Our muffler for 40cc 4-stroke ATV, dirt bike, and mini bike engines will replace a damaged or missing muffler This type of muffler is used on some 4-stroke engines powering mini bikes, pit bikes, dirt bikes, and small ATVs. Our muffler for 40cc 4-stroke ATV, dirt bike, and mini bike engines will replace a damaged or missing muffler. This type of muffler is used on some 4-stroke engines powering mini bikes, pit bikes, dirt bikes, and small ATVs.