Pushrods

I note the Commando Combat 750 pushrods are shorter than stock 750's by .034"
I suppose the logic is since the head is shaved .040" to increase compression that the pushrods needed to be shortened to help keep the related geometry in sync.

However the relationship of the centerline of the rocker spindle to the point of rocker contact at the valve stem is fixed, independent of pushrod length, and any difference in pushrod length is negated by the adjuster screw when setting valve clearance.

Can someone explain the valve geometry that justifies the different pushrod lengths.

Thanks
Bob

Theoretically any lever that rotates around a fulcrum will transfer any movement from one side of the lever to another in a fairly linear fasion when parralell but the further the lever goes from parellel then there is not only up and down movement but also some sideways slip. I believe this sideways slip is what the shorter rods is trying to mitigate. I am also a bit surprised that such a small difference in dimensions would make a difference but apparently it does and when I built my Combat spec engine I optained the specified push rods.

Best leverage with least out of line loads is obtained by getting rocker to be 90* and centered to valve at mid lift. Can think of this like hitting a nail by hammer at 90* impact arc vs hitting it before full swing or after full swing, can still drive nail but tends to bend nail or not drive it as far per strike per force applied.

I wouldn't even try to describe what the motion should look like without pictures. Fortunately Jim's got video in the spintron post that shows rockers in action. I'll shoot for what he's got then try a little longer and shorter to see for myself.