and what won't. Any and all advice would be greatly appreciated.
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The main intercompatibility issues with cranksets are the connection between the crank arms and the bottom bracket spindle.
Most bikes from that age (not to say all mountainbikes) use square taper bottom bracket, for which is very easy to find good quality cranksets.
These look like the leftmost spindle in the picture above (which also show newer types, that are less likely to be what you need):
You might also take a look at the page from where that image came, in the excelent website of Sheldon Brown:
Other compatibility issues might be related to the length of the bottom bracket and the length for which the crankset was designed. Using different-sized bottom brackets and cranksets might result in a misaligned chainline, an ill-working front derailer, or in extreme cases a chainring running too close to the frame. This is so for tapered square, but I am not sure about the other types.
From the description of the problem --- rounded teeth on the chainring --- there is no need to be replacing the crankset. Simply replacing the chainring in question should take care of the problem. It's almost certainly a 5-bolt 110mm BCD chainring which should be available in any bike shop.
It looks like this bike likely came with a square taper bottom bracket and crankset. I'd recommend taking the dust caps off your crank arms and verifying this first. You may also need to remove the bolt holding the crank on the bike. If it is indeed a square taper (you'll see a square box sized hole in the crank where the bolt was), then that's good news. It's really easy to find cranks that will fit a square taper bottom bracket.
Something like this FSA Crankset would probaby be a decent replacment for not a lot of cash http://www.amazon.com/FSA-DynaDrive-Sqare-Crankset-22-32-44/dp/B001PT7TM2/ref=pd_sim_sbs_sg_3
Square taper comes in subtypes:
To some degree, these can be used interchangeably. However, the resulting small misfit will push the chainline in or out. Sheldon Brown covers this:
You can Google Velobase for all the possible Square Taper ISO cranks