Thomas Borchert wrote
NO, NO, NO!!!

Running "overlean" does NOT hurt cylinders!

It does at takeoff power. At takeoff power, a VERY rich mixture
(probably 200 degrees rich of peak, or more) is normally and properly
used. An induction leak can cause the mixture to be significantly
leaner, meaning much closer to peak. Running at peak EGT at takeoff
power most certainly will hurt the cylinders.

If the induction leak was bad enough that a cylinder was running well
lean of peak, that would probably be OK - but in that case, there
would be noticeable roughness and loss of power. The insidious thing
about a slight induction leak is that there is no loss of power - in
fact there is a slight gain - and smooth operation is maintained right
to peak on the affected cylinder. Without all-cylinder EGT, this is
undetectable. The design of the induction systems in certified
aircraft engines is such that they are prone to this problem of having
one jug running at close to peak due to an induction leak while the
other jugs run well rich. Thus they are, by design, unreliable junk.

I'm sorry if you found the term "overlean" confusing. In this
context, it simply means leaner than is proper for the operation.

Michael