re, the actual culprits were the fasteners clamping the rod to the crankshaft. They disintegrated under the strain of transferring the engine's power to the crankshaft.
"The biggest concern in the connecting-rod business is the fastener," says Jack Sparks, president of Carrillo Industries, San Clemente, Calif. "The fasteners are typically the weakest parts in the rods."
Carrillo rods are constructed of forged chrome molybdenum, a high-strength alloy commonly used in aerospace applications. Fred Carrillo had worked in the aerospace industry and, through his connections, was introduced to SPS Technologies. The Jenkintown, Pa., company manufactures high-strength fasteners for aircraft engine and airframe components.
Material selection was an important factor in choosing fasteners for the connecting rods, but Carrillo also needed to keep costs down. "We were a small company at the time," says Sparks. "Our application could hardly justify proprietary bolts. We went to SPS product engineers and they assured us they had alloys that would meet our requirements."
SPS engineers recommended Multiphase alloys MP35N and MP159. These alloys, patented by SPS, combine nickel, cobalt, chromium, and molybdenum. They are tested to 260,000-psi tensile strength.
"In addition to material selection, engineers at SPS suggested changes in the bolt design," adds Sparks. "An asymmetric thread design developed by SPS improves the fasteners' reliability and increases their life. This gives us a wider safety margin."
The bolts are preloaded up to 13,000 lb, which is a lot for a 7/16-in. reduced-shank bolt. "The bolt will function and run at yield, but we build in a lot of forgiveness," says Sparks. "There are actually four areas on the rods with less strength than the bolts."
To verify strength requirements, unique bolt-fatigue parameters were jointly developed by SPS and Carrillo Industries for each connecting-rod bolt. The test criteria have been modified several times to keep up with advancing technology in connecting-rod designs.
October 7, 1999