Data Point on Factory Balance

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From talking to scat it is a balanced crank with whatever is needed and bob-weights. Everything is numbered and ready to install. It is a la318 so internal balancing. That is where I get confused on the balancing thing. If a machine does it, do they add my flywheel and harmonic balancer to the assembly and balance the whole thing? I didn’t think they did? So what does internal balancing etc matter. They just rotate the crank on a machine and make sure it is balanced? Or is it more complicated than that. I understand the piston and rod weights on the big and little end, but not clear on what they do with the crank. Sorry to stupid questions, trying to understand it all.
 
An LA318 was internally balanced from the factory. This means that the external parts like damper and flexplate/flywheel/TC are each balanced by themselves and do not supply any extra balance weight to the overall assembly. When you bolt those individually balanced external part to the internally balanced rotating assembly, the overall engine remains balanced. They are not used in the crank balance process.

A so-called 'external balance' engine has those external parts designed to be intentionally imbalanced in a certain way. Those imbalanced parts work with the internal counterweights to balance the whole engine overall. So the crank balance process in that case HAS to incorporate those external balance parts as part of the final crank balancing.

Theoretically, an aftermarket assembly could be done either way. But I see SCAT only lists 'unbalanced' and 'internally balanced' parts numbers for your 390 stoker kit, so both have been designed for the original 318 type of external parts, which are each individually balanced. So you will get your kit, either balanced by SCAT or locally, and then just bolt those external parts onto it.

No need to supply these external parts to anyone for an internally balanced rotating assembly. That is one advantage: less parts being passed around. Another advantage is a broader selection of such inherently balanced parts.

The actual balance process is done in 2 parts:
  • Part 1 is piston and rod weighing, and then drilling and grinding on them to match closely. The main things to make match are rod big end, rod small end, and piston weights. (The pins, bearings, ring packs, and pin locks are also weighted but are usually pretty darned close anyway.) The final matched numbers go into computing what is called a 'bobweight'. At that point, any work with the pistons, rods, etc., is done except for marking and packaging.
  • Part 2 is balancing the crank. Actual bobweights with the values from part 1 above are setup and attached to each of the 4 crankpins. (Look up what a bobweight looks like...) Then the crank is spun and balanced to a certain tolerance with those bobweights. The crank balance process is performed in a way to minimize certain types of crank vibrations.
 
BTW, my point on the external parts effecting a balance job's accuracy is just this: If you have a super well balanced rotating assembly, and then bolt on a standard production external part that was made on a Monday by a new production balance trainee, then that supposedly 'balanced' damper (or flexplate, flywheel, TC, or clutch pack) can be out of balance enough to make all the $$ spent on that super-precise balance job a waste of time and $$. So, if you are doing standard hot-rod work (like many/most of us), then getting beyond +/- 2 grams of balance accuracy seems rather unnecessary....IMHO.
 
the thing about internal or external has everything to do with the weights of the pistons...pin and small end of the rod ...vs the weight of the counter balance of the crank and big end of the rod...such in a cast crank which is lighter due to less dense steel...the counter balance does not weight enough...so heavy metal must be added to the counter balance to balance the engine....

lighter the piston and rod....less weight is needed for counter balance.....

my 4 inch scat forged crank still took 2 slugs of mallory to balance it...my 4 inch forged K1 crank took one small slug of mallory... 'they both use the same rods and pistons..

more than likely Scat will use the weights on the boxes and balance the crank...where as the machine shop will take each piston....pin...rod ....and weight them individually...
You can see that on the balance sheet I posted....
 
@70aarcuda that is pretty much what scat says they do. They said they don’t see much variation in piston and rod weight, maybe 2 grams. But going back to the topic of this post. Isn’t that going to be way better than factory?
 

