[Supras] oil pan noise: cracked off piston skirt?

[berniek at technicaldevelop.com]

Last night I wrote about a noise like pebbles dropping on the extreme rear
of the oil pan.  I could feel random pulses in the stethoscope diaphragm
holder also when the pebble noise occured.

The pistons were stock and I'm running 15-17 PSI boost, and am on the rich
side.  Just changed the first oil after 400 miles.  Analysis said not too
much aluminum in the oil (some lead though, presumably from the camshaft
soft coating after refininshing the bearing caps).  but I did find some
flakes of aluminum in the oil filter.  Did not seem abnormal, however.

The noise was found later, by about 50 miles.  There was also one instance
when revving the engine under the hood I did hear a muffled "crack" which
could have been anything, I suppose.  I'm half tempted to buy a borescope
and look in the pan through the drain opening or through the dipstick tube
(one is sold with a 90 degree mirror).

Has anyone had a skirt crack off a stock piston?  In looking for excessive
pistion rock or slap when warm, I squeezed closed the ISC hose to the
extent possible to reduce engine speed to just above he stall point.  No
noise other than normal valve noise was produced.

As always, any thoughts would be appreciated.  I do not know yet if the
noise is getting worse.

Thanks.

Bernie