Rotating assy balance

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Tays340

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Hey Guys, More naivety,
So I got some H116cp to go in this 360 I'm trying to build. I took it all to the local guy for balancing.
Tonight I was going to start ringing up the pistons. As I was pulling the parts out, I noticed some huge differences in the rod caps so I pulled out the ol' kitchen scale and started weighing things. I weighed everything three times.
I thought everything should be within a gram or so. This is shocking! Somebody please tell me this is how it's supposed to be. I won't believe you, but try. Note how the machinist says I need
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to switch some of the caps to different piston/rod assy's, but only four, what about the other four? None of the weights make sense to me. WTF?
If I assemble it with the correct caps in place the weight is 8 grams difference from low to hi. With the difference in material missing from the rod caps, how can the big end of all the rods weigh close to the same? Visibly 20 grams between 5 & 6. What does that do for the reciprocating balance? This doesn't look right to me from any angle. Help!
*That's supposed to be #1 cap on #7 assy.
 
For starters you should not be switching rod caps around. A connecting rod and rod cap is machined and align bored as an assembly. Switching caps around would require the rod to at the very least be checked with a T gage and a micrometer to be sure its in spec. At the worst, align bored and honed to get it in spec. Otherwise the clearances can be too tight or too loose on the rod. Both lead to disaster.

This all being said, the machined pad on the rod cap is where the excess material is removed to balance a connecting rod. If your machinest wanted all that moved around, he should have done it, resized the rods with the issues, and re marked the rod/cap assemblies.

I have taken at least a dozen engines apart in my lifetime and i never have seen rod caps with varying heights all in one engine like that, but that's not to say this didnt ever happen.

How far off was the crankshaft? Was there a lot drilled out of the counter weights? A lot added in the form of mallory slugs welded in?

You probably payed good money to have this done with your rods. The whole magnaflux, check for straightness, shot peen, etc route. Honestly you would have been miles ahead with a new set of Eagle forged I beam rods with bushed small end and ARP bolts.

Maybe somebody else can chime in with more information on this than what I got. Heck, I'm an amateur though, I dont assemble em weekly like some of the racers around this site do, what the hell do I know.

Hope this helps
Matt
 
You are well within normal weight tolerances... I see 70-80 grams pretty often. The new powered metal stuff is reall close on weight but the old forged rods are not.

Run it...no one I know balances within a gram!
 
Krooser what do you recommend on the rod caps. I think he should keep each cap with it's original rod, and not switch them.
 
disagre,
Krooser said:
You are well within normal weight tolerances... I see 70-80 grams pretty often. The new powered metal stuff is reall close on weight but the old forged rods are not.

Run it...no one I know balances within a gram!
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-disagree , thats terrible spread on the balance
I `d be trying to get it done right some way --------and dam sure aint gonna switch rod caps around !
My 505 kit was race balanced by hughs engines , on a solid motor plate , no rubber bushings , u cant feel it running !
 
Agree that the rod caps need to stay with the rod they were machined to.
On the weight issue, you also need to consider the weight of the pistons and pins. Match up heavier pistons with lighter rods and see where that takes you for the piston and rod assembly. When considering the weight of the rod to do the balance 100% correctly, the upper part of the rod travels in an up and down motion and should be balanced to the piston, rings and pin as a unit. The bottom of the rod moves in a rotating motion and is not included with the piston weight.
 
my68barracuda said:
Agree that the rod caps need to stay with the rod they were machined to.
On the weight issue, you also need to consider the weight of the pistons and pins. Match up heavier pistons with lighter rods and see where that takes you for the piston and rod assembly. When considering the weight of the rod to do the balance 100% correctly, the upper part of the rod travels in an up and down motion and should be balanced to the piston, rings and pin as a unit. The bottom of the rod moves in a rotating motion and is not included with the piston weight.
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Race balance means all the pistons are brought to the same weight , then the rods big end and small end, then the total assembly spun balanced -----not average balance jobs .
 
The last 2 I had balanced, my machinest made sure ALL the pistons and rods weighed exactly the same, and drilled the crank where needed. I asked if it needed any special order EX. certain piston and rod on any certain journal. He said no. They weigh all exactly the same. Put it together.

Will be doing a hot teener for sonny next. It will have a 340 forged crank, Eagle I beams, KB167s. A matched set of Eagles usually weigh with within a gram of one another. Ditto for the KB167s. Since this setup is lighter than stock, The one that will likely get an adjustment will be the crankshaft.

