The rotary aircraft engines of World War I (engines with the crankshaft fixed to the airframe and rotating cylinders) were notoriously difficult to keep cool when operating. Thus, their very thin walled steel cylinders would become distorted to an unacceptable level. Obturator rings, made of brass, were fitted to compensate for this out-of-roundness in much the same way as a leatherwasher does in a bicycle pump. Wear on the rings was considerable and they only lasted a matter of hours. By the time the war ended, technology and materials had advanced sufficiently to do away with these obturator rings.