Rotating Assembly Balance Question

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middleagecrisis

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I'm putting together a 360 shortblock that I bought completely disassembled. The rotating assembly looks to be internally balanced with heavy metal in the crank throws, material removed from the rod big end balance pad and an 318/340 style internal harmonic balancer. I weighed each assembled rod, wrist pin, piston and rings and came up with a difference of 2.1 grams between the lightest and heaviest of each assembly. The pistons were not marked for the cylinder they came out of. Is 2.1 grams difference acceptable, or do I need to try and get it lower? Obviously, I'll be measuring the piston to cylinder walll clearance for the best fit of each piston, but do I need to try and match the lightest/heaviest pistons on the same rod throw, front to rear, etc. or just where they fit best and don't sweat the 2.1 gram difference? The last engine I built, I measured each piston, pin and rod separately and managed to get each assembly to within a gram of each other after assembly. Smoothest running engine I've ever owned! That being said, 2.1 grams difference seems a lot, but is it really considering each assembly weighed ~1619 grams? Thoughts?
 

Sounds like you're all over it. But when I put my 340 back together I had a new eagle forged crank going in and even though everything had been prior balanced I had to flywheel and rotating assembly spun balance to the new crankshaft. It's a piece of mind that's well worth the 300 bucks.
 
middleagecrisis said:
I'm putting together a 360 shortblock that I bought completely disassembled. The rotating assembly looks to be internally balanced with heavy metal in the crank throws, material removed from the rod big end balance pad and an 318/340 style internal harmonic balancer. I weighed each assembled rod, wrist pin, piston and rings and came up with a difference of 2.1 grams between the lightest and heaviest of each assembly. The pistons were not marked for the cylinder they came out of. Is 2.1 grams difference acceptable, or do I need to try and get it lower? Obviously, I'll be measuring the piston to cylinder walll clearance for the best fit of each piston, but do I need to try and match the lightest/heaviest pistons on the same rod throw, front to rear, etc. or just where they fit best and don't sweat the 2.1 gram difference? The last engine I built, I measured each piston, pin and rod separately and managed to get each assembly to within a gram of each other after assembly. Smoothest running engine I've ever owned! That being said, 2.1 grams difference seems a lot, but is it really considering each assembly weighed ~1619 grams? Thoughts?
Click to expand...


You don’t weight the rods assembled.

At any rate it doesn’t matter. Send it.
 
middleagecrisis said:
They were already assembled and I wanted to see how close each piston/rod assembly weighed to each other.
Click to expand...


I get that. But 2.1 grams isn’t much. And where does the 2.1 grams come from?

The pin end?
The Big end?
The pin?
The piston?

All of them?

If you take the above 4 things that’s .5 grams per measured part.

It’s tolerance stack up.

Like I said, send it.
 
You have 2 guys here who have balanced lots of cranks saying its fine, like I said before, run it.
 
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