Are all mountain bike cranks the same size because my 1980s specialized hard rock has a discontinued shimano bio pace crank set and it is trashed so can i just throw any crank set in it?

Answer 1

No.

Crank arms have a few different lengths, and then the way the crank arms meet up with the bottom bracket (AKA crank axle) can be different in several ways.

Back in the days of Biopace, the domineering model was square taper, so that's what you should be looking for.

Try matching the tooth count as well, for least hassle at assembly.

You'll need a crank puller to get the old ones off.

Make sure to use the right torque at assembly, as square taper cranks are notorious to wobble and get destroyed unless mounted right.

But check first if you can't remove & replace the chainrings only, that might be an easier/cheaper option.