This SB involves corrosion in the front of the crank. Even though these engines drive predominately fixed pitch prop'd planes, the crank is hollow. Sludge, moisture collect in the front and corrode the crank, expecially if the engine sat idle for long periods of time.
My A&P was able to remove the plug and hone out the corrosion while keeping the crank thickness within limits. If I recall correctly, if the corrosion is too deep or there are cracks, the crank has to be replaced.