My balance shop I found will balance a crank for a minimum of $75 IF you give him one complete piston and rod pack and tell him that all your parts are balanced to each other (pistons all weigh the same, rods, etc) If its a press pin piston, you need to give it to him apart so he can weight the big end seperate. There is an official jig that you put the rod on to balance the big end only (so it hangs from a pivot in the rod bearing bore at 90 degrees) but I find it interesting that the rod caps are the small end weight themselves. There is a youtube machine shop that does alot of race stuff that balances rods by just weighing/equalizing the capless rods, then weighing/equalizing the caps and grinding them from their balance points which are the pads at the ends.
Eagle adds 5g per rod for oil and 15g per rod on big blocks, they say those values work good, so that's alot of wiggle room per tolerances because they say that weight cannot be computed as it will change over RPM's and even location of rod. That tells me those weights are probably trivial to the balance as they are dynamic. Eagle cast 4" stroker crank bobweight is 2050 for an example. They make them a little heavy so the chances of adding $$ metal are reduce. I broke 1g worth of oil ring land (about 1.5 inches) on a piston and have to press the damn thing off to weigh the piston (and piece) again to balance the new one. I hate press fit pistons.
My MP cast stroker 4.00 X 4.00 pistons bare are 478.9, and the set had a 6g range! Try removing 6g from a stroker piston, near impossible so I had to grind some off the press fit pin. Not pretty but got them all to within .1g. I did that to all the reciprocating parts minus the rings..They were curiously all the same weight! Duh......:eek:
Rover balanced their final assemblies to within 1oz-inch/72g-cm (whatever that computes to) at 500 rpm..externally after the internal balance was stacked up by welding 'bits' to the crank pulley and flywheel. 50 to 80g-mm is the target for a sub F1 competition balance job, anything more was money wasted.
 
pishta said:
My balance shop I found will balance a crank for a minimum of $75 IF you give him one complete piston and rod pack and tell him that all your parts are balanced to each other (pistons all weigh the same, rods, etc) If its a press pin piston, you need to give it to him apart so he can weight the big end seperate. There is an official jig that you put the rod on to balance the big end only (so it hangs from a pivot in the rod bearing bore at 90 degrees) but I find it interesting that the rod caps are the small end weight themselves. There is a youtube machine shop that does alot of race stuff that balances rods by just weighing/equalizing the capless rods, then weighing/equalizing the caps and grinding them from their balance points which are the pads at the ends.
Eagle adds 5g per rod for oil and 15g per rod on big blocks, they say those values work good, so that's alot of wiggle room per tolerances because they say that weight cannot be computed as it will change over RPM's and even location of rod. That tells me those weights are probably trivial to the balance as they are dynamic. Eagle cast 4" stroker crank bobweight is 2050 for an example. They make them a little heavy so the chances of adding $$ metal are reduce. I broke 1g worth of oil ring land (about 1.5 inches) on a piston and have to press the damn thing off to weigh the piston (and piece) again to balance the new one. I hate press fit pistons.
My MP cast stroker 4.00 X 4.00 pistons bare are 478.9, and the set had a 6g range! Try removing 6g from a stroker piston, near impossible so I had to grind some off the press fit pin. Not pretty but got them all to within .1g. I did that to all the reciprocating parts minus the rings..They were curiously all the same weight! Duh......:eek:
Rover balanced their final assemblies to within 1oz-inch/72g-cm (whatever that computes to) at 500 rpm..externally after the internal balance was stacked up by welding 'bits' to the crank pulley and flywheel. 50 to 80g-mm is the target for a sub F1 competition balance job, anything more was money wasted.
Click to expand...


Take a look at some stock mopar crank pulleys for internaly balanced engines and you will find that they have pieces of metal welded to them aswell.
 
stumblinhorse said:
@70aarcuda that is pretty much what scat says they do. They said they don’t see much variation in piston and rod weight, maybe 2 grams. But going back to the topic of this post. Isn’t that going to be way better than factory?
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To answer your question: YES. For one set of 340 rods, I found 5 gram variation in the total and small end weights, and 3 grams variation on the small ends. For the 273 rods, there was a 7 gram variation in total weights, a 12 gram variation in big end weights, and 10 grams variation in small end weights in my 1 batch. I am sure the factory work was worse overall as this is just in one sample set of each of these rods. AND then there is how accurately the crank is balanced which is a completely separate process.

The +/- 2 grams is what comes from piston mfr's like UEM and the others. The KB's we got for a 340 had a +/1 gram weight variation, and even SpeedPros are (per their spec's) pretty closely matched. For the rods, when SCAT makes them, they have a bunch made, and then weigh them all, big end and small and total, and match up 8 rods to make a weight matched set. The actual big, small, and total weights can vary more BETWEEN rods sets, butWITHIN a set in a box, they are all very close. As aar70 said, when you get a box, the small end and big end weights for that batch of 8 are marked right on the side. (See pix below)

DSCN2523 (Large).JPG
 
70aarcuda said:
the thing about internal or external has everything to do with the weights of the pistons...pin and small end of the rod ...vs the weight of the counter balance of the crank and big end of the rod...such in a cast crank which is lighter due to less dense steel...the counter balance does not weight enough...so heavy metal must be added to the counter balance to balance the engine....

