The problems with impact wrenches are ...
1) They need a lot of air.
2) The torque they supply isn't continuous.
Both bit me in my adventure discussed above. I bought one of [link|http://www2.northerntool.com/product-1/370269.htm|these] at Lowes. "625 ft.-lbs. max torque in reverse." Should have been more than enough to take off the bolt, but even with multiple tanks at 130 psi, with 5/8" pneumatic hose in between, I couldn't get it to budge. It claims it needs 5.4 cfm (at 90 psi IIRC, 21 CFM under load). Note that it says that it handles bolts up to 5/8" - I don't know if that's head size or bolt diameter. (The actual head size on my balancer bolt was 1 1/8".)
In my attempts, I would get ~ 2 s of full-power impacting before the air pressure had dropped too much. I tried about 5 cycles of refilling the tanks and hoses and trying the impact before giving up.
Even if I had enough air, it still might not have been able to do the job because the 625 ft-lbs of torque is only for a tiny instant in time each impact. Continuous torque from a wrench (and a long handle), or from the flywheel is probably the most reliable way to do it. That's what worked in my case anyway.
[edit:] An illustration of how torque from an impact is different from torque from a wrench: You can easily hold a socket on a 625 ft-lb impact and keep it from turning, while you'd have a pretty hard time holding a socket attached to a torque wrench with 20% of that torque applied.[/edit]
[link|http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=6970&productId=371242&R=371242|This] would probably have been able to do it (says it can handle 1 3/8" bolts), but at 12 CFM average (64 CFM under load!) it would have taken a 450 pound [link|http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=6970&productId=767&R=767|compressor the size of the car] just to keep up. Almost.
Oh, I also tried an electric impact wrench. I broke it within about 5 s. It sounded like a pin sheared inside it. Luckily, Home Depot took it back.
It was a big bolt. The threads were perfect in my case - no K5 at all. ;-) It was just very very tight.
Cheers,
Scott.
(Who enjoys working on cars when he can get something accomplished in a reasonable amount of time and hates it when he can't.)
