Actually, 10 grams per cylinder weight different over what you pulled out is significant, so precision balancing is necessary unless you were just throwing the engine together to sell the car, and since the new components are lower weight, final dynamic balance can be easily achieved by removing a little metal from the crank end cheeks.

I recall he OE rods are 570 grams, but that might be for the 350 rods that are likely a bit heavier than both the first and second design 327 rods.

It's good to hear that the Eagle SIR5700s are +/1 one gram. That likely means the set was weight-matched by Eagle and sold as a balanced set. One of the balancing issues I may not have mentioned is that rod and piston mass should be equalized before the final dynamic balancing operation, and +/1 one gram is sufficient precision.

Duke

 

That's not how it works, taking a 10 gram difference and multiplying by 8.

This is how a balance card is configured.
Piston 650
Rings. 65
Pin. 165
Rod recip. 182

Rod rot. 415 X 2. 830
Rod bear. 42X 2. 84
Oil. 10

Equals a bob weight of 1986. These are just numbers I pulled out of my @¥£, but this is representative of a SBC. Once you come to this bob weight you put it on the crank and spin it up. What you'll notice is the big end of the rod is considered rotating weight and the small end is reciprocating weight. When balancing you double the rotating weight, which consists of the big end of the rod, the rod bearing, and estimated oil weight. So for example the stock rod weighs 597 grams, rotating weight plus reciprocating weight, 182+415=597. If you change to an aftermarket rod that weighs 600 grams but the recip weight is 170 and the rotating weight is 430 this is how it would affect the bob weight

Piston. 650
Rings. 65
Pin. 165
Rod recip 170


Rod rot. 430X2 860
Bear. 42X2 84
Oil 10

For a bob weight of 2004. So even though the rods only weighed 3 grams different they changed the bob weight 18 grams. This would result in needing to add about 5 grams to each end of the crank. It doesn't work out that when the bob weight is X amount over you add the same amount to the crank. If the bbweight came in within 20 grams of a known bob weight on a balanced crank I wouldn't bother to rebalance. Just me. If for some reason it come in lighter you could go quite a ways and not affect the engine. This is where you get into over balance and under balance. The main thing is if all of the components are matched i.e. rods all the same Pistons all the same bottom line is it doesn't affect the motor that much. If you have one rod considerably out it will cause a vibration. Remember reciprocating weight is doubled. Hope this helps clarify balancing in some people's minds. I didn't say how the rod weights were figured. The rod is weighed first total weight of the rod then it is hung from a fixture and the bearing end is weighed. The bearing end is rotating weight. The bearing weight is subtracted from the total giving you the reciprocating weight, 600-430=170.

 

My 66 427/425 engine is about to go to the machine shop. What components of this engine would be subject to balancing..I do not know whether or not is it internally or externally balanced

 

Its internally balanced. If its manual I'd take the flywheel and clutch

 

Excellent. Thanks.

"The rod is weighed first total weight of the rod then it is hung from a fixture and the bearing end is weighed."

The rod is weighed first to determine the total weigh of the rod. Then the swing or rest weight of each rod end is weighed, and the total rod weight is proportioned based on the swing weights. Fix one end to swing on a journal, and weigh the other end with it resting on a scale. The total of the big and small end swing weights combined is more than the total rod weight, so you need to calculate the proportion that is rotating weight (big end) and that of the reciprocating weight (small end) from the total swing weight, and then calculate the percentage of the true rod weight assigned to the rotating and reciprocating weights to add into the bob weight.

Some guys just swing and measure the big end swing weight, and subtract it from the total true rod weight. The difference between the two methods is that the simpler method results in a higher bob weight due to the 2x multiplier, and heavier crankshaft counterweight (an overbalanced result).

Over balanced is a condition where the crankshaft counterweights are heavier than needed for neutral balance. A lighter replacement rod/piston assembly usually results in an overbalance condition. Most balance Technicians believe over balance is preferred to an under balanced assembly. Some engine builders believe the heavier than neutral balance counterweights have some type of flywheel effect that alters the natural order vibration frequency bandwidth to help keep the oil clearance in the journals consistent approaching the top of the stroke where the clearance slack opens up from the piston changing direction (YMMV). The beauty of engine building is that things like over and under balance are still outside the scrutiny of profitable and regulatory science, and as a result experience and empirical factors like the 2X rotating weight factor are still open to experiment.

The take-away is to remember with both methods is that a rod upgrade from 3/8 bolts to 7/16 bolts on identical weight rods will result in a higher percentage of the total rod weight on the rod big end, with more rotating weight & less reciprocating weight, and a resulting change in bob weight. A similar but opposite effect occurs after material is removed to resize or recondition a rod.

As Robert61 mentions, "the main thing is if all of the components are matched i.e. rods all the same Pistons all the same bottom line is it doesn't affect the motor that much. If you have one rod considerably out it will cause a vibration."

 

This may fit here. Years ago, I had to replace a single piston in the 350 Chevy, 360 Mercruiser engine. I bought a replacement, noticed it was heavier than the OEM piston it was replacing. Two days before Memorial Day and I wasn't about to hunt down a shop to match weights.

When I started the engine, it had a vibration it never had before. Not a big one but I could feel it. Nobody else noticed.