They engineered it so all of the components put together would have a acceptable balance variation range . They knew what the rods weight range ,pistons etc. etc. and it would balance within a certain range using components made to work together.
This probably what one should concentrate on in understanding the balancing procedure utilized by Ford (and others) in this period. In other words for example the O.E.M. would engineer, say for a connecting rod, establish what was required dimensionally for function, this would lead to the resultant mass and typical weights in manufacturing, and then with the variables exhibited in the manufacture of one component versus another it is necessary to balance or one might prefer the term to 'equal-weight' them in the appropriate fashion, this generally via the removal of the parent material through a grinding or milling operation in the deemed appropriate and provided for locations. Then even though the weight differentials have been narrowed their still exists a resultant tolerance value remaining necessary for mass production efficiency so then units are often batched by simular observations to further reduce the differential; all of this in each step with "acceptable" tolerance specifications, with individual units often being tossed (or redirected from Production to Parts & Service
) when failing to equal such along the way. This process most represents the "reciprocating" (stuff that moves up & down
) balancing effort of the connecting rods (big-ends & small-ends) pistons, gudgeon pin, and known sums for the rod bearings, piston ring set and piston pin retaining rings/locks if applicable. This providing a "bob-weight" value, typically in the American V8 (but not so in other engine layouts
) a sum of 50% of the reciprocating (rod small-ends, pins, locks, ring set, etc.) and 100% of the revolving (rod big-ends & bearings), in the balancing effort.
The "revolving" (stuff that goes round & round
, crankshaft, damper, flywheel or flex-plate, etc.) balance process requires a 'spinning' operation with establishments of the effects of the reciprocating values in place and then corrections to create the desired effect, this via removal of excess material provide for as before. Unlike the local machine shops where it is required to establish "bob-weight" fixtures to simulate the effects of the rods & pistons etc., here the O.E.'s for efficient manufacturing have tooling set-ups that create the imbalance effect and the revolving units (crankshaft or crankshaft dampers or flywheels and flex-plates) are balanced, or perhaps actually imbalanced to counter this establishment, or what we in the balancing business say: to "neutral" the assembly, and whether the net result on the component is neutral balanced or providing a counter-weighted imbalance value is dependent on the set-up. But again, this to a tolerance outlined by the O.E.; one which at the local machine shop is suppose to be bettering.
And then yes, as stated above, all of the appropriate components are then assembled, having never seen one another previously, this unlike the procedure required for the local machine shop balancing effort, and it all 'should' fall within the manufacturers' acceptable tolerance; but it is the potential 'stacking' of tolerances exhibited in each individual component that really causes the problem, and here is where the factory wasn't able to compete, and even though the local balancing effort 'should' provide a narrower deviation in weights and balance, as we have all of a singular assembly in front of us and invest a greater time element.
"Balancing" is a rather complex subject, and there are differences of opinion on what a "balanced" unit might really be, so read up on it if interested, cause I type way-to-slow to be able to present all of the nuances here.
Scott.