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I've never balanced an engine but it still seems to me that any differences in the notches between pistons would be very small if any at all. And the notches, while bigger than I had envisioned, are still only a few % of the overall piston. So a tiny % difference in a few % is basically zilch and probably nothing to worry about. .
It’s not the piston to piston variation that is as important as the proper crank counterweight balancing that is key.
That is is a lot of piston weight removed and the crank balance counter weights need to be reduced to match. After weighing the pistons, pins and big and little ends of the connecting rods a formula is used to add balancing weight to the crack throw. Then the crank spun and like a tire, the machine says were to add or remove weight. Adding is done with metal having a density more than steel in holes drilled in the counterweights. Reduced weight with drilled holes in them.
As far as piston variations, recall the ‘51 Olds engine I had bored for 1/8 inch larger for ‘55 Olds pistons the machine shop would not start until they had the pistons! They miced each and honed the cylinders to provide the same clearance, cylinder to cylinder! That was in ~1959! Recall I got the pistons and block back with the pistons numbered for location.
Think I would use a machine shop who did the same today.
It’s not the piston to piston variation that is as important as the proper crank counterweight balancing that is key.
That is is a lot of piston weight removed and the crank balance counter weights need to be reduced to match. After weighing the pistons, pins and big and little ends of the connecting rods a formula is used to add balancing weight to the crack throw. Then the crank spun and like a tire, the machine says were to add or remove weight. Adding is done with metal having a density more than steel in holes drilled in the counterweights. Reduced weight with drilled holes in them.
As far as piston variations, recall the ‘51 Olds engine I had bored for 1/8 inch larger for ‘55 Olds pistons the machine shop would not start until they had the pistons! They miced each and honed the cylinders to provide the same clearance, cylinder to cylinder! That was in ~1959! Recall I got the pistons and block back with the pistons numbered for location.
Think I would use a machine shop who did the same today.
Yep, last motor I had in the machine shop, flat 6, they said they do each cylinder to match each pistons OD for the best fit and longevity. The clearances are also set based upon the alloy of the piston. They still do the crank balancing the same way, as internal combustion engines haven't changed all that much fundamentally, and physics is still the same. 1/10th of a gram balancing should be the goal, or better. I think I did mine to .05g just because. Working in a machine shop really makes tolerances out to 4 decimal places seem like not a lot some times. The temp at which things are mic'd, based upon the material as thermal expansion rates differ, is important also, which has always perplexed me when motors use billet steel main caps bolted to aluminum blocks. I only have a BS in BS so I don't know what I am talking about though. lol
Yep, last motor I had in the machine shop, flat 6, they said they do each cylinder to match each pistons OD for the best fit and longevity. The clearances are also set based upon the alloy of the piston. They still do the crank balancing the same way, as internal combustion engines haven't changed all that much fundamentally, and physics is still the same. 1/10th of a gram balancing should be the goal, or better. I think I did mine to .05g just because. Working in a machine shop really makes tolerances out to 4 decimal places seem like not a lot some times. The temp at which things are mic'd, based upon the material as thermal expansion rates differ, is important also, which has always perplexed me when motors use billet steel main caps bolted to aluminum blocks. I only have a BS in BS so I don't know what I am talking about though. lol
Thanks, as you say physics hasn’t changed since that Olds engine I built in 1959! At 17 my car knowledge came from my Dad and Uncle plus in the case of the Olds engine assembly Hot Rod etc magazine! Recall I used Plastigauge to cheek bearing clearances-all I had available! I assembled that engine (I bought all in parts from a buddy for under $25) without a manual etc. Won't touch the Vette without consulting the Service Manual now.
SIDE BAR
I think I have read every significant car mag since I was ~13! Was enamored with cars since that time as the older teenager across the street owned a Barris chopped ’50 Merc (real lead sled as it had lots of cracks so he was removing the lead and trying to use mostly hammered metal as a replacement) and the house next to him there were twins who came back from the Army and bought matching 1954 Austin Healeys. Since then have always liked sports cars and Hot Rods! Currently have subscriptions to ~6 auto mags as well as two SAE pubs and get most of my car info from them. Frankly all the theory is fine but learning from folks who actually build engines/cars is much more valuable! Have found the same in my field (welding) during my carrier.
