i have always put new shells in when the opportunity arose and consequently have a few sets of used yb shells which display expected wear patterns from a good engine.

recently measured them using accurate gear and can say that there is a greater variation in the crank pin diameters and ovality than measurable wear on the shells. in fact cosworths acceptable limits for main pin diameter vary by 0.5 thou from memory and non of the shells measure half thou of wear so without expert explanation on the benifits of why they should be replaced mine will be reused in future.