Any machinest that tells you to go swapping rod caps around is suspect. Sounds hokey to me. Go buy a matched set of Eagles for $400 and use them.
 
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bob mCcarty said:
Race balance means all the pistons are brought to the same weight , then the rods big end and small end, then the total assembly spun balanced -----not average balance jobs .
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Last 2 I had done, were done exactly that way. With the crankshaft bobweight speed balanced and drilled where needed. The 2.2 turbo engine, and a 408. Both he said assemble whichever rod on whichever piston. It didnt matter since they weighed all exactly the same.
 
moparmat2000 said:
Krooser what do you recommend on the rod caps. I think he should keep each cap with it's original rod, and not switch them.
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100% do NOT EVER swap caps. You can’t just weigh the caps and rods separately like that. You have to have a real scale and a hanging fixture. The OP doesn’t have the tools or knowledge to screw with what he’s doing. That usually ends up very bad.
 
Tays340 said:
Hey Guys, More naivety,
So I got some H116cp to go in this 360 I'm trying to build. I took it all to the local guy for balancing.
Tonight I was going to start ringing up the pistons. As I was pulling the parts out, I noticed some huge differences in the rod caps so I pulled out the ol' kitchen scale and started weighing things. I weighed everything three times.
I thought everything should be within a gram or so. This is shocking! Somebody please tell me this is how it's supposed to be. I won't believe you, but try. Note how the machinist says I need
View attachment 1715747163 View attachment 1715747162 to switch some of the caps to different piston/rod assy's, but only four, what about the other four? None of the weights make sense to me. WTF?
If I assemble it with the correct caps in place the weight is 8 grams difference from low to hi. With the difference in material missing from the rod caps, how can the big end of all the rods weigh close to the same? Visibly 20 grams between 5 & 6. What does that do for the reciprocating balance? This doesn't look right to me from any angle. Help!
*That's supposed to be #1 cap on #7 assy.
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How do you know the rods and caps were numbered correctly? I’ve had them come in numbered wrong. Guys take **** apart and throw it in a box and then slap that same **** back together and number it. It can take hours to sort it out.


If you have questions, why not take that stuff back to the machine shop and have him explain why he is telling to do what he is telling you to do? Anything else is just rabid speculation.
 
Keep the caps on their rods! Auto shop 2nd week stuff. Do this: take the pistons and weigh each, then take material out to make all equal to your lightest *. Then do the rods, weigh each top and note the differences. The top of the rod has a balance pad, take material out there to equalize all to the lightest rod, do the same for the caps. then you really should weight the ends on a hanging scale as YR suggests. Your gonna have a hard time getting a repeatable measurement unless you have a dedicated fixture and scale but you can get very close balancing the assembled rod on a knife edge at the cap seam and then setting the scale at a 90 degree height. Do the same with the big ends on the scale and the little ends on a floating pin if bushed. Press fit will need a close fitting dowel or something as the slightest variation of the support point will give you different weight readings. With the little ends and caps all balanced, your balanced weight should be pretty darned close. * My MP magnum stroker pistons were all over the place, one was so heavy I had to take material off the pin as I ran out of piston material, likewise with the lightest piston, I added a couple shots of MIG wire to the inside of the pin to add a few grams to get it close to the others. All pins were equalized, even weighed the bolts, nuts and ring packs. I had a nut that was +1g! Its easy to get all the parts neutral balanced, much harder to get the rods ends to balance on the jig, but your not looking for perfection, your only looking for maybe +- a few grams. I think static and dynamic balancing was incredibly loose back then off the line and they still ran pretty smooth.
 
When I pulled the engine apart I stamped each rod and cap.
I am so on a budget here, New rods? They said the pistons would be destroyed if had to be changed. He did weld up the crank some and add a hole. When I put #8 cap on #5 rod it binds before it seats. I don't like it.
I thought the whole purpose of balancing was to get all the components to weigh the same before you made bob weights to balance the crank. This looks like it would shake apart and kill every bearing on cam break-in. But i'm naive. Still 13 grams between #3 journal and #4.
I guess I'm headed back to the machinist for some explanation, but, in my opinion, this is not right.
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Pressed pin pistons can break if you try and press them back out but they can be saved if he uses the proper support. If he balanced the crank, he balanced it to something as you cannot just balance the crank to itself, it needs up to 2050g of bobweight (total reciprocating and non reciprocating weight formula) to balance out. (example, 2050g is what Eagle builds into their cranks to give the balancer plenty of weight to work with and is not balanced to any piston rod assembly out of the box). Usually weight needs to be removed from crank on lighter aftermarket stuff as it is far cheaper to remove weight than to add tungsten at $75 a slug. Mismatched rod and cap is a guaranteed lock up after torquing them down, happened to me too on a very light stamped 3 and 8 cap. It was a toss up where it went and I rolled craps on that one. Swapped them and it was night and day when I went to rotate the lubed crank after retorquing them down.
 
