Yep, no problem. I've used transmission coolers and heater cores from cars...although heater cores are much more like typical watercooling radiators in design and can be easily used for good performance. However, there isn't a common 'size' and there are not any real fittings that work for PC watercooling components, so once you buy one, you still have to do a lot of modification to get it to work as you want.