what happened with us was that a crack formed in the cylinder head. As the engine warmed up it expanded. The cylinder head then pinched the pushrods causing the pushrods to break and blow through its case. As the cylinder head continued to separate cooling fins broke free and entered the compression chamber. The top of the cylinder was held on by one side of the jug.
Other than the oil trailing off the nacelle, the only indication that something was wrong was the fact that they couldn't lean the engine. Every time they tried it started to backfire. There was no sound change.
I've lost several R2800s in C-118s and a C-131. Usually, a cylinder suffers a failure (heads blow off, or partially blow off), con rods break or any number of various mechanical failures. However, the engine will make good power on the remaining 17 cylinders. Your biggest risk is fire. If feasible, feather the prop and shut it down.
Depending upon the problem, you may need to change the engine, or more commonly, replace the affected cylinder. That will be determined after the post-flight inspection.