383 rod/piston balance??

[Brent1960]

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What is the acceptable balance for the piston/rod assemblies for a stock rebuild?? This is a daily driver street engine application.

Master re-build kit from machine shop that did the grinding. Cast 366NP pistons .030 over, steel crank, RV cam. Got the hardware home and decide to check some weights. There is 32 grams difference from lightest to heaviest. The rings and bearing are dead on. I found 12 grams diff in the caps from the light to the heavy.

Should I run with it?, grind on the big ends until close or I run out of metal, or go back to the shop for a balance act and risk damage to the pistions and pins during dis-asssembly?

Rookie mistake I won't make again.

THanks

[rampage_82]

Yeah fortunately for us Mopar's factory balancing was one of the worst. Anytime you rebuild you need to have it balanced... Sorry Bud..

[Brent1960]

Too late in the game I read that mopar factory balance was an issue. The machine shop has a great rep but is more GM oriented. I've spoke with some who feel everything must be perfect and on the other side I've had a tech guy from a major piston supplier and a guy who builds HEMI engines who feel +- 15 grams is not an issue.

After making a jig to support the piston and small end of the rod parallel to the scale it seems 90% of the im-balance is in the big end. I'm considering covering up the piston and grinding the humps, bumps and some of the balancing pad and try to get it down to the range of the other 7. The machine shop does not seem to thrilled about trying to removed the press pins without damage to pin or piston.

Any ideas about the pros/cons of this action?

[Rich Kinsley]

That sounds like something I might try if I was in the same situation. I can't see how it'd hurt the rod if you don't have to get carried away. Keep things as clean as possible.

[cudabob496]

A properly balance rotating assembly helps the engine last longer and make more power. If plan on any serious racing or power, I'd tear it down and have it balanced. 7 to 15 gram diff seems excessive. I know that when my engine was assembled, they had the rod/piston combos within 1/2 gram. A good balance job may help your engine last twice as long. Has your crankshaft been balanced?

[toad490]

First thing is if your machine shop can't re and re the pins get another shop. Second thing is matching the weight of the pistons and rods is one thing, you then have to balance them to the crank, on a forged crank that ivolves adding or removing weight from the counterweights, internal balancing, on a cast crank I believe it's done with adding weight to the flywheel/convertor, not real sure on that.

Obviously if your using the same rod and crank combo that the engine came with that part of your "balance" is fine, the variable is the difference in weight of your stock pistons compared to the pistons your using. Personally I'd find a shop that can re and re your pins and balance it, but if your pistons are with in a few grams of the stock pistons I wouldn't worry to much about it. Lets remeber guys, it's a daily driver, street application.

[rampage_82]

Nope even on cast external balance you still add or remove from the counterweights, but not the case here it should be a forged unit, unless he put in a 400 crank or it's a 72 model...

[toad490]

Okay, so what is the difference between external and internal balance rotating assembley? Not arguing, just asking

[cageman]

external is using a weighted balancer and flex plate and or weight on the converter. Internal you modify the crank to whatever spec it needs.
Whats so hard about the pins, and I too would find a diff shop. Balancing is cheap.

[Brent1960]

Thanks for all the input so far.
It is the original internal balanced forged crank that just needed polish and original rods that were re-conditioned.
I made up another jig to support the big end and weighed the piston and small end and only have about 2 grams difference. A complete assembly with rings is within 2% of the weight of an original assembly.

[JVMopar]

The difference in internal and external balancing is that, for internal balancing there is enough weight in the crankshaft alone to balance it out, and for external balancing there isn't enough mass in the crankshaft to balance it out. So they added weight to both the torque converter and balancer to get it to even out. Mainly it's because of the weight differences between a forged and a cast crank, early 340's were internal balance and when they went to the cast crank they were external. And you can have a external balanced assembly internally balanced but they have to add heavy metal into the crankshaft to do that.

Again If I remember correctly I read that it was +- 10 grams of the stock piston weight. Then you don't need to rebalance.

I agree with Toad if your machine shop can't press out the pins without damaging the pistons. Find a good machine shop. Heck even at the GM dealer I work for we have a jig for pressing piston pins without damaging the piston or rod.

[Rich Kinsley]

Hmm, maybe you should leave well enough alone then. It's not customary to balance a plain Jane engine but it's only about $75 here and to me it's a nice upgrade/precaution.

