A recent AD forces the owners of Lycomings to take any sort of
propstrike seriously, as the bolt that holds the gear on the back end
of the crank tends to fail or loosen during a sudden RPM drop. I wonder
if Lycoming will design the next engine with some different means of
retaining that gear...
Here's the AD text. Note Lycoming's definition of "prop
strike."

Dan

2004-10-14 Lycoming Engines (formerly Textron Lycoming): Amendment
39-13644. Docket No. 89-ANE-10-AD. Supersedes AD 91-14-22, Amendment
39-6916.

Effective Date

(a) This AD becomes effective June 25, 2004.

Affected ADs

(b) This AD supersedes AD 91-14-22.

Applicability

(c) This AD applies to

Lycoming Engines (formerly Textron Lycoming),
direct-drive reciprocating engines

(except O-145, O-320H, O-360E, LO-360E, LTO-360E, TO-360-E, O-435, and
TIO-541 series engines).

Unsafe Condition

(d) This AD results from a change to the definition of a propeller
strike or sudden stoppage. The actions specified in this AD are
intended to prevent loosening or failure of the crankshaft gear
retaining bolt, which may cause sudden engine failure.

Compliance

(e) Compliance with this AD is required as indicated before further
flight if the engine experiences a propeller strike after the effective
date of this AD, as defined in paragraphs (i) and (j) of this AD.

(f) Inspect, and if necessary repair, the crankshaft counter bored
recess, the alignment dowel, the bolt hole threads, and the crankshaft
gear for wear, galling, corrosion, and fretting in accordance with
steps 1 through 5 of Lycoming Mandatory Service Bulletin (MSB) No.
475C, dated January 30, 2003.

(g) Remove the existing gear retaining bolt and lockplate from
service, and install a new bolt and lockplate, in accordance with steps
6 and 7 of Lycoming MSB No. 475C, dated January 30, 2003.

Prohibition of Retaining Bolt and Lockplate

(h) Do not install the gear retaining bolt and lockplate that were
removed in paragraph (g) of this AD, into any engine.

Definition of Propeller Strike

(i) For the purposes of this AD, a propeller strike is defined as
follows:
(1) Any incident, whether or not the engine is operating, that
requires repair to the propeller other than minor dressing of the
blades.
(2) Any incident during engine operation in which the propeller
impacts a solid object that causes a drop in revolutions per minute
(RPM) and also requires structural repair of the propeller (incidents
requiring only paint touch-up are not included). This is not restricted
to propeller strikes against the ground.
(3) A sudden RPM drop while impacting water, tall grass, or similar
yielding medium, where propeller damage is not normally incurred.

(j) The preceding definitions include situations where an aircraft
is stationary and the landing gear collapses causing one or more blades
to be substantially bent, or where a hanger door (or other object)
strikes the propeller blade. These cases should be handled as sudden
stoppages because of potentially severe side loading on the crankshaft
flange, front bearing, and seal.