The Stat-O-Seal is very simply the best sealing washer available. It consists of a synthetic rubber O-ring mechanically locked to the I.D. of an aluminum washer. When tightened, the O-ring is compressed, forcing the sealing surfaces around the bolt shank. They will form a positive seal over a temperature range of -85° to +450° F. They provide the advantages of an O-ring type seal without the necessity of machining O-ring grooves into the part. In any application where you are now using a copper or aluminum sealing washer, the Stat-O-Seal will do a better job.