New pistons 7,6% lighter, do I have to rebalance?

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Djakken

Djakken

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In the middle of a piston swap, std 273 to domed 10,5:1.
Just messured the weight, the old one is 796 gr, incl. pin.
The new, domed, is 736 gr, incl. pin.
Lighter pistons is good, but do I need to rebalance the crank?
 
I would say yes you just shaved a 1lb off your rotating assembly. Best to get it balanced. 1lb is 453g you just took off 480g total.
 
Question from a non-tech...if all the pistons are lighter, wouldn't they still be equal weights and then everything would still be in balance? Or are there variations and that's why the rotating assembly needs to be rebalanced? Could someone explain it to me? Thanks in advance.
 
No.
Now your counter weights on your crankshaft will weigh more than your rods + pistons that they are trying to balance out. I would say that your weight savings is enough to warrant a balance job. At least I would.
 
goldfish65 said:
Question from a non-tech...if all the pistons are lighter, wouldn't they still be equal weights and then everything would still be in balance? Or are there variations and that's why the rotating assembly needs to be rebalanced? Could someone explain it to me? Thanks in advance.
Click to expand...

The counterweights on the crankshaft are there to balance the rotating weight vs. the reciprocating weight. The reciprocating weight includes the pistons, circlips (if any,) piston pins, and rings. I know the bearing inserts are in this equation, too.

The counterweight has to act to nullify the weight of all that stuff so the engine doesn't vibrate as the pistons, etc., go up and down.

Change the weight of any of that stuff, and the equation changes.

Lighter pistons will require a lighter counterweight.

I'd get it balanced, if it were mine...:read:
 
Thank you for the explanation, now I get it, it's because of the reciprocating/rotating dynamics.
 
Balancing an Engine​

To balance a V-8 engine requires 50% of reciprocating weight plus 100% of rotating weight, for a V-8 engine.

Reciprocating weight: Piston (1), rings (1 set), wrist pin (1), and small end of connecting rod (1).

Rotating weight: Big end of connecting rod, rod bolts & nuts, rod bearings, & oil supply to crankshaft bearings. Because there are two rods to each journal then the rotating weight is x 2.

The pistons are all machined to weigh the same. The big end of connecting rods and small end of connecting rod are machined to weight the same (separately). Then the weights of the pistons, big end of connecting rods, small end of connecting rod, one set of rod bearings, one set of rings, one wrist pins and oil weight (usually 6 grams) is recorded and the bob weights are made up from the formula for the type of crankshaft being balanced. Then the bob weights (4 for a V8), dampener, flywheel (flex plate) are bolted to the crankshaft and spun to balance the left side and the right side of the crankshaft. Then spun with the left side and right side coupled together to make sure the crankshaft is still in balance.
The crankshaft is either welded on the counter weights (or heavy metal installed) or drilled on the counter weights (left and right) to bring the crankshaft into balance.
 
Oklacarcollecto said:
Balancing an Engine​



To balance a V-8 engine requires 50% of reciprocating weight plus 100% of rotating weight, for a V-8 engine.

Reciprocating weight: Piston (1), rings (1 set), wrist pin (1), and small end of connecting rod (1).

Rotating weight: Big end of connecting rod, rod bolts & nuts, rod bearings, & oil supply to crankshaft bearings. Because there are two rods to each journal then the rotating weight is x 2.

The pistons are all machined to weigh the same. The big end of connecting rods and small end of connecting rod are machined to weight the same (separately). Then the weights of the pistons, big end of connecting rods, small end of connecting rod, one set of rod bearings, one set of rings, one wrist pins and oil weight (usually 6 grams) is recorded and the bob weights are made up from the formula for the type of crankshaft being balanced. Then the bob weights (4 for a V8), dampener, flywheel (flex plate) are bolted to the crankshaft and spun to balance the left side and the right side of the crankshaft. Then spun with the left side and right side coupled together to make sure the crankshaft is still in balance.
The crankshaft is either welded on the counter weights (or heavy metal installed) or drilled on the counter weights (left and right) to bring the crankshaft into balance.
Click to expand...

That's a great explanation that even ~I~ can understand!!!

Thanks!!!:cheers:
 
Thank's guys for the information.
But I did a mistake, forgott to messure the the weight of new pistons with the new rings. The weight incl. rings, pin, etc is equale for new and old pistons.
I must have been tired last night.
 
Djakken said:
Thank's guys for the information.
But I did a mistake, forgott to messure the the weight of new pistons with the new rings. The weight incl. rings, pin, etc is equale for new and old pistons.
I must have been tired last night.
Click to expand...


HOW DARE YOU!!!!:violent1:


lol!!!
 
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