Because engines create very high temperatures, valve rings that were used in these engines were commonly made with asbestos for its heat resistance. Because of this, repairing and removing these valve rings became a hazard for those to whom this job fell. Many valve rings made before the 1980s contained asbestos before its dangers became publically known.
Generally, the asbestos in valve rings is not harmful, since it is bound up with other materials. However, when the valve rings are removed or become damaged, they may release deadly fibers into the air. Once breathed in, these fibers remain in the lungs and other body tissues, damaging nearby cells and, sometimes, causing them to become cancerous. Exposure to airborne asbestos fibers is the only confirmed cause of mesothelioma, a deadly cancer of the lining of the chest and abdomen.
Consequently, thousands of lawsuits have been filed against those companies who made asbestos products. This includes the manufacturers of valve rings, pistons, bearing packing, and a number of other parts used in engine building. A good portion of these lawsuits have been successful since the dangers of working around asbestos were known and most employers made no effort to either warn and protect their employees or make the public aware.