John, your comment regarding the transmission type is spot on. The normal automatic transmissions, ST, TH350. and TH400 all used a flexplate mounted torque converter which would place a twisting force on the flexplate under load. The Tempestorque version of the powerglide had the torque converter mounted on a shaft behind the transaxle and was driven by a long shaft or "ropeshaft" as it is commonly referred to, that was bolted directly to the crankshaft. So, with this particular design there was never any twisting force applied to the flexplate (flywheel) except when the started motor was engaged. My guess is that the engineers were worried about metal fatigue in the thin metal flexplate when they switched from the older Jet Away / Slim-Jim design with an actual flywheel to the new Turbo-Hydramatic Transmissions. with only flexplate/ring gear set up for the new block mounted starters.
Thanks, JK
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