This recommended practice establishes equipment and procedures for testing spark arresters used on medium size, single position internal combustion engines, normally used in transportable, stationary, and vehicular applications, such as highway trucks, agricultural tractors, industrial tractors, and other highway equipment. This recommended practice provides two uniform methods of testing (laboratory testing and engine testing) which may be used by spark arrester manufacturers, governmental bodies, and engine spark arrester users. Under the present state-of-the-art and under normal operating conditions, taking into account the speed of the engine and exhaust flow rate, a spark arrester having an efficiency of 80% as determined by this procedure is considered adequate for most conditions and areas. Arresters meeting this standard should not be expected to arrest sparks adequately when tilted more than 45 deg from their normal operating position. This procedure recommends testing spark arresters, except turbochargers ³ , at flows causing back pressures up to 2 in. Hg (6.8 kPa). Should spark arrester applications be considered where back pressure above 2 in. Hg (6.8 kPa) are developed, it is essential that additional testing be completed at the higher flow rate necessary to develop the actual maximum back pressure.