While the bolts we stock are not "aircraft-certified", they have been approved for use on certain FAA-certified aircraft. They have superior toughness and ductility compared to the standard AN aircraft bolts. They are machined to close tolerances, and heat treated under strict control so that their high tensile strength does not result in brittleness. AN bolt tensile strength is 125kips and our bolts are 180kips. Over 60,000 of these have been used on experimental and certified aircraft for the past 33 years with a perfect safety record. The heads are single-drilled if required by your application.
Most importantly, these engine and prop bolts have a shorter grip and a LONGER THREAD, which provide a much greater safety margin against bottoming out the threads. This is critical because each time you re-torque your wooden PROPELLER, you may use up more of the available thread, and risk bottoming out the bolt before achieving proper grip between the extension face and prop face. This is also important because some engine models have lugs installed which have unusually long thread lengths which can allow a standard bolt to bottom out before reaching proper extension face to engine face grip.
SOME SIZES OF GRADE 9 HAVE BECOME UNAVAILABLE AND WE NOW USE GRADE 8.