As you say the piston alloy is important. The large forged aluminum alloy pistons in the 502 cid BB in my ProStreet Rod expand more than cast alloy pistons. Piston to cylinder clearances are large. It sounds like a sewing machine with piston slap for a few minutes until the combustion temps get them hot. Then it sounds fine.
Aluminum, although light has several undesirable properties. One it bends ~3 times more for a given load than steel (about its weight savings) and the other it does not have an “infinite fatigue life” as steel does if loaded to a max half the ultimate strength. One reason you seldom see aluminum connecting rods and never an aluminum crank (they change those aluminum AA/Fuel connecting rods every few races.) Yep 7000 series wrought aluminum is strong but small “imperfections” will slowly grow as the part is stressed and eventually cause failure. Steel, if loaded less than half its ultimate strength, will blunt those small imperfects from "growing" and can cycle load an “infinite amount of time.”
Also a wrought aluminum cap would still expand at a different rate than the cast aluminum block. It’s possible that excess bending is another reason not to use aluminum caps. The only way to reduce the bending is to have 2 to 3 times the thickness so there would be no weight savings!
Not an engine designer so all the above is theory! Although technically correct, if my wife was listening she’d remind me as she does at times of the 3 my BS degree is best!
Pistons came in from Wiseco and look great! The notch is only on one side of the piston and in my opinion the notch is smaller than what was reflected in the first image I showed earlier in the thread. Heres some pics so you get the idea:
Pistons came in from Wiseco and look great! The notch is only on one side of the piston and in my opinion the notch is smaller than what was reflected in the first image I showed earlier in the thread. Here are some pics so you get the idea:
Ok, update time! So the pistons came in, and I got a small scale that is accurate enough to weigh down to a tenth of a gram. I weighed all the pistons, as well as re-weighed the wristpins and connecting rods and compared them to what was listed in the original balance sheet. The result is that all of the components are within 1.2 grams or less of what each cylinder assembly was balanced to and a grand total of a 5.0 gram variance for all combined. This is very very low. As in, I don't think I am going to have them rebalanced as I don't think the difference is enough to call for it. Here is a snippy of the details below:
It sure seems like the difference in the weights of the pistons are within standard error in measurement.
I gotta be honest. I would have a hard time not filing away trying to get everything within 0.1g, but I am absolutely positive that the point of diminishing returns tells me that would be a waste....... and you would really need a scale that goes one more decimal place to get there.
It sure seems like the difference in the weights of the pistons are within standard error in measurement.
I gotta be honest. I would have a hard time not filing away trying to get everything within 0.1g, but I am absolutely positive that the point of diminishing returns tells me that would be a waste....... and you would really need a scale that goes one more decimal place to get there.
So I already tried swapping pistons around to balance the weights out to be as low as possible and that's where I ended up. Remember there are 4 left and 4 right pistons so I can't swap them to the wrong sides. Overall I'm pretty happy how my worksheet and results turned out
It sure seems like the difference in the weights of the pistons are within standard error in measurement.
I gotta be honest. I would have a hard time not filing away trying to get everything within 0.1g, but I am absolutely positive that the point of diminishing returns tells me that would be a waste....... and you would really need a scale that goes one more decimal place to get there.
I agree ANT it looks from just looking at those pistons & what was cut away that it really would be off a lot more that what is has come out too. As always buddy great write up & job on her..No more than that I would let the engine builder run with it also..In your case get her done. Robert
FWIW a key point in balancing a piston, rod and crack assembly is the vibration or out of balance caused when pistons etc rotate especially at higher rpm. That is what the counterbalance weights on the crack are there to offset piston pin and rod weighs. The piston etc variations from cylinder to cylinder are secondary to the overall balance.
