Engine balancing

[Mrbill426]

Regarding engine bottom end balancing at what point does it become a "MUST DO" for high revving race intentions as apposed to a daily driver?   Our 340 has been bored over .020",  the crankshaft turned down .010" on the rod & main journals, and the rods have been resized with small ends re-bushed and new rod bolts installed.  The stock (1972) pistons were I believe cast aluminum 8:1 flat tops while the replacements are forged 10:1 with domes and are quite a bit heavier but within a few grams weight of each other. 
Question is does a piston change like this make it a MUST balance or would balancing be more of a nicety??

Thanks

[jimynick]

If the pistons have a noticeable weight difference to the cast ones, I'd re-balance it. Think of it this way, what component is going to see more stress, and go accordingly. Money well spent in my opinion. Just my 

[Mrbill426]

They are definitely heavier than the cast ones were.

If the pistons have a noticeable weight difference to the cast ones, I'd re-balance it. Think of it this way, what component is going to see more stress, and go accordingly. Money well spent in my opinion. Just my 

[Pinionsnubber]

Always balance new pistons 

[Mrbill426]

I'm referring to the whole rotating assembly, but yeah I am leaning that way.  Will be interesting to see how long it takes now.  It took five months for what was done so far; the longest wait was for the local crank grinder to get to it. 

Always balance new pistons 

[HP2]

Consider it in tire/wheel terms...if you only drove 30 mph, 6 blocks to the store, once a week, you could get away with  wheels that weren't spun balance. If you commute on the highway daily, you want them within fractions of an ounce of correct. If you hit triple digits on a race track, you want them as close to ideal as possible.

Old engines are similar. They were built to go down the highway just fine for the vast majority of people the vast majority of the time. If you want to exceed those standards, then the more precise you make the internals, the better it will perform, the smoother it will run, and the longer it will last.

[Mrbill426]

I'm convinced; thanks for all your input folks   

Consider it in tire/wheel terms...if you only drove 30 mph, 6 blocks to the store, once a week, you could get away with  wheels that weren't spun balance. If you commute on the highway daily, you want them within fractions of an ounce of correct. If you hit triple digits on a race track, you want them as close to ideal as possible.

Old engines are similar. They were built to go down the highway just fine for the vast majority of people the vast majority of the time. If you want to exceed those standards, then the more precise you make the internals, the better it will perform, the smoother it will run, and the longer it will last.

[Shane Kelley]

I would recommend balancing with the piston weight change. Rod bolts and resizing effected the big end of the rod weight as well. Balancing will increase your bearing life and the engine will run and feel nice and smooth.

If you just assemble the engine will it run fine? Yes
Will you notice anything wrong? Maybe/maybe not
But why take the chance at this stage of the build? IMO

[JonH]

On top of all that has been said, balancing is very cheap in the grand scheme of an engine rebuild. If you are in a time crunch find a different/better machine shop....

[tparker]

I think it really depends how out of balance it is and how that weight stacked up. The higher the RPM would exacerbate the issue as well. It's going to push some stress on the components if it is off. And I agree the cost isn't that great.

Did the factories balance each engine? It seems like that would be a time intensive process.

[Mrbill426]

Yep, taking it all in next week to be balanced.

[Racer57]

How much does it cost to balance an engine ? I realize there's lots of variables, but I've never heard anyone say how much they spent.

[Chryco Psycho]

Usually $200 range but it can vary depending how much it is out of balance , how much weight has to be removed or added , Mallory metal is very heavy & expensive .
Most balancing involves beam polishing the rods & grinding metal for the rod cap pad most of this you can do yourself with a basic scale.
the piston can be lightened in the right places if needed & hopefully you end up lighter with the piston & rod assy than the factory weight , then the crank weight just needs to be lightened to match the weight .
Adding weight is much more difficult so if the piston & rod somehow end up heavier or you are trying to internally balance & external balanced engine & weight needs to be added  this is where a lighter piston & rod can really help , strokers typically remove about 6+ lbs from the rotating assy .  Sometimes just welding the crank is enough to add the needed weight , if mallory is needed the price just jumped up .
SO overall it really depends how far out of balance things are & how much labor is required to correct it .

[HP2]

I've got the fixtures that allow me to balance connecting rods myself.  This takes a fair amount of time as an enthusiast without milling equipment, but it also takes labor out of the balancing costs.

Once I have all the weights of the rods equalized (new pistons are usually dead nuts equal), I take these weights along with the pistons, rings, bearings, weights to the shop with the crank. They then perform the bob weight balancing on the crank.

The last race 360 I built was converted to internal balance. Using the process above, I still had around $400 bucks in just the crank (and this was in the mid 90s) thanks to the mallory costs.

[Mrbill426]

@Racer57  I was quoted $250 but that is surely the base charge and as mentioned before it can go up from there.  I am convinced it will be worth it in the end if I am avoiding expensive headaches in the future.

How much does it cost to balance an engine ? I realize there's lots of variables, but I've never heard anyone say how much they spent.