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The aircraft in question below is a 1940 BC-65 with an A-65.
Question: What is the correct type and size for the engine mount to firewall attachment bolts, and what is the proper torque range for fastening with a castellated nut? For the purpose of looking up a torque spec, are these bolts considered loaded in tension or shear (I assume tension since that is the loading in flight, but sitting there on the ground, seems more like shear)?
Rambling Backstory: I had my A&P replace the worn out exhaust system two years ago, which as we all regret requires unbolting the engine mount from the firewall. I've been flying it since then until recently when that mild steel exhaust developed a crack, and I decided to replace it with stainless steel this time, as many have done. (I hired Custom Aircraft Parts in El Cajon, CA to fabricate a new stainless system from the old mild steel with excellent service and results. They were recommended by Dawley, who no longer performs the service.) Upon installation of the new exhaust, my mechanic discovered significant deterioration of the threads and scoring on the shank of the upper left engine mount to firewall bolt. The scoring appears as if the bolt had been rotating in position from vibration. I have no idea if this happened in the two year interval between exhaust replacements, or if it was present prior and went unnoticed at the last exhaust swap. Hard to believe it happened in the interim since I usually give the engine mount a good tug on preflight, and I never noticed any play or unusual vibration in the air that would indicate the bolt was loose. But equally hard to believe the mechanic did not notice it when it was apart last time. The A&P replaced the damaged bolt with a new one this time. I am asking about the torque spec thinking maybe it was not torqued correctly last time, and planning to double-check it this time. Other than incorrect torque on the bolt, can anyone think of an explanation for the damage described?
Thanks,
Lance
Orlando, FL
Question: What is the correct type and size for the engine mount to firewall attachment bolts, and what is the proper torque range for fastening with a castellated nut? For the purpose of looking up a torque spec, are these bolts considered loaded in tension or shear (I assume tension since that is the loading in flight, but sitting there on the ground, seems more like shear)?
Rambling Backstory: I had my A&P replace the worn out exhaust system two years ago, which as we all regret requires unbolting the engine mount from the firewall. I've been flying it since then until recently when that mild steel exhaust developed a crack, and I decided to replace it with stainless steel this time, as many have done. (I hired Custom Aircraft Parts in El Cajon, CA to fabricate a new stainless system from the old mild steel with excellent service and results. They were recommended by Dawley, who no longer performs the service.) Upon installation of the new exhaust, my mechanic discovered significant deterioration of the threads and scoring on the shank of the upper left engine mount to firewall bolt. The scoring appears as if the bolt had been rotating in position from vibration. I have no idea if this happened in the two year interval between exhaust replacements, or if it was present prior and went unnoticed at the last exhaust swap. Hard to believe it happened in the interim since I usually give the engine mount a good tug on preflight, and I never noticed any play or unusual vibration in the air that would indicate the bolt was loose. But equally hard to believe the mechanic did not notice it when it was apart last time. The A&P replaced the damaged bolt with a new one this time. I am asking about the torque spec thinking maybe it was not torqued correctly last time, and planning to double-check it this time. Other than incorrect torque on the bolt, can anyone think of an explanation for the damage described?
Thanks,
Lance
Orlando, FL
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