A radiometric method has been developed to measure apex seal wear in rotary engines. Seal wear rates are measured in ranges of μg/min without engine disassembly, and at various speed and load conditions using a dynamometer. The method incorporates radioactive apex seals in the front rotor of a Mazda 10A engine. Neutron irradiation of commercial carbon-aluminum apex seals produced radioisotopes of iron and chromium. Experimental carbon-aluminum seals, containing terbium oxide as an additive to improve the nuclear properties of the seals, were also used.The radiometric method involves the following steps: 1.operation of a rotary engine at any desired engine condition with radioactive apex seals, 2.collection of the seal wear debris by filtering the exhaust gas, and 3.the quantitative determination of the resulting radioactivity on the filter medium.The method is capable of measuring apex seal wear at a specified engine condition in approximately 25 min (three min sampling time, and a maximum of 20 min counting time). With the experimental (terbium oxide additive) seals, the counting time is reduced to six min. Precision of the method is ± 10%.Apex seal wear was measured with several engine oils and automatic transmission fluids; the wear data were used as performance criteria to rate these lubricants. The data indicated that automatic transmission fluids were as good as engine oils for apex seal lubrication in the Mazda 10A engine, and that kinematic viscosity of the lubricant may not be the important variable in carbon-aluminum apex seal lubrication.