RST Engineering wrote:
There aren't a lot of possibilities, are there?

1. The case crack set up a vibration or torque that overstressed the
bolt -- and since it happened on startup when things are running less than
smooth in a Lyc, I'd bet on this one.

2. The bolt was WWAAAYYY overtorqued on installation. You'll never know
about this one. However, a through bolt shearing and a case crack by
another through bolt leads me to check the calibration on somebody's torque
wrench.

3. Something else in the engine was vibrating ... not likely as the whole
AIRplane would have been vibrating to shear a through bolt.

4. Defective bolt ... not likely as throughbolts get magnafluxed or x-rayed
at overhaul.

5. (Add yours here)

5. The engine had cylinder work done. Either the cylinder bases were
painted while off, as mentioned earlier, and the paint eventually
squeezed out beneath the fasteners leaving them loose, or, the mechanic
neglected to loosen and retorque all the through bolts, in proper
sequence, as Lycoming recommends. Both are good bets. Failure to
retorque ALL the through bolts in sequence after installation of a
cylinder is a VERY good bet, based upon my observations.


Oh, and BTW, mechanics cannot ground airplanes. IAs cannot ground
airplanes. The FAA (unless they pull the airworthiness cert.) cannot ground
airplanes. I know it is a common phrase, but the mechanic simply suggested
that it would be less than wise to fly the airplane in its current
condition.

Jim

FAA inspectors have a form (more of a tag, really) that is designed to
be attached to an aircraft that the inspector believes should not be
flown. If one removes that tag, and flies the aircraft anyway, one may
lose the privilege of flying for some set period. Not immediately, but
in due course. I knew a pilot who did so. He flew the aircraft three
states away from the site it was tagged. The FAA inspector traced the
plane, found its location, notified the local FSDO, got the assistance
of the local sheriff, who chained and locked the aircraft to the ground.
All in one day. And merely because one of the elevators had a 12"
crack.

Dale