As a grade 8 will have 190,000 (?) tensile strength (straight pull) and a grade 5 at 120,000 (?) yet when side loaded the grade 8 will break where the 5 will bend first.
We have 20" screws moving tons of rock 24-7 and use grade 3's to 5's.
Grade 3? are you sure? Are you reading 3 radial markings on the head as a grade 3? There is no such thing as a grade 3.
As for grade 5 bending before breaking.. In a shear application (which this is), there won't be any bending of the bolts prior to failure. A grade 5 bolt will simply shear under less load than a grade 8. A grade 8 is superior (150,000psi), yet may not be required depending on the amount of loading.
OE Powerglide and TH350 are 3/8-24 Gr5. (GM 3799830) However, Ford use larger 7/16-20 Gr8 in the same application, so "overkill" is not really matter here. Just make sure you torque according to both diameter and grade.
Let me go check my COPO torque converter bolts. Be right back,,,
Grade 8. There's six on mine. But my Super Street Fighter only uses 3 grade 5's. (35 pounds torque) Go figure?
Fred, the amount of torque used to preload the bolt has nothing to do with it's shear strength. The shear strength of a 3/8 Gr5 is still ~7700lbs per bolt
(plus ~5" of leverage they have over the crank centerline. Basically, it's a lot!
Being such a short fastener, the type of bolts used in this application are "place bolts" that use the head of the bolt to apply most of the preload when torquing.
You will note that these bolts have radial slots in the heads or like ARP, have dimples in the center.
These slots and dimples are to allow the head of the bolt to flex at a specified torque (35ft/lbs in your example) for bolt retention.
These bolts are really too large in diameter and too short in length to stretch properly.