Will I have to balence the rotating assembly on forged rods and pistons?

[UtOBS]

I’m planning on putting high comp forged rods and pistons but keeping bore and stroke stock. I had someone tell me I will have to get my rotating assembly balanced. Is this true? I thought it was only if you change the stroke and bore to a 377 or 383? Thanks, Korbin.

 

[tayto]

changing pistons or rods or both will require balancing unless they are similar weight to stock, but getting balance checked is good practice reguardless

 

[Schurkey]

The GM balance is not that wonderful to begin with.

Change the weight of the rotating/reciprocating components, and all bets are off.

Don't let the balance shop screw-up the damper or flexplate/flywheel when they should be working on the crankshaft to achieve balance. This is a common "hack" job.

For the record, the original rods are either forged mild steel, or forged powdered metal. Either way, the stock rods are forged.

 

[Hipster]

I’m planning on putting high comp forged rods and pistons but keeping bore and stroke stock. I had someone tell me I will have to get my rotating assembly balanced. Is this true? I thought it was only if you change the stroke and bore to a 377 or 383? Thanks, Korbin.

yes, get it done. Smoother running and longer lasting. Different sets of rods can have different weights on each end than factory,( weighed at each end separate , then comparatively for total weight) pistons and pins can be different weights, they calculate ring pack weights , bearings, etc. Rotating weight is calculated separately from reciprocating weight, a little more math and come up with a bob weight for the crank to be balanced at. It could be way off from stock, as said, Gm was not close factory but not critical in low rpm engines. They need the balancer and flexplate to do this. I normally do them new depending on how old, or what condition the old stuff is in. Maybe sfi flexplate if I plan on some romping. No need to mess with balancer with deteriorated rubber and crack prone stock umpteen 1000's flexplate. I know of only one application where a higher cr pistons and it's pin is within a few grams of oem, with stock rods they can be run, but balanced is still better. Don't know your plan but every set of aftermarket rods I've used needed the pin ends sized if bushed for floating pins. Undersized to allow custom pin fitment seems to be the thing. Street engines typically get balanced at a slightly different balance factor than a race engine that lives at high rpm from 5-9k

 

[GoToGuy]

Are you assembling the engine?

 

[b454rat]

IMO, any time an engine is torn apart, pistons have been changed, rotating assembly needs to be balanced. New balancer and flexplate too.

 

[Erik the Awful]

Digital Scale

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My first SBC build, I just threw the engine together without balancing. It runs fine. My second and subsequent engines I balanced with a cheap Harbor Freight scale. There's a significant difference in vibration. When I sold my Suburban, it had a cracked block (buyer was aware), but the buyer said it was the smoothest running small block he'd ever seen.

 

[Schurkey]

Just be glad you guys don't have a Buick. Goofiest OEM balance-job I know of. And when you replace the flywheel/flexplate, the crappy factory balance is lost entirely. If you replace the damper, and don't remove the balance pins from the old damper, and pound them into the correct positions in the new damper...the crappy factory balance is lost entirely.

 

[Hipster]

Just be glad you guys don't have a Buick. Goofiest OEM balance-job I know of. And when you replace the flywheel/flexplate, the crappy factory balance is lost entirely. If you replace the damper, and don't remove the balance pins from the old damper, and pound them into the correct positions in the new damper...the crappy factory balance is lost entirely.

Can get into a whole bunch of stuff. forged/cast cranks, early/late models of same engine, counter weighted balancers/flexplates,etc. A guy I use weighs the pins and puts light pins in the heavy pistons before working on the pistons. pins bores and pins od's seem to be extremely precise, even on the name brand cheaper pistons.. Have come across a few mismatched disasters.

 

[Supercharged111]

Just be glad you guys don't have a Buick. Goofiest OEM balance-job I know of. And when you replace the flywheel/flexplate, the crappy factory balance is lost entirely. If you replace the damper, and don't remove the balance pins from the old damper, and pound them into the correct positions in the new damper...the crappy factory balance is lost entirely.

C5 Corvettes are balanced like that too, but the weights go in the flywheel.