The 66E and the early are identical in every way except the P&C. What I've heard is the 66E is a dog compared to the 70E.....the P&C on the 66E is very similar in design to the 621 or 80/90......which means it a high dome piston with two rings. Though I have several 66Es I have never run one myself. The 70 E is, like Bill said, a more modern design with a single ring and a flat top piston.....and the later versions with mounting bosses cast into the cly to mount the spark coil. My opinion would be if you were looking for one you should pick up the later version 70E # 158500 and later, mainly as this gets you away from the plastic fan Bill was alluding to. That said these are pretty hard to find.....many, many more early versions out there.....not only can the fan be problematic the ign modules are hard to find and expensive when you do find one. The only parts available for any of these saws are sparkplugs, recoil springs and carb kits.....so you'll be scrounging for bits everywhere if you get a "project" saw....
Visit the Jonsereds Sticky.....some good info on the 70E in the last couple pages.