[Tarrbabe]

Any machinist that has a problem r&r ing piston pins scares me.
The weights your talking about sounds excessive and can shorten the life of the engine. 32 grams don't sound like much but spun at 6000 rpm is an impressive load when those weights are moving in conjunction with each other. I vote to get the crank/rods/pistons balanced.

Just thinking out loud.

[Paul Precht]

The piston difference will be fine and won't hurt the engine. The cap weight is part of a bigger picture called bob weight, a motor can be balanced correctly and have caps that differ in weight, Paul.

[Cudadrag]

ok. on the same subject I am freshening up my 451. light hone. rings and bearings. 2 pistons being replaced by same make model and pins. Crank and rod/pistons were balanced when introduced to the stroker crank. The question is how touchy is the balance? I didn't think it would need again. This would be very slight if any adjustment. Am I wrong?

[Dirt Driver]

If the balancing requires adding weight it can be expensive for the material and the work. Normally the weighs are drilled on an the outside edge and the heavy metal welded to the crank. The outside is easiest to work on but I like to have the weighs drilled in the side so the force of the rotating crank isn't trying to break the welds. At high RPM's the welds can break and the added weight fly off.

The earliest 440's were carefully balanced. The 1966 and 1967 440's went through some sort of special balancing. Mopar wanted their largest engine to be smooth. The Imperials and New Yorkers competed with the other big cars that had their own special big engines at the time. Also just the size of the engines' parts may have had caused balancing issues the first years. They didn't do this for their other engines and I think the 440's stopped getting the special treatment in 1968.

[pishta]

I heard 10g is about the average on a production motor. You can get very close by weighing the rods without caps and equalizing to lightest by the top balance pad, then do the same with the caps, equalize to lightest. When you are balancing pistons, sometimes it is more efficient to lighten the pins than the pistons. Try removing 12g from an aluminum piston, from where? Also if you cant get everything balanced, remember that the paired rods are added to find the bobweight so you really only need to equalize 4 pairs of rods to balance. so a heavy and a light rod can be paired along with 2 equal rods with no ill effect as long as the 4 pairs weigh the same.
Pressing out the pin while saving the piston is basic, just use the proper support across the flat of the piston ajacent to the pin and of course leave a relief for the pin. Use an old pin that has been reduced in diameter for a mandrel. While pressing out,Its going to make a scary POP when it finally gives way. Be brave, its been done before....

[Cudadrag]

I'm using Ross 99484 pistons with stock 440 1851535 rods. The rods looked great and measured straight and true. The cylinder walls looked very good and honed nice. Average bore still very near 4.375. (Any various are in the 1/10,000 range. Outside my micrometers range.) The bearings and rings looked very good under 6x magnification. Crank measured great, well within tolerance. The pistons looked like shit. These 510-gram lightweight pistons looked chewed. Small hole in one, 3 with noticable top marking and all have some visible side wears. I'm guessing these pistons have about 80 to 100 runs on them. Is this the normal life of 510-gram pistons on 550-horse motor? It's really remarkable they came out of the same block that is now ready for re-assemble. (Once piston issue is addressed and secured.)

[Brent1960]

Originally Posted by pishta

I heard 10g is about the average on a production motor. You can get very close by weighing the rods without caps and equalizing to lightest by the top balance pad, then do the same with the caps, equalize to lightest. When you are balancing pistons, sometimes it is more efficient to lighten the pins than the pistons. Try removing 12g from an aluminum piston, from where? Also if you cant get everything balanced, remember that the paired rods are added to find the bobweight so you really only need to equalize 4 pairs of rods to balance. so a heavy and a light rod can be paired along with 2 equal rods with no ill effect as long as the 4 pairs weigh the same.
Pressing out the pin while saving the piston is basic, just use the proper support across the flat of the piston ajacent to the pin and of course leave a relief for the pin. Use an old pin that has been reduced in diameter for a mandrel. While pressing out,Its going to make a scary POP when it finally gives way. Be brave, its been done before....

I spend a lot of time confused so I'm just trying to clarify here. As best I understand from rebuild books, internet etc.. is that the pistons/rods/pins are balanced to each other to +/- X grams. Once they all equal each other one piston, rod, pin, rings are weighed and 4 grams is added to this weight for oil film. Then a % is added to this for rotation forces. This is the "bobweight".

The crankshaft is then balanced front to rear to itself and then weight assemblies that are the equivalent of the "bobweight" of one assembly are added to all rod journals. Lets say 2600 grams for giggles. Weight is then added or removed from the crank to meet the "bobweight" of the piston assemblies.

