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
:wrenching:
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 :alan2cents:
They are definitely heavier than the cast ones were.
Quote from: jimynick on April 27, 2021, 07:38:39 PM
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 :alan2cents:
Always balance new pistons ;)
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. :huh:
Quote from: Pinionsnubber on April 27, 2021, 08:22:01 PM
Always balance new pistons ;)
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.
I'm convinced; thanks for all your input folks :twothumbsup:
Quote from: HP2 on April 28, 2021, 06:29:31 AM
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.
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
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....
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.
Yep, taking it all in next week to be balanced. :bigthumb:
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.
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 .
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.
I don't recall how much I was charged for it but it was around 200-300. It had the whole motor done. But there wasn't any discussion about that number changing because it was harder, more labor intensive, or needed more weight. As far as I know it didn't need any balancing and they kept the 200 or so bucks for no work. LOL. Not sure if this all changes if you take it in just for balancing though.
My shop had the price sewn in for balancing. My NM 440 needed a .020. Small money I gladly spend as I hope this is the last time I see the lower end apart, especially in pieces in the middle of a huge oil scatter if you catch the drift.
Why would you choose new pistons that are heavier than the originals?????
Loosing weight is how you make an engine rev quickly & at the same time it's easier on parts because your throwing less weight around....
Quote from: 1 Wild R/T on April 29, 2021, 03:48:38 PM
Why would you choose new pistons that are heavier than the originals?????
Loosing weight is how you make an engine rev quickly & at the same time it's easier on parts because your throwing less weight around....
I was thinking the same thing. Some of the older aftermarket forged pistons were heavy. But, nowadays, most forged pistons are lighter than the factory stuff.
The machine shop I took my '68 AMX 390 engine rotating assembly for balancing required everything from the harmonic damper to the fly wheel/flex plate. He discovered one of the new pistons was 6 grams heavier than the lightest piston. He said, that is unacceptable. We both contacted the piston manufacturer and they over-night-ed a new piston. When I picked up the rotating assembly, the owner said it is balanced within 1 gram from the harmonic damper to the flexplate. :stayinlane:
Quote from: IRON MAN on April 30, 2021, 10:59:30 AM
The machine shop I took my '68 AMX 390 engine rotating assembly for balancing required everything from the harmonic damper to the fly wheel/flex plate. He discovered one of the new pistons was 6 grams heavier than the lightest piston. He said, that is unacceptable. We both contacted the piston manufacturer and they over-night-ed a new piston. When I picked up the rotating assembly, the owner said it is balanced within 1 gram from the harmonic damper to the flexplate. :stayinlane:
If your engine balancer requires the clutch, flywheel, damper, pistons, rods, crankshaft and bearings...then you NEED to very closely supervise them, and specify how you want your engine balanced.
The fastest way to balance the rotating assembly is to calculate a bobweight, add the mass to the rod journals, add the damper, flywheel, and clutch...and spin it up. Then you an add weight to the clutch to bring it to spec. This is the absolute WRONG way to balance an engine, unless you are a 1/4 mile jockey and you run your engines for 10 seconds at a time.
The correct way to balance an engine is to balance the components individually, against a calibrated standard. First you match and balance the components...then you add and subtract metal to balance the crankshaft. Then you add the damper flywheel and balance those.
You didn't say whether your 340 was a forged or cast crank. If it's a forged crank 340...those are internally balanced...and you should NEVER see an external weight added to the clutch, flywheel, or damper. Those pieces are all neutral balanced, and should never need weights. If they do need weights, then they should be replaced.
I have pulled apart numerous engines that were balanced as a matched set. When the clutch needed replacement, the engine needed to be re-balanced.
External balance engines are a little different...because mounting location and rotation angle becomes important. For those, you do need to balance them as an assembly...but ONLY after first balancing the crank individually with a calibrated flywheel.
If you are going to race your MOPAR, and rebuild the engine every season...then the balance strategy is completely different than the person that is building his engine to last for years. Many Race shops don't think this way. All OEM manufacturers think this way.
To use the tire and wheel analogy...imagine that you balanced your tires by WELDING a weight to the back of the wheel...then very carefully marked where to mount the tire on the rim. As long as you use the same tire/wheel combination...it works just fine. but if you ever replace your tires...you would then need to throw away your rims.
My last balancer told me not to worry about longevity...because I could always just bring the parts back in and he would re-balance them for me. He didn't see that as a problem...
The Only situation where I can see choosing a heavier piston is if you were removing a weaker piston such as a Hyperutectic & replacing them with a forged piston which could be heavier or taller adding weight with compression , otherwise I agree why ever go to a heavier piston !