I have a 93 Euro and I just changed the front pads and rotors and I was going to change the back ones also. My question is to collpase the pistons on the calipers can I use a C clamp or do I need to screw them in with a special tool??

They screw in with a special socket, you can get one fron AutoZone for about $10 or less.

Turn them a little, then look at the piston and make sure it's going in, not out. Wind it out, and the piston pops out and now you need a caliper. Some are reverse threaded. Can't remember if these are one of them.

Make sure the surfaces where the calipers slide on their mounts are well lubed with a hi-temp, waterproof lube like Syl-Glide, or they'll sieze and turn into junk real quick.