If you measure a pushrod using a 5/16" ball but order new pushrods and they send them with 3/8 balls, the length won't be what you wanted...that's really all I was saying. I actually will only use a Manton checking pushrod when ordering Manton pushrods, and a Smith checking pushrod when ordering from Smith. Then, I mail them the checking pushrod and tell them to match it. That's overkill on a lot of engines, but it's also surprising to see how much variation can creep in otherwise.
As for the motion lost to flexing....when you are already buying the best pushrod there is, and in the largest size/stiffness, you run into a wall. Pushrod flex is exacerbated by angularity....a straight-shot pushrod won't bend as much as one that being pushed at an angle. There's a lot of math going on when you start including angles.
