A spark arrestor is on the exhaust system and a flame arrestor is part of the crankcase venting system to prevent a flame from inside or outside of a crank case from propagating into/out of the crankcase. Flame arrestors are common on engines that are used to drive natural gas compressors. Gas leaking by the compressor rings or the engine rings can enter the crankcase and flame arrestors prevent an internal fire from escaping or an external fire from entering a crank case that might have gas in it.
On slow speed diesel engines a flame arrestor is also incorporated into the starting-air line in case the start-air admission valve fails or passes. It is generally situated as close to the cylinder head as possible for obvious reasons!