It is hard for me to believe that weight is added or removed to the crankshaft to make it match up to individual piston assemblies. I know you are supposed to keep rods matched to their respective locations if you do not get the crank machined and rods reconditioned. But if you did get the mains machined and the rods reconditioned and use new pistons it should not matter where they are installed the first time. Right? You just re-stamp the caps and move on.

The mains and rod journals were polished and the rods were reconditioned.

I have already installed the rods to check clearance with Plastigage and everything was exactly the same.

My feeble brains seems to think I could mix and move caps around or put assemblies in new locations and get close.

Or grind on the big end until balanced.

Or have the machine shop take the pins out have the pieces balanced to each other.

OEM assemblies were about 1800 grams and my assemblies range from 1785 to 1815 so if the piston/rod assemblies are balanced I should not need to balance the crank.

Once again I am about 30 grams out of balance and it all seems to be on the big end. How close is close enought?

+/- 4? +/- 0.5?

This is just a stock engine with an RV cam and just planning to cruise around, not race.

I am probably completely wrong here so tell me if I am I was was blessed with a thick skin and am not easily offended so blast away.

Thanks

[Cudadrag]

Originally Posted by Brent1960

I spend a lot of time confused so I'm just trying to clarify here. As best I understand from rebuild books, internet etc.. is that the pistons/rods/pins are balanced to each other to +/- X grams. Once they all equal each other one piston, rod, pin, rings are weighed and 4 grams is added to this weight for oil film. Then a % is added to this for rotation forces. This is the "bobweight".

The crankshaft is then balanced front to rear to itself and then weight assemblies that are the equivalent of the "bobweight" of one assembly are added to all rod journals. Lets say 2600 grams for giggles. Weight is then added or removed from the crank to meet the "bobweight" of the piston assemblies.

It is hard for me to believe that weight is added or removed to the crankshaft to make it match up to individual piston assemblies. I know you are supposed to keep rods matched to their respective locations if you do not get the crank machined and rods reconditioned. But if you did get the mains machined and the rods reconditioned and use new pistons it should not matter where they are installed the first time. Right? You just re-stamp the caps and move on.

The mains and rod journals were polished and the rods were reconditioned.

I have already installed the rods to check clearance with Plastigage and everything was exactly the same.

My feeble brains seems to think I could mix and move caps around or put assemblies in new locations and get close.

Or grind on the big end until balanced.

Or have the machine shop take the pins out have the pieces balanced to each other.

OEM assemblies were about 1800 grams and my assemblies range from 1785 to 1815 so if the piston/rod assemblies are balanced I should not need to balance the crank.

Once again I am about 30 grams out of balance and it all seems to be on the big end. How close is close enought?

+/- 4? +/- 0.5?

This is just a stock engine with an RV cam and just planning to cruise around, not race.

I am probably completely wrong here so tell me if I am I was was blessed with a thick skin and am not easily offended so blast away.

Thanks

Thanks for the details. Does make sense even the adding of weight. I have wondered how they balanced but if you start with a trim up (or down) then you can shave to that weight. I will be bring my rods/pistons/pins and crank in for such balance especially after new pistons are introduced. As my last post said they got chewed. I'm surprised the block did not reflect but I guess that is the difference of aluminum and steel. The pistons are the the only aluminum part in the equation thus they took the brunt of the abuse. So drag racing is tough on your motor? Lol!

Thanks again for showing me this is not the time to be cheap!

[pishta]

If your 40g heavy on the big end, remember that the big end is the rotational mass. The little end and piston pack are the reciprocating mass. The recip. mass is easy to equalize, just weight each piston pack and the little end of the rod (balancing off the center of the big end, like on a knife edge at the seam) then grind at the little end balance pad until you are close as you want. Its hard to get repeatable weights of the little end without a fixture. The big end is all rod cap as that is what you can weight accurately. Remember the crank is balanced to bobweights that are determined by the total weight of the rotational and reciprocating mass. 2250g bob weight is what an Eagle cast crank has. My bobweight from the pistons and rods ended up being 2265 and that was just by equalizing all components to the best of my instruments. so in theory, I just need to remove 15g from each throw. Im alsmot there with oil tolerances.

[JVMopar]

DO NOT mix and match caps. Each cap has to stay with it's rod. They are a matched set. Precision machined. Infact if you replace your rod bolts you should have the big end resized because the cap gets shifted around.

If you do you'll probably won't even be able to turn the engine over and if you can it won't take long to burn a bearing out of it.