Question on piston and rod balancing

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OriginalDart

OriginalDart

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I'm building a 1978 440 for my dart.
The block was in great shape with low milage so I'm reusing almost everything.
I did however swap out the stock pistons for a set of KB hypereutectic with valve relief.
But still using stock rods and crank.
I was so excited to get everything back from the machine shop I just started assembling.
I realized after everything was torqued down that I didn't weigh my rod/piston assemblies.
This motor isn't going to doing anything crazy just city driving...maybe the strip once or twice a year.
So my question is...

Do you think I'll be OK or will these new pistons **** everything up?
 
IDK about 78 pistons, but the 69s were about 858gms. The hypers vary from 765 to 775. So this is over 10% lighter,and so yeah the engine will shake.And you're really gonna notice 88 gms per
I don't think you can cure this by modding the TC or aftermarket flexplate.
 
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The pistons would need to be removed from the rods to facilitate a proper balance job. The reciprocating and rotating ends of the rod get weighed seperately. Some shops will not guarantee removal without damage.
 
What George didn't say,lol, is that the engine has to come out and apart.
I might take a chance on the rods all being close enough for your application, but you still need the separated rod to calculate the crank mods. I'm not a BB guy so I don't know about shortcuts for you.
 
I know the KB pistons are a bit lighter but I figured coming from the factory they would all be close in weight. So if the rod weight didn't change and the pistons all weigh the same that would make the weight across each rod/piston drop the same? Does having a lighter rod/piston throw anything else out of whack?
 
You may be looking for one of us to say send it, like the Chevy guys who will go to the scrap yard and grab another core for $150 to destroy. I will caution you on this. It needs to come apart and go back for full balance. Will it run like it is, sure, but even a grocery getter will wear bearing surfaces unevenly and vibrate. You need crank, rods, pistons, balancer, and flexplate to do it right. Plus or minus flexplate. More so for externally balanced small block stuff. I had $1800 in machine work, balance, heads on my last 440. Yes it will cost money, but a new motor is $6500. And my advice and $5 will get you coffee. Good luck
 
Comments in the quote
OriginalDart said:
I know the KB pistons are a bit lighter but I figured coming from the factory they would all be close in weight. The KB pistons I have seen are really close to each other in weight, but no where near the factory numbers, and that's the problem. Hypers are lightweight items, designed for moderate performance.
So if the rod weight didn't change and the pistons all weigh the same that would make the weight across each rod/piston drop the same? Yes. I think I know where you're going with this, and so; no; One piston has to be separated from the rod so the rod itself can be weighed on each end, and the numbers plugged into a formula to determine how much weight to remove from the crank. In your slow-performance application, you might be tempted to cheat, by supplying another same kind of rod. But if you're wrong, you'll be taking it all apart....again. I suppose your machinist might know a shortcut, but who knows if it will work.
Does having a lighter rod/piston throw anything else out of whack? Yes.The engine can get to vibrating so bad it can hammer out the bearings, and break the balancer, and the crank often follows.. And that same vibration can make the car undriveable as well, and IDK what else. Oh yeah; 88 grams per piston is HUGE
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Ok so someone here knows the answer to this; how much will 88grams be, swinging on the end of that rod at say 3500 rpm, in terms of hammers I mean, lol. Is that about like hammering 3 inch nails into 2x4s? or more like a splitting firewood? or like driving fenceposts with a sledge-hammer?
I don't have the math for that.
 
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OriginalDart said:
I know the KB pistons are a bit lighter but I figured coming from the factory they would all be close in weight. So if the rod weight didn't change and the pistons all weigh the same that would make the weight across each rod/piston drop the same? Does having a lighter rod/piston throw anything else out of whack?
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It would mean the crank is balanced heavy. There's never much discussion from the pro's on balance factors, but it's all trade off's to minimize harmonics in the engines designed rpm range. I know when I used to be involved in drag bike engines we used different/heavier balance factors for engines that ran higher rpm ranges then say a street engine. maybe not apples to apples but to give an example, build identical 2 Harley engines with the same use in mind. One balanced @ 50% factor for street and the other %60+/- drag set up. The %50 will idle and perform better in the lower rpms. Put it on the track running to 7k rpms it will come undone. The one balanced at %60 will actually smooth out as the rpms come, make a touch more power, and stay together better.

But like I said it's not a subject that is discussed as openly in the automotive side.

I know of no shortcuts either. Unless you have actual numbers you're just guessing. You need to separate the rotating mass from the reciprocating mass to calculate.
 
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**** me right? Thanks for all the feed back..except for that lumberjack talking about swinging hammers. Since I'll be bringing it in for a balance...anything else I should know or have done while it's apart?
 
OriginalDart said:
**** me right? Thanks for all the feed back..except for that lumberjack talking about swinging hammers. Since I'll be bringing it in for a balance...anything else I should know or have done while it's apart?
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The lumberjack is a pretty good dude once you get to know him lol I don't know the math either other then saying an 800g piston weighs a metric **** ton when changing directions at 3500 rpm.
 
Pistons are reciprocating weight - not rotating weight. So that makes it less important to be "right". But that's a lot to be "heavy" and I would totally expect it to break itself. Also if you haven't, make damn sure you read up on the top ring gap setting on those pistons.
 
George
Yeah, I hear ya on the balance factors. When I sent my parts out, I asked for the parts to hang together at 7000 cuz I was a smartiepants and wanted to spend time up there.I didn't care about low-rpm smoothness. And for a few years everything went smoothly,lol. Then my driving style changed,as I got older,(I built her in 99 at age 46), and by then I had forgotten all about that. One summer, several years later,the low speed vibration got to me, and after several hours of searching, I concluded that the vibration was coming from the engine................. And That's when I remembered.
Ohhhhhhhhh! I said.
When I first put it together, it had 3.55s and 25.6" tires so 65mph was 3033rpm and it was a stick.. And it was dead-smooth. Four years later it had morphed into 4.30s and a GVOD, and 65 was now 2837. The following year I made some more changes and 65 was now 2273, and that's when the vibration became noticeable. Man did I hunt for it! But it was so small, and I said; Oh well, I'll know better next time. That was over 10 years ago,lol.....I don't think there will be a next time.....
 
OP
Maybe if you stroke it, the balance will be cheaper to achieve,lol. 88 grams per is a lot of correction.Check your deck clearance after the heads come off, maybe there's room,lol.
And for sure; heed Mopers warning. Specs are in the KB catalog. It's all too easy to make things too tight. I ended up taking my 367 apart to loosen it up a little. It liked to "seize" up every time I shut it off. I had to wait sometimes a half an hour with the hood up, before even a new mini-starter couldn't crank her over. A thou on the skirt and two on the gaps, and I never again had that problem. I had used re-useable gaskets so it didn't cost me much more than time, and I outsourced the resizing.
But I tore that engine down,4 winters in a row, to check up on it. Those gaskets got cheaper every time lol.
Your BB IDK. I'm an SBMer.
 
Thanks again for the input. I filed the ruins to the kb spec fuss came with shipping. I'm talking with a company near me called loge stein racing. Mopar guys. I figured while I had it back apart for balancing I might as well opt for a steel crank and internal balance.
 
I broke a piston ring land on a rare piston and found a replacement after a year of looking. Everything was balanced and now I have to press the old one off and balance the new to the old. I hope it presses off in one piece or ill have to collect them all and weigh em in a pile.
 
You have to support the pin boss when pressing it out - not the side and the ring lands. If you support the piston right, it will come off without damage. If you don't it will be in one piece, but junk.
 
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