Bolts are funny things, and more complicated than you might imagine. Over design is always a good thing in amateur applications! Take, for example, the following chart:
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That chart is for grade 2 NF fine thread bolts. You can see the safe working limits are much lower than the absolute shear strength when calculated as 60% of tensile strength for a given bolt area. For example, that 3/8 bolt in grade 2 the calculated shear strength would be
2897.4 lbs actually is at its safe limit at
510 lbs. Bolts that are subjected to shear loads that are variable, intermittent (cyclic) also have another failure mode called fatigue failure which lowers the bolt capacity even further - you will often see this discussed on airplane propeller bolts, since it is important that the propeller does not fall off while in the air! The design of the threads actually has an large impact on fatigue strength of a bolt - a round, smooth thread root is much stronger than a thread that is cut, or has a sharp thread "root." Unfortunately I could not find you a simple calculation for fatigue strength, probably because the forces are dynamic, and the calculations more complex and rely on other factors beyond simple strength calculations.
Grade 8 bolts have approximately 2x the absolute strength of a grade 2 bolt (55,000 psi vs 120,000 psi). If you design within grade 2 parameters, you will always have an extra margin of safety if you use a grade 5 or 8 fastener.
So, we know you use 6 bolts in a "lucky mod" flywheel. I also went out and measured my old DMF and found the bolt circle
radius to be about 2.5" for those 6 bolts. We commonly measure engine torque in ft-lbs or spoken "pound-feet". Simply, this is how much force the engine can apply 12" (one foot) from the axis of rotation (center of the crank). This is important because when move a bolt
closer to the axis of rotation (crank) the more force it must be able to withstand, think about how much torque you can apply with a 3 ft cheater bar vs a stubby wrench - you need a lot more
oomph to move the same bolt with the shorter wrench for the same bolt.
Static torque of a 600 ft lb engine (safe overestimate) on those bolts would be
12" / 2.5" = 4.8 times the shear force on those bolts, or
600 x 4.8 = 2880 lbs. If you divide that by 6, you see that each bolt needs to take
480 lbs of torque
each in this appication for a 600 ft. lb. engine. Pretty close to that 510 lb static limit for a grade 2, eh? Now put those bolts in a dynamic, cyclic environment like a flywheel and you can imagine you are operating pretty close to or even exceeding their fatigue strength specs. Now, of course we drilled our holes so accurately that each bolt is sharing the load
equally and one is not subject to more force than the others, what if one bolt hits the "wall" of the drilled hole before the others? It has to take all the force until it deforms enough that the other bolt(s) can then share the load, assuming it doesn't break first.
So, ensure you have very highly rated, traceable strength bolts in there to give yourself some more margin, or like you said, add more bolts! Also make sure you put the proper torque on each fastener which would be 22 ft lbs. for a dry grade 2 3/8 bolt, or 50 ft lbs. for a dry grade 8 bolt. This ensures that the clamping force exceeds the shear force on the bolt, in which case you can avoid bolt fatigue all together (the bolt creates friction between the two surfaces that is greater than the shear torque load on the joint) A grade 8 3/8 NF bolt torqued to 50 ft lbs can create 7900 lbs of clamping force, well in excess of the calculated safe shear loads for the bolt.
Pretty cool, if not extremely boring! I always found this kind of stuff fascinating, hope it helps!