lighter the piston and rod....less weight is needed for counter balance.....

my 4 inch scat forged crank still took 2 slugs of mallory to balance it...my 4 inch forged K1 crank took one small slug of mallory... 'they both use the same rods and pistons..

more than likely Scat will use the weights on the boxes and balance the crank...where as the machine shop will take each piston....pin...rod ....and weight them individually...
You can see that on the balance sheet I posted....
Click to expand...
FWIW AAR, for the SCAT cast 4" stroker crank, here is a FABO reference to those that seems to say they will internal balance without heavy metal if you have around or under 1700 grams of bobweight....I called SCAT, and they said with their kit bobwieghts of 1760 grams, they still have to add a 'small amount' of heavy metal.
Balancing a Scat cast crank
 
nm9stheham said:
when you get a box, the small end and big end weights for that batch of 8 are marked right on the side. (See pix below)

View attachment 1715253287
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Did you weigh the cap on that rod, was it actually 173g? My theory is the cap weight is the small end weight. >> NOTE << They ask for the 'insert' weight on the big end, wonder if that is the complete rod bearing, not just the shell on that side. they dont list an 'insert' on the little end. I would think the entire bearing would be rotating weight like the little end.

180 crank guys got it made, they only need to balance out their reciprocating weight. new Heavier pistons, no prob, drop em in and go.
 
pishta said:
My balance shop I found will balance a crank for a minimum of $75 IF you give him one complete piston and rod pack and tell him that all your parts are balanced to each other (pistons all weigh the same, rods, etc) If its a press pin piston, you need to give it to him apart so he can weight the big end seperate.
Click to expand...
That is the trick to doing balance jobs for low cost: do the piston+rod work yourself; that is where most of the labor cost lies. I have been doing this lately, and just handing the crank and a bobweight number to the shop. I dig for data and weigh parts to work out the bobweights well ahead of time so I know where things will end up when it comes to crank balance time.

pishta said:
There is an official jig that you put the rod on to balance the big end only (so it hangs from a pivot in the rod bearing bore at 90 degrees)
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Here is a link that shows purchased and home built jigs. Connecting Rod balancing question | Grumpys Performance Garage

I made my own on the same principle; the biggest trick was in finding out how to make the scale behave properly with this; the tiniest side load on the scale from the jig would throw it off, but I found that and how to make it 'settle in' properly.


pishta said:
but I find it interesting that the rod caps are the small end weight themselves. There is a youtube machine shop that does alot of race stuff that balances rods by just weighing/equalizing the capless rods, then weighing/equalizing the caps and grinding them from their balance points which are the pads at the ends.
Click to expand...
I am sure they have figured out a process to do it that way but the cap is part of the big end weight, not the small end weight. There is some misconception above about the rod cap.

pishta said:
Eagle cast 4" stroker crank bobweight is 2050 for an example......
My MP cast stroker 4.00 X 4.00 pistons bare are 478.9, and the set had a 6g range! Try removing 6g from a stroker piston, near impossible so I had to grind some off the press fit pin. Not pretty but got them all to within .1g. I did that to all the reciprocating parts minus the rings..They were curiously all the same weight! Duh......:eek:
Click to expand...
That is the smart way to do it IMHO. And, I am finding almost no variation in rings, pin locks, and bearings, and a gram or less in pins. I still weigh a batch of each part at a time and divide the total by the number of items weighed to get a more accurate average.

BTW, that 2050 gr target bobweight for the Eagle cast 4" crank is for an external balance crank design (per their catalog) ... it won't balance internal only. Their forged one IS adverised to balance intenrally at or below 2050 gr.

pishta said:
Rover balanced their final assemblies to within 1oz-inch/72g-cm (whatever that computes to) at 500 rpm..externally after the internal balance was stacked up by welding 'bits' to the crank pulley and flywheel. 50 to 80g-mm is the target for a sub F1 competition balance job, anything more was money wasted.
Click to expand...
As I understand it, inch-ounces are the unit to which crank balance machines work. I have read that SBC production cranks were balanced to a limit of 2 in-ozs; that works out to over 30 grams at the crankpin distance! So that shows how poor factory crank balance work could be.

DSCN2525 (Large).JPG
 
pishta said:
Did you weigh the cap on that rod, was it actually 173g? My theory is the cap weight is the small end weight. >> NOTE << They ask for the 'insert' weight on the big end, wonder if that is the complete rod bearing, not just the shell on that side. they dont list an 'insert' on the little end. I would think the entire bearing would be rotating weight like the little end.

