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New 383 motor in '32 Ford pickup in stock boxed frame. The motor mounts are located at the front of the motor by two (one on each side) bolts attached to the frame. Under hard acceleration the driver's side bolt stretches or breaks. Probably not the best way to mount a motor but I understand that the bolts wren't meant for 400+ power. I'm considering using aircraft grade bolts. I haven't tried Grade 8 bolts yet and that's my first step. I need bolts that minimize the " stretch aspect" . Any help or advice will be appreciated.
 

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If you're talking about " don't" mounts with the bolt through the center ,you either have mis alignment trouble or a junk bolt , a 7/16 fine grade 5 bolt has a tensile strength 10500 lb , 7450 proof load , your sbc aint going there ! The bolts in mine came from chassis engineering , with the mount cushions , 20+ years & 50k+ miles ago ...
 

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Grade 8 bolts are a must, but, sounds like there is another issue going on. Unless your SBC is putting out like 600 foot pounds of torque and youre pulling the front wheels 4 feet high off the trans brake, I'm guessing something is moving. Maybe the trans mount is busted, and the engine/trans combo is floppin around like a fish out of water?
 

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If you break ( pull apart) a 7450 lb tensile strength bolt , you've bent / broken the frame. &/or the mounts , why do you feel a higher strength bolt ius needed ?
 

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It seems you are talking about the front engine mounts used in the early SBC. Those are notoriously weak at best, you need to change to side mounts, or , if side mounts are not possible, some sort of engine plate mount. Also, if your trans is only supported by its rear mount, the distance between front engine mount and rear trans mount is also way too long. Using side trans mounts (using the bellhousing bolts) could help your problem.
 
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