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 negative 85 degrees to positive 450 degrees Fahrenheit. 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.