On a 340 that has the crank already balanced would you or would you not rebalance it with the flywheel,stock rods and dampner?

Thanks Moparts

the question would be...how much has the weight of the rods ...pistons have changed?

Need more info. Is the recip assy all matched up from a previous build? Is this a stock engine with a 6000RPM redline? Balancing is nice to do and cheap however Chrysler engines of the muscle period were notorious for having terrible balance from the factory, they were also known for kicking the crap out of and also outlasting the competition so I guess that has to tell you something.
I know everyone will now tell me what a hack I am, but thats the truth of it all.
Sheldon

The engine was already balanced but had very high compression pistons and I don't know what for rods. I'm going to put about 9:5 to 1 pistons in it and stock rods .I want to make it a little more then stock but not to wild since it is the original motor.

I just bought a cheap Harbor Freight flat gram scale for ten dollars and change and it weighs as accurate (so far ) as my buddys high dollar one. rig up a fixture in addition to your dial indicator stand & get the rod level sideways and horizontally (bore centerlines level) and weigh the small end plus seperately weigh the piston/rings/pin/locks for a reciprocating weight figure then reverse the rod/level it & weigh it w the bearing halves for rotating weight. Dont forget to zero the scale for the tare weight of the fixture you rig up. post these figures or call your machinist. this will let you if you choose to (or on a future build) polish the forging marks out of the sides of the rods and a bit of judicious grinding on the piston etc all to lighten everything then just need to hand over the crank & your math & your bill will be MUCH cheaper. The scale/fixture & 2 wheels to slip on the rod to hold the 2 rod bore sizes level all is less than the labor for a shop to weigh everything since you are now paying just for spinning/drilling the crank. I'm w Sheldon I'm a stickler for it being balanced. Yes depends on the difference between the old/new. Maybe OK (close enough) as is but I cant leave anything to chance. If you do this post your differences: (4) figures

Anything hotter than production motors, balance it. Motor lasts longer and makes more power.

Balance it.Its not that expensive and will make the motor happy.If balancing wasnt important they would not have balanced the Hemi from the factory.I agree factory wasnt great on balancing the lower end motors.Changing to different weights will make a difference in rotating weight.Rocky

Yes, you MUST re-balance.

I would rebalance it.

The whole point of balancing it is to match the counterweights on the crank with the weight of the rods & pistons so that the engine rotates smoothly without vibration. If you have no idea what pistons or rods were in it you have no idea if the balance would be correct with the new pistons & rods.

If you don't rebalance it you may have a nasty vibration that won't go away without a complete teardown and balance. The cost of balancing now it is cheaper than the cost of a teardown.

If you are going back to STOCK rods and 9.5:1 pistons, then try to find a piston that are as close as possible to the weight of the STOCK pistons. You may be able to bring them closer to stock weight if they are a little bit heavy, and if so, balancing won't be necessary. If they are heavier then the stockers by more then a few grams, then yes, balance it.

This assumes that the crank and rods you use have not been molested. I have also heard that the stock balance was poor, but, I've also read that stock balance was pretty close. I think that's a toss up.

I had my 273 rebalanced when I had the machine work done. I think it was either 50 or 75 and will be well worth it. For me the main reason to balance is longevity of the engine. An out of balance lower end will wear out bearings faster and cause rod bearing failure sooner.

Even using stock .030 over 10.5:1 pistons, I had to change out one rod, and rebalanced not only for that rod, but because the pistons were +-.05 grams. Rick took a fair amount off the back of the #1 rod.