Rating Torque on Air Impact Wrenches

You see them all over - 1/2" drive air impact wrenches with " 250 ft/lbs
of torque " at very attractive prices. After getting his bargain-buy
home, the novice mechanic finds that he still can't budge his sprocket
nut, or his clutch hub nut, or his sense of frustration. I myself find
that it takes a rating of almost 400 ft/lbs for a 1/2" impact to be
really useful, the weight of the internal rotary hammer probably being
the key factor. Although there are differences between manufacturers, I

How many ft/lbs are you trying to apply anyway ?

Most parts of my motorcycle can be thoroughly destroyed
with way less than 100 ft/lb. Lots of aluminum, not a
whole lot of cast iron in most bikes.

don't think I have ever seen a 1/2 " air impact that could achieve even
half of its rated torque in the real world ( 3/8" butterfly wrenches are
a somewhat different story ). Anybody have an inside on how these
things are rated ?

For what it's worth. Years ago I rented a heavy duty electric impact
wrench and I kind of thought it was worthless for doing more
then putting together swing sets and childrens' toys. Maybe it would
have been OK for a motorcycle but it was impotent working on my ~5 ton
dump truck. When I decided I needed one full time around the house I
looked at prices and decided to go with air tools.

I ended up buying a compressor and a relatively cheap <$80 Husky H4110
1/2 air impact wrench and over
all I am satisfied with it's performance. It is a little much for
working on cycles and not quite consistantly enough for the dump truck
but still three times the wrench the electric was. That's pretty much
what I would do if I was in the market anyway: Divide the electric's
rating by three to come up with the air equivalent.