This the most probable reason about cracking. the plain of rotation of the bolts surface is not in parallel with the plain of rotation of the ring gear..so in short it is actually wobbles while you are driving. The cure for this is to check the flywheel with and dial indicator while lying flat on a inspection table. The reading should show zero on all around the bolt surface. If there is any deviation, you must bring it to an auto machine shop capable of surface grinding it flat up to plus and minus zero tolerance. Grind the surface that attached to the crankshaft's end.
Lastly, check the allowed torque for the bolts and the pattern of tightening it. Some do it by triangle or square pattern and make sure not to use an impact wrench. Gradual tightening of all bolts is a must..never go to the final torque right away, it has to be by step. Like 5ft lbs then 25 ft lbs and to the final torque.
It will also help to check the clearance between the ring gear and the starter's pinion gear. Not enough clearance will hammer the ring during starting inducing a wobble. Any rotating wheel with a wobble will vibrate perpedicular to the plain of rotation and will stressed the hub whereby cracking starts at the bolt holes which is the weakest point. Also, if the manufacturer failed to temper the flywheel correctly, no amount of correction will help you. Hope, this can give you a picture. In addition , countersink the bolt holes prior instalation. Bye.
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