Just like a tire balance machine the crack is spun with bob weights added to each crack throws to match the weights of piston, pin and big/little end of the rod. The weights added are based on a formula that takes into account the distances of those weights from the spinning crack. That is why little and big end rod weights are measured separately. Pic below shows an assembly ready to be spun, out of balance measured and holes drilled in in counter balance weights to lighten where needed. If more weight is needed the holes are drilled then filled with a material of higher density than iron (Mallory Metal is twice as heavy.) Just like adding balance weights to a wheel/tire.
Making some rough calculations estimating the "U" shaped half circle removed as ~3/4 inch diameter ~3/16 inch thick that is about 1.8 grams of material removed.
Assuming whoever sells a "balanced" assemble less crack is trying to match the new piston and rod weights with the OEM so the crank does not have to be rebalanced - that is a lot of weight difference.
Rotating assembly balancing is the crankshaft counter weights to piston weights delta, which should be zero, so removing mass/weight from the pistons throws the whole thing out of balance if the crank counter weights are not equally attended to. I would get it all re-balanced at a good machine shop as it doesn't cost all that much really. Good luck with it all and I hope it turns out to be a great build.
FWIW a key point in balancing a piston, rod and crack assembly is the vibration or out of balance caused when pistons etc rotate especially at higher rpm. That is what the counterbalance weights on the crack are there to offset piston pin and rod weighs. The piston etc variations from cylinder to cylinder variations are secondary to the overall balance.
Just like a tire balance machine the crack is spun and using a formula with bob weights added to each crack throws to match the weights of piston, pin and big/little end of the rod. As mentioned there is a formula that takes into account the distances of those weights from the spinning crack. That is why little and big end rod weights are measured separately. Pic below shows an assembly ready to be spun, out of balance measured and holes drilled in in counter balance weights to lighten where needed. If more weight is needed the holes are drill and filled with a material of higher density than iron (Mallory Metal is twice as heavy.) Just like adding balance weights to a wheel/tire.
Making some rough calculations estimating the "U" shaped half circle removed as ~3/4 inch diameter 3/16 inch thick that is about 1.8 grams of material removed.
Assuming whoever sells a "balanced" assemble less crack is trying to match the new piston and rod weights with the OEM so the crank does not have to be balanced - that is a lot of weight difference.
Back in the 70's and 80's GM's tolerance if I remember right was 4-10 grams which is why 100k miles was about it for them then, or a big contributing factor . Meanwhile Honda had a tolerance of 1 gram.
FWIW a key point in balancing a piston, rod and crack assembly is the vibration or out of balance caused when pistons etc rotate especially at higher rpm. That is what the counterbalance weights on the crack are there to offset piston pin and rod weighs. The piston etc variations from cylinder to cylinder are secondary to the overall balance.
Just like a tire balance machine the crack is spun with bob weights added to each crack throws to match the weights of piston, pin and big/little end of the rod. The weights added are based on a formula that takes into account the distances of those weights from the spinning crack. That is why little and big end rod weights are measured separately. Pic below shows an assembly ready to be spun, out of balance measured and holes drilled in in counter balance weights to lighten where needed. If more weight is needed the holes are drilled then filled with a material of higher density than iron (Mallory Metal is twice as heavy.) Just like adding balance weights to a wheel/tire.
Making some rough calculations estimating the "U" shaped half circle removed as ~3/4 inch diameter ~3/16 inch thick that is about 1.8 grams of material removed.
Assuming whoever sells a "balanced" assemble less crack is trying to match the new piston and rod weights with the OEM so the crank does not have to be rebalanced - that is a lot of weight difference.
I’ve seen the vids online on exactly how they do it. Like said, just like balancing a tire. Based on the fact that all the parts for each cylinder are 1.2 grams or less from zero, you still think I need a rebalance?
