Lead flakes sound like main bearings disintegrating. I don't know of any
other use for lead in an aircraft engine. Main bearings are soft, solid
bearings which use babbitt for bearing surfaces. Babbitt typically is a
combination of tin and lead, sometimes with a layer of silver underneath and
a hard shell of cast iron or steel. The hard steel crankshaft rotates
inside several sets of these bearings, typically one on each end and a third
in the middle. These bearings are said to be friction bearings (as opposed
to rolling bearings, like ball bearings); however, the few thousandths
clearance between the bearing surface and the crankshaft allow the crank to
hydroplane on a surface of oil.

Here's an article which will help you understand how these bearings work and
why you should be careful that they don't disintegrate:

http://www.eaa49.av.org/techart/mobil01.htm

"Les Sullivan" wrote in message
om...
I have a 1972 Cessna 172 with a Lycoming 0-320-E2D engine and it runs
[...]