180 crank guys got it made, they only need to balance out their reciprocating weight. new Heavier pistons, no prob, drop em in and go.
Click to expand...
No, I did not weigh the caps, and that is a misconception on the cap being any part of the small end weight. I'm gonna guess that they use the cap weight to correct the small end weight measurement that they do make; that small end weight measurement will be off without the rod cap and bolts being in place and snugged up, due to the center-of-gravity of the rod being moved when the cap is off.

I do believe the 'insert' is the bearing as they refer to it; for standard bobweight computations, it is both halves of the bearing, so I would assume that is what they are asking for. Yes, the bearing is part of the rotating weight; the small end and piston, pin, etc., are part of the reciprocating weight.

Yes and no on the flatplane cranks... but they suffer from 2nd order (2nd harmonic) vibrations that cannot be totally eliminated. As I understand it, the crossplane cranks like we have for SBM's inherently balance out the 2nd harmonic vibrations. You might like this article:
V8 Engines - Craig's Website at Backfire.ca
 
BTW, Pishta, I just weighed a 273 rod cap to test your theory: 179 gr, and 194 grams with the nuts. Actual small end weight for that rod is 225 grams. Another fine theory blown to hell...hey, we all have them LOL
 
so total rod weight (ie, 758g) does not add up to big end and little end weight? I tried making a jig and I could not get a repeatable weight. ended up using a knife edge and hanging the rod on the knife edge at the parting line, at 90 degrees, inside the bore. I got repeatable results that way. if they were legit,...?
 
cool....well, how do you measure the little end then? same jig setup?
Argh, I got a perfect set of recon'd balanced ARP fitted rods but they are floaters....and my pistons dont have lock grooves. :BangHead:
 
Yes, the small and big end weights do add up to the total weight. Not sure I understand the question....?? If you are referring to SCAT box pictured, their total weight is not anywhere close to the standard factory rod weights (758 or 726 grams for heavy and light factory rods, respectively).

The jig I showed is measuring the little end as pictured. I change an adapter, re-zero, flip the rod around, and then measure big end weight with the big end over the scale. The fixture has registration marks so I can repeatably position the rods in the same exact location, time after time. It works on the same principle as the ' vertical hanging' fixtures:
  • Support the rod so that the rod's center axis along its length is perfectly level. (Which is why the hanging type has a vertical adjustment on the support; you change that when you change ends.)
  • Support one end exactly at the center of it's hole by some means; the Grumpy's link above shows good examples of how this is done. It has to be supported exactly at the center line of the supported hole to give precise, repeatable results.
  • The other end of the rod rests on the scale. The resulting weight on the scale will be that end's weight for for using in bobweight computations.
  • As noted, scales can/will show errors if their is the slightest side load (i.e., sideways pull or push) on the rod so you have to detect that and find a way to make it 'settle in'. I found that tapping lightly on one end of the support table would do this; this is a scale issue, not a fixture issue.
IMHO, the hanging methods are probably easier to build and make work consistently than mine. I have not tried a hanging fixture. But the sideload errors on the scale was the biggest matter to find out how to correct.
 
dartfreak75 said:
Yea that's what I figured. That is pretty crazy. Wonder if that is why newer cars seem to last longer miles? They are better balanced than The older engines
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That's only a very small part of why they last longer. What has a bigger effect on lasting is (to name a few)

1. better fuel ratio control through fuel injection
2. better oils
3. precision metallurgy in today's parts
 
fishy68 said:
That's only a very small part of why they last longer. What has a bigger effect on lasting is (to name a few)

1. better fuel ratio control through fuel injection
2. better oils
3. precision metallurgy in today's parts
Click to expand...
1) I agree
2)That is interesting. Do you think today's oils are better? Because it seems like people say the opposite.
3) I agree that today's metallurgy technology is more advanced and accurate. But I also think that due to economics and companies goals of top profit the metal quality has decreased significantly. I think they make steel now a days just enough to pass as the minimum standard. Look at how thin body panels are now and how fast they rust. My 01 is a rust basket case! I have a friend with a 08 Chevy the besides and bumper along with cab corners and rockers are gone rusted completely thru! That is less than 10 years. You see cars from the 50s 60s 70s yea they have some rust but not like the newer ones are rusting! I have a coworker with a 2013 Ford f150 he bought it brand new and the wheel arches are already rusted thru. That's just ridiculous!
Now I understand that metal used to make engines are not the same as body panels I'm just using that as an example.
I dont know about engine bearings and the liking maybe they are made better now?
 
