Balancing an engine

[jos51700]

For me crankshaft rotating assembly balance is NOT important, you only waste time and money for it.

Before the engine assemble, engineer has calculated how part will be made, weight, length..etc to make sure that when they put them together, they will achieve the BEST balanced. As long as you re-use the crank and rod, even you has your cylinder bore and use a new piston, this will not effect bottom end.

Think about it, the motor have 8 piston and firing order go by 1 8 4 3 6 5 7 2 , ASSUME at every 90° angles, there are explodes stroke occur, so that mean the crankshaft does not turn freely but force by combustion 4 time in 1 revolution(360°) , how do you "mechanically balance" this? This can only be done by engineer by using math and physics formula to calculate at which angle(position) the crankshaft will has much/less force, then balancing the whole thing by drill a hole in balancer . The machine shop can only balance the bottom end when crank is free, no force, its a joke.

Think: "Equal and opposite reaction". You don't "balance" the combustion forces, that's precisely what cylinder heads do. The piston/crank catches one end of combustion force and the head catches the other. And to push a point, it's not a precise "explosion" (unless something is wrong), it IS just a "push" as mentioned previously.

This was not a case of am I going to get it done. I am. I was just wondering if there was any set rules as to when to have it done. I have never heard of any.

Jack

Fact is, it's either balanced or it isn't. The only thing that changes when unbalanced is where the harmonics occur. The production tolerances don't have much to do with it, the factory doesn't really care if it's in balance or not, only the NVH guys do. (Granted, this was in the old days when the engineers weren't looking at every last thousandth for economy and emissions).

If the customer didn't ***** about vibration, it was "balanced" enough.
For the wider RPM ranges that we tend use our cars at, I'd balance every engine I built. As mentioned previously, a small amount of imbalance increases significantly both with distance from the crank centerline and with RPM.