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That crank has been balanced. Im a little confused. So are the 116 pistons already on the rods? Was the crank balanced with the 116 pistons or was it balanced before? If the 116 pistons are on the rods and those go to that crank put the correct rod caps with the correct rod and run it!! It will be fine that isn't gonna be enough difference to hurt anything. I'm running kbs with factory rods and crank and didn't even rebalance the crank at all .
 
Tays340 said:
When I pulled the engine apart I stamped each rod and cap.
I am so on a budget here, New rods? They said the pistons would be destroyed if had to be changed. He did weld up the crank some and add a hole. When I put #8 cap on #5 rod it binds before it seats. I don't like it.
I thought the whole purpose of balancing was to get all the components to weigh the same before you made bob weights to balance the crank. This looks like it would shake apart and kill every bearing on cam break-in. But i'm naive. Still 13 grams between #3 journal and #4.
I guess I'm headed back to the machinist for some explanation, but, in my opinion, this is not right.View attachment 1715747234 View attachment 1715747235 View attachment 1715747236
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If the machinist swapped caps to the get balance job done you’ll be starting over. That is ridiculous.

Also, getting everything down to a gram or less on each end is mental masturbation and losing money. You do all that work and then what happens to all the oil slinging around in the crank case and hanging onto the parts.

If the rods are within a few grams on each end and the crank is within 10 grams I’d send it. And I lost money trying to balance this junk to under a gram everywhere. 2 plane balancing isn’t as accurate as one would think.

I’ll see if I can find a link to a really good webinar on crank balancing and I’ll post it here. Watch it as many times as you have to so you can get a better understanding of what is happening. That way you can have a logical discussion with the guy doing the job. And he won’t be able to bullshit you the better educated you are.
 
Ok, I suck. I can’t post a link to the webinar for some reason. There are two of them by the same guy. Both are worth watching. So here’s how to see them.

Go to pera.org.
Click on the “webinars” tab.
On the first page that pops up you will see a webinar given on December 18th 2020. Watch that one first, and then watch the one from May 23rd 2021. That will help you understand what is going on better than i or anyone I know can explain it.

BTW, any of the webinars by PERA is free to anyone and you can watch any or all of them you want. You have to give your name and email address but there is a TON of FREE information in those webinars on a wide variety of topics. Well worth the time of you like to self educate.
 
yellow rose said:
Ok, I suck. I can’t post a link to the webinar for some reason. There are two of them by the same guy. Both are worth watching. So here’s how to see them.

Go to pera.org.
Click on the “webinars” tab.
On the first page that pops up you will see a webinar given on December 18th 2020. Watch that one first, and then watch the one from May 23rd 2021. That will help you understand what is going on better than i or anyone I know can explain it.

BTW, any of the webinars by PERA is free to anyone and you can watch any or all of them you want. You have to give your name and email address but there is a TON of FREE information in those webinars on a wide variety of topics. Well worth the time of you like to self educate.
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Im gonna have to check that out also thanks for sharing!
 
I don't have a pic of the piston rod assy. The rod end has been heated to get the wrist pin in. I don't think I can easily press them out, but, naive.
The big end has been machined so probably align bored. Looking at the numbers more, maybe he is trying to get each journal weight close. Still, his sharpie only tells me where 4 of the caps go. Not a happy camper.
 
dartfreak75 said:
That crank has been balanced. Im a little confused. So are the 116 pistons already on the rods? Was the crank balanced with the 116 pistons or was it balanced before? If the 116 pistons are on the rods and those go to that crank put the correct rod caps with the correct rod and run it!! It will be fine that isn't gonna be enough difference to hurt anything. I'm running kbs with factory rods and crank and didn't even rebalance the crank at all .
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Hey Kevin, hows your 360 running. I havent seen any build thread updates.
 
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