The descent procedure for the Scouts (180 HP Lycoming) at my club in Alberta was to throttle back to 2100 rpm, maintaining the tow speed ~70 mph and dump full flap. After one minute or CHT below 300, reduce to 1700 rpm (below the resonance zone) and descend at 90 mph with full flap.
I'm not aware of any of the Scouts at Alberta gliding clubs ever having any cracked cylinders.