I'm re doing my old Turbocharged Engine I built in the mid 1980's that's been sitting around since I built a replacement. I'm building a conventual higher compression SBC 350 for Autocross and I'm trying to both keep the costs down and use up a bunch of parts I have laying around. So I need to replace the Pistons to get the Compression up. New Pistons can be had that are much lighter than the ones I'm removing. I have a set off H beam 6" Rods laying around that are old enough to still be in the Heavy era Rod era that I would like to use. So, according to my Postal Scale I can have a New Piston Pin and Ring package that weighs 650 Grams and still keep the total Weight of the Piston-Rod combo the same or less than what I have now. Yes I realize that I am going to have to re balance the Rotating Assembly, but I'm trying to avoid adding weight to the Crank. I thinking that rather than spending a lot on a heavy metal balance job I'd be better off buying lighter Rods.
Ok so here is my question. When it comes to balancing can I just subtract the new Piston- Rod Combo from the old, and as long as it is less, then a less expensive removal of weight from the Crank can be done rather than adding weight ? Or is it not that simple ?

 

Read this article and pay attention to the table

https://www.hotrod.com/articles/ccrp...ine-balancing/

this article too

http://www.neponsetvalleymachine.com...balancing.html

from that, you can see how the simple formula works ... and how you can accurately compare what was balanced to what you want to swap in.

You'll need a scale with accuracy & precision within 1 gram.

here's a decent example of a bobweight table

If your old bobweight is heavier than you new bobweight, Shop can probably lighten crank by drilling/cutting ... if old BW is lighter than new BW, then Shop adds metal

but use the tables formulae ... it's Not simple what is a rod's total weight ... it's big end weight & little end weight and then a percentage.

 

Call Mike at QMP racing in Chatsworth Ca he will give it to you straight. Nice guy does real high end chit

 

Thanx for the heads up. Interesting Link.

 

One potential problem is that most 6 inch rods are made for radius ground cranks.

 

You need to do the bobweight sheet then have the crank spun on a balancer to know for sure.
Chances are that if the piston is much lighter you will only have to drill it.....
Take your pieces to a reputable shop and have the sheet done and crank spun up.
If they tell you that you need mallory.....compare the price per slug to a new set of rods.
Additionally, if you are staying under 7000rpm and 600 horspower....any of the newer Eagle or Scat I-beam rods are much lighter and a LOT stronger than people give credit for. Do not hesitate to purchase a set....they can be had for about $250 a set.
Trying to make old stuff work is more work than it is worth sometimes......but if you have a high dollar old school crank.....it might be worth it. But consder that a premium rotating assembly balanced, ready to go with rings and bearing can be had for $2000 these days.

Jebby