Originally Posted by Ghostnotes
Back in the 70's and 80's GM's tolerance if I remember right was 4-10 grams which is why 100k miles was about it for them then, or a big contributing factor . Meanwhile Honda had a tolerance of 1 gram.
4-10 grams across the whole crank? Or for each cylinder?
Back in the 70's and 80's GM's tolerance if I remember right was 4-10 grams which is why 100k miles was about it for them then, or a big contributing factor . Meanwhile Honda had a tolerance of 1 gram.
Yep, particularly for the small block Chevy GM has spent hundreds of millions in the factory in Tonawanda NY so it's one of the highest quality low cost engine plants in the world (for the volume.) Apparently one reason they keep bore centers the same as the original 265 cid 1955 V8! No doubt much smaller tolerances today on the C7 engines. I think about that as my 378 cid LT1 spins at 6600 rpm!
This is a pic of how the counterweights are drilled to lighten based on spinning on a crack assembly balance machine. That is easier than adding heavy metal so assume the OEM crack counterweights are designed a bit heavy as metal removal is easy. If needing to add weight based on the balance machine info Mallory Metal can be added to a drilled hole and spot welded. Or particular for a stoker crack where the counterbalance weights are limited in diameter to clear the pistons it can be added on the sides.
I’ve seen the vids online on exactly how they do it. Like said, just like balancing a tire. Based on the fact that all the parts for each cylinder are 1.2 grams or less from zero, you still think I need a rebalance?
4-10 grams across the whole crank? Or for each cylinder?
“The fact is pistons are balanced within 2 grams,” Lieb says. “The rods are balanced plus or minus 2 grams end for end. Most people don’t know what a gram is. One gram is 1/28 of an ounce. The actual weight of a gram is the weight of a dollar bill. When people talk about balancing within half a gram or balancing to zero, it’s nearly impossible to do that with 100 percent accuracy. If they take that same thing that they supposedly balanced to within half a gram and put it back on the scale, they might find out they’re half a gram off. You have to understand the repeatability of the scales of the equipment you’re using and how minute it is.”
Not true. If you have repeatability differential/problems at 1/2 gram range, you need a new scale. I know plenty of industrial Mettler Toledo scales that can repeat less than 1/2 gram.
Now, Lieb stresses that engine builders shouldn’t be discouraged from striving for doing the best, but balancing within half a gram is unnecessary.
Tell NASCAR builders that.
Most rotating assemblies from a manufacturer come with a basic balance that has the rods at a 1.5-2 grams variance and the pistons at .75-1 gram variance. While some people might claim that isn’t balanced properly, the truth is, you’re not going to get it much closer. “Nobody in the industry does that,” White says. “You will not believe how few cars on the planet get a closer balance than that. When you buy a rotating assembly from anyone, they keep their tolerances usually around 2 grams on rods and pistons. We check all those and if they’re over that then we correct it, but we’re not getting down to a half gram on all those parts unless they pay us to do so.”
These 2 sentences almost contradict each other.
I just don't agree unless this is just covering drag racing, quick run then shutdown. My last 2 builds my G8 and my current C7 were balanced to less than 1/8 and was repeatable. They are also endurance engines. I put a heavy *** ultra billet crank in both. When asked why, I would say mine does not run at a more or less fixed rpm. My load...frequency...harmonics are constantly changing. A heavier crank, as opposed to a lightweight crank can handle the load changes better. A tighter balance covers the exponential increase and sudden changes in load and the harmonics throughout the rev range.
Last edited by Ghostnotes; 10-31-2018 at 01:27 PM.
10-22-2018, 09:57 AM
Thanks, as you say physics hasn’t changed since that Olds engine I built in 1959! At 17 my car knowledge came from my Dad and Uncle plus in the case of the Olds engine assembly Hot Rod etc magazine! Recall I used Plastigauge to cheek bearing clearances-all I had available! I assembled that engine (I bought all in parts from a buddy for under $25) without a manual etc. Won't touch the Vette without consulting the Service Manual now.
