Lycoming cylinder barrel wear signs

Here's some not uncommon cylinder barrels.

First notice I've placed a white piece of paper to reflect the light onto the barrel. Now you aren't looking down a dark hole.

In the next picture the yellow arrow points to a dark patch. Dark patches are almost always areas of corrosion pitting. Next look at the red arrow. Notice two things; the rainbow colors and the shinny appearance. The rainbow colors is a heat tint that forms at approximately 500 degrees F. You should not see any hint tinting in a Lycoming or Continental cylinder barrel. The heat tint tells us the barrel at this spot got too hot. Notice that the heat is local. The heat is caused by blow-by of combustion gas past the piston rings.

Here is a closeup of the dark patch clearly showing corrosion pitting and the heat tint below. 

Corrosion pitting is a difficult call. If it is concentrated in one spot it will trap oil that will oxidize and glaze over creating a tan colored patch on the barrel that will hinder the proper operation of the piston rings. If the pitting is more general then usually it does not adversely effect the operation of the piston rings. 

When you look at a pit you might have the impression that "it doesn't look too bad" or that it is "real small" but you are only looking at the entrance to a hole. Pits penetrate deeply beneath the surface and they weaken the barrel. Proceed with caution and consult the manufacturer for rejection standards when it comes to any corrosion on a structural surface. Remember a cylinder is a pulsating pressure vessel. All the torque that turns that propeller is due to the piston pressing against the cylinder wall for leverage. On the other hand if you start rejecting cylinders with pits on the cylinder wall you will probably ground half our entire fleet of aircraft. That being said, reject any cylinder with corrosion pitting on the outside barrel next to the hold-down flange. See C-152 O-235 barrel pitting.