Here are the facts!
Alright! I'm Mechanical Engineer, not engine builder...so let's set the facts straight! I am going to explain the facts from a mechanical standpoint, not as an experienced engine builder.
The forged rods and pistons stock are heavier than stock. Scaling the pistons and rods are only part of the balancing process.
YES, matching the reciprocating weights, pistons and rods, IS important but as stated is only PART of the process.
I know but thats a pure DIYable stuff, a grinder and a good scale and grinding the good place...
DO NOT DO THIS!!!! I cannot stress this more! A forged part is made in such a way that the discontinuities/grains in the material are lined up and at specific places. This is why they are stronger than a cast part. Grinding on ANY portion of the piece introduces irregularities in the part causing stress concentrations in that area therefore creating a place for failure to occur.
Just because everything weighs the same doesnt mean that everything is perfectly balanced. Its where the weight is on the rotating assembly that is important too as it will affect how forces are applied to the crank.
This statement is also completely correct. The rotating assembly, ie the crankshaft, has to be balanced as well. Reciprocating and rotating assemblies are two COMPLETELY different animals and require individual attention. The crankshaft should be balanced when rebuilding to ensure that it is indeed balanced and will work with the new reciprocating parts. It should also be noted that the changes in the reciprocating part weights will change the harmonics of the rotating shaft further increasing the need to be sure every moving part is precisely balanced.
Performance built engines REQUIRE precise balancing.
You don't balance it with the flywheel or crank pulley. They are balanced independently. If you replaced the parts or machined the flywheel you would throw off the balance, thus is why the engine is internally balanced and should stay that way.
The forged rods and pistons stock are heavier than stock. Scaling the pistons and rods are only part of the balancing process.
Just to reiterate, someone else saying the same thing. Balancing a crankshaft and scaling pistons BOTH need to be done in order to avoid catastrophic failure.
As a final note, by reading this post I will assume that you are rebuilding your current engine. This would mean to me that you are reusing your crankshaft. As anyone with any experience with two parts touching each other with relative motion between them will tell you, things wear out! No matter how good the factory balance is or how expensive the oil you use is, over time the mating surfaces will wear out. Because of the variations in the weight the pistons and rods attached to the crankshaft, wear is uneven. Uneven wear will cause the balance to be thrown off, even if only slightly, it will be off. Again, reason to balance your crankshaft.
Plus, you are building your engine, I would assume to create more power. Replacing FACTORY pieces with AFTERMARKET ones. If this is the case, then why would you trust a FACTORY crankshaft balance to be adequate?
Just listen to what everyone is telling you, balance the crankshaft AND scale the pistons. But don't get it done at that shop, way too expensive.
