Today a Pawnee had a wheel bolt lose its nut and back out of the wheel
enough to strike the brake caliper. I know of two other ocasions where this
locked the wheel leading to a groundloop and to severe damage to the tug.

In this instance, the bolt bent about 45 degrees producing a loud ticking
from the wheel which called attention to the problem. It's a good idea to
check the nuts on wheel bolts each preflight.

It's an aeronautical tradition to install bolts with the heads up, forward
or outboard depending on the location such that gravity will tend to keep
the bolt in place if the nut is lost. If the bolt heads had been on the
outboard side of the wheel, it wouldn't have been possible for them to
strike the caliper. Unless the maintenance manual says otherwise, I'd
suggest installing wheel bolts with the heads outboard.