I agree with precision engine building is the way to go. and unless u are revving to 10,000 balancing to 1/2 gram is more than enough. and years ago I asked Ed Hamburger about replacing 600 gram pistons with 660 in a 1970 steel crank 340 , he said pistons within 10% was not bad , as I was on a budget. I ran it for 7 years to 7000 with solid engine mounts and it was fine.
 
That makes sense MH due to what is actually the main thing being balanced out in these crank designs. That balance factor is what is called a 1st order imbalance (meaning it happens once per rotation), front-to rear. As I understand this....

Imagine a heavy rod poked straight through one side of the engine and out the other, so that the engine can rotate CW and CCW on the rod when viewed from the side. The vibration we are trying to balance out is one where, as the front up the engine pitches up, the rear pitches down and then it reverses half a rotation later, and that continues over and over, once per rotation. This vibration is due to the front and rear pairs of pistons always moving in equal but opposite directions and is large due to the distance that they are from the center of the engine. The heavy counterweights at the front and rear of the crank are their to counterbalance that.

The inner pairs of pistons at the 2nd and 3rd crankpins also move in equal an opposite directions, but they are much closer to the center of the engine, so they do not contribute nearly as much to the vibration described above.

If you get the pistons/rod ends/etc. pretty much matched up and uniform, then the 2nd and 3rd crankpin vibration is pretty small. However, if you are 10% off in piston weight, then the bobweights on the front and middle and rear crankpins will be about 3% off (edited to correct that %), and, if you don't touch the end counterweights to compensate, then there will be 1st order vibration in the engine. With solid mounts and the leverage of the trannie working to stablize the engine position, it sounds like the felt vibration felt in a sporty car might not be too bad for a gearhead. But there will certainly be some extra loading on main bearings, block, etc.

And, if you get the piston weight all matched, even if they are off target bobweight, then what is known as a 2nd order vibration (happens twice per rotation) will be balanced; that it inherent in this crank design.
 
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dartfreak75 said:
1) I agree
2)That is interesting. Do you think today's oils are better? Because it seems like people say the opposite.
3) I agree that today's metallurgy technology is more advanced and accurate. But I also think that due to economics and companies goals of top profit the metal quality has decreased significantly. I think they make steel now a days just enough to pass as the minimum standard. Look at how thin body panels are now and how fast they rust. My 01 is a rust basket case! I have a friend with a 08 Chevy the besides and bumper along with cab corners and rockers are gone rusted completely thru! That is less than 10 years. You see cars from the 50s 60s 70s yea they have some rust but not like the newer ones are rusting! I have a coworker with a 2013 Ford f150 he bought it brand new and the wheel arches are already rusted thru. That's just ridiculous!
Now I understand that metal used to make engines are not the same as body panels I'm just using that as an example.
I dont know about engine bearings and the liking maybe they are made better now?
Click to expand...
Since were getting a little off topic I'm going to message you and we can continue there
 
tried that hanging fixture and about hung myself trying to get it to work. Going back to my knife edge because I can control the vertical axis as its hanging on it horizontally. If I can get my big end to 563 Ill be right at the 2050 bobweight target of my crankshaft. My measurement are all over the place, from 550 to 577, cant repeat with that hanging trapeze thing I built. I got a balance shop a mile down the road, maybe Ill drop by tomorrow and ask him to give me a big and little end weight for $5. About 20 seconds of work. I got my pin pressed out for $5, glad to pay him that. The pop always makes me jump a little.
 
LOL.... Did you tap the table on which the scale was sitting after the rod stopped moving? That is the problem with some scales; there tends to be some residual side load (push or pull) on the scale, and that need to be 'settled out'.

What are you working on? I'm a bit baffled as much lighter than stock pistons would be needed to get to a 2050 bobweight with that big end weight. That big end weight looks considerably higher than either the light or heavy SBM factory rods.

And if you are working with an Eagle 4" cast stroker crank that was mentioned, that 2050 gram weight does not apply for internal balance only, at least per their catalog info.
 
Machine shop called this afternoon...he had spun the crank and showed it need weight added to counterweights....wanted to know what I wanted to do...external or internal balance...I replied internal.
The front counterweight needs 130 grams and rear needs 113 grams...should be ready in the morning..
 
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