I am told by my local shop that they cannot balance my rotating assembly with the pistons assembled on the rods. Not sure why that makes a difference but that what they say. The issue is that they also feel like if they remove the pressed in pins it will destroy the hypo pistons. My question is does this sound correct, the part about not balancing because the pistons are assembled to the rods? I agree that most likely the pistons will be damaged if they press the pins out. Second, is it worth buying a new set of pistons just to have the assembly balanced? If I go that route I will have to have the bores redone as well in the block to match the new pistons. Maybe not if I go with the exact same piston? If I have to buy a new set I may go with a forged set.

I thought that's exactly what you had to do. You weigh each assembly (piston and rod) to get the lightest bobweight. Then you match the heavier ones by lightening them. Then you balance the crank to the bobweight. Maybe I'm wrong.

I think they are correct, because you have to weigh the big and small ends of the rod so that they can calculate how much weight will be on each end. The scales that Ive seen used require that the pistons be unattached to the rods. Someone correct me if I'm wrong.

I thought it was something like that that. This shop is widely recommended in my area as being the place to go for good work. How about my second question is it worth the cost of a new set of pistons to get the assembly balanced?

Well you can get a set of hyperuetetic pistons from Speed Pro for about $100, I would say yes.

Pistons and rod are weighed separate, machined into pairs, assembled then the crank from harmonic blancer to fly wheel is dynamically balanced.
The issue is that they also feel like if they remove the pressed in pins it will destroy the hypo pistons.
There is always a risk of distorting pistons and bushes. This operation should be carried out in special jiggs with experianced assemblers. The pistons are measured in several places then checked afterwards....in the hands of a good assembler, this is generally OK...thu not garanteed to not to have to replace a piston.
At this piont I question, why did they not explain this to you, and do they have these jiggs and experise?
Second, is it worth buying a new set of pistons just to have the assembly balanced?
Well I would 1st be looking for someone experianced in removing, go that route, and if pistons distort, be prepared to get a new set...
A fully balanced dynmaically engine is worth every penny, be it a racer or a daily....having a smooth running engine, that revs out fast, and makes a huge difference to the long lasting and reliablity
If I go that route I will have to have the bores redone as well in the block to match the new pistons. Maybe not if I go with the exact same piston?
Measure you bores, they have most prob been std oversized bored and measured for final hone...Generally pistion machining is very accurate from a good manufacturer....Check the new pistion diameters before purchase.
There is also a slight diff between brands in piston specs and diameters...
I have/are going thru a similar issue at the moment...thu different reasons...
I found the Aussie ACL pistions are slightly larger and are designed to work with less tollerance....
eg Sealed power are more fogiving piston to bore from 1 to 4/1000
ACL are are slightly bigger in diameter with a tollerance between just under 1/1000 to max of 1.5/1000
I believe the ACL is avalible in the States now...and it is a very nice hi performance piston....and very piston is identical in weight and dimensions out of the box.
U can do what I do with Camaro parts form the States, buy from NZ or
Aaussie and get shipped

Greg,
The pistons can be easily removed without hurting them if you have the correct fixture that actually goes into the piston and supports the rod itself instead of the piston. I don't have one, but have a friend at another shop that does and on the rare occasion that I need to remove pistons without hurting them he does it for me. Most of the Sealed Power Hypereutectics are basically impossible to remove any other way without hurting them :(

As far as balancing with he pistons on the rods goes ..... I might get smacked for this but .... if the rods are good rods, such as Scat, Eagle etc ... I have managed to get a weight on the big end of the rod with it still attached to the piston, that and the total weight of the assembly can get you pretty darn close, within a gram or two. Now if they are GM rods, forget it, they vary too much and the rods need to be balanced individually.

Now if they are GM rods, forget it, they vary too much and the rods need to be balanced individually.
Yep.

I gotta comment!!

First off, what rods do you have?

Second off, you should have what the pistons should spec at in weight! NOT what each one weigh's, but what they want them to weigh.

Check this weight against a stock piston so that you know how much diff there is between them and if they differ by more than a small amount, then re-balance!!

That said, I ran my old junk301 a long time HARD and it was made out of unbalanced well, junk parts and cheap W/JCW cast pistons!!

And I'm talking up to my points cut out and I figure that was about 7500rpm!!

Not the best, but I still did it as my daily driver way back then..

pdq67