Balancing rotating assembly
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Senior User
Joined: Mar 2002
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From: Charter Oak, Iowa
Balancing rotating assembly
What are the thoughts on balancing the rotating assembly as part of the overhaul process. Is it necessary for relatively low RPM engines (4500 tops)? I'm rebuilding a 302 and doing a relatively mild build up. I am willing to spend the bucks if I will end up with a smoother, longer running engine but don't want to waste money either. I know some guys do balance their engines and some don't. Any thoughts?
Randy
Randy
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Joined: Feb 2001
Posts: 6,624
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From: Missouri
Balancing rotating assembly
I just rebuilt a 289 for the neighbor. He didn't have the cash to balance everything and it ran fine even with a new crank. He sees over 5,000rpm about every day. Ford was pretty good about getting their parts all pretty much equal as far as cranks, balancers and flywheels go. I myself would never rebuild one without balancing it. It is by no means a waste of money.
Senior User
Joined: Apr 2002
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From: cedar rapids usa
Balancing rotating assembly
I just spent the last three months, and a good part of my holiday spending money, building a 406 out of my old 351M. While I don't have the time to go into all the details here and now, I well say that I would never run an engine that has not been balanced. If automakers thought it wasn't a big deal they wouldn't balance every engine, no matter its use, from the factory. Your friend might be having fun now, but when you figure that (quoting Einstein) mass gains like mass squared to the speed and acceleration of the mass or E=mc(2). For every gram an engine is out of balance @ 5000 rpm’s it is like having a 20lb hammer beating on the bearings, crank, rods and so on.
Lead Driver
Joined: Nov 2002
Posts: 9,557
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From: Concord, NC
Balancing rotating assembly
>Your friend might be having fun now, but
>when you figure that (quoting Einstein) mass gains like mass
>squared to the speed and acceleration of the mass or
>E=mc(2). For every gram an engine is out of balance @ 5000
>rpm’s it is like having a 20lb hammer beating on the
>bearings, crank, rods and so on.
Imbalance causes a considerable force on the bearings, but you must also consider the stresses applied by each of the 8 pistons and rods are applying to the crank each revolution (even in a balanced engine). As for the Einstein, quoting a friend of mine, "the only place we see mass energy relations is in nuclear reactors, we dont operate at high enough energies to see it otherwise. It's classical mechanics there, don't need einstein" Don't get me wrong, being out of balance is most definitely not good for an engine, but I think using Einstein's theory is a bit unneccessary.
>when you figure that (quoting Einstein) mass gains like mass
>squared to the speed and acceleration of the mass or
>E=mc(2). For every gram an engine is out of balance @ 5000
>rpm’s it is like having a 20lb hammer beating on the
>bearings, crank, rods and so on.
Imbalance causes a considerable force on the bearings, but you must also consider the stresses applied by each of the 8 pistons and rods are applying to the crank each revolution (even in a balanced engine). As for the Einstein, quoting a friend of mine, "the only place we see mass energy relations is in nuclear reactors, we dont operate at high enough energies to see it otherwise. It's classical mechanics there, don't need einstein" Don't get me wrong, being out of balance is most definitely not good for an engine, but I think using Einstein's theory is a bit unneccessary.
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Joined: Aug 2001
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From: Nevada
Balancing rotating assembly
If you've ever pulled the pan on an engine to view a lot of hard work, money and enthusiasm grenaded into tiny pieces, you might better appreciate extreme descriptions and words of caution that may appear to be excessive.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
Balancing rotating assembly
I never said not to balance the engine. I completely agree that all engines should be balanced - always. But Einstein? Please. My point is, that it doesn't take Einstein to tell me what general mechanics will - if it's not balanced, it will break - simple as that. Just my 2 cents.
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Joined: Aug 2000
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From: PacNW
Balancing rotating assembly
>What are the thoughts on balancing the rotating assembly as
>part of the overhaul process. Is it necessary for relatively
>low RPM engines (4500 tops)? I'm rebuilding a 302 and doing
>a relatively mild build up. I am willing to spend the bucks
>if I will end up with a smoother, longer running engine but
>don't want to waste money either. I know some guys do
>balance their engines and some don't. Any thoughts?
The only time I would consider not balancing an engine during a typical rebuild is if I were not replacing the pistons. If you're doing a serious "budget" overhaul i.e. a straight "ring and valve" job then you can get away with not balancing. However if you replace any hard parts in the rotating assembly of the engine, i.e. crank, rods, pistons, harmonic balancer or flexplate/flywheel, basically anything besides the rings & bearings then it's cheap insurance to get the whole assmbly balanced again.
>part of the overhaul process. Is it necessary for relatively
>low RPM engines (4500 tops)? I'm rebuilding a 302 and doing
>a relatively mild build up. I am willing to spend the bucks
>if I will end up with a smoother, longer running engine but
>don't want to waste money either. I know some guys do
>balance their engines and some don't. Any thoughts?
The only time I would consider not balancing an engine during a typical rebuild is if I were not replacing the pistons. If you're doing a serious "budget" overhaul i.e. a straight "ring and valve" job then you can get away with not balancing. However if you replace any hard parts in the rotating assembly of the engine, i.e. crank, rods, pistons, harmonic balancer or flexplate/flywheel, basically anything besides the rings & bearings then it's cheap insurance to get the whole assmbly balanced again.
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Joined: Aug 2000
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From: PacNW
Balancing rotating assembly
>why balance the pistons?they don't rotate and as long as
>they are all the same wieght it shouldn't matter, or what?
That's the key...they aren't all the same weight when they're cast or forged. Remember that in a 4 stroke motor only 1 in 4 strokes is a power stroke, the other 3 the crankshaft has to push or pull the piston. If all of the pistons are a little bit different weight it sets up vibrations in the crankshaft that over time are detrimental to the engine.
>they are all the same wieght it shouldn't matter, or what?
That's the key...they aren't all the same weight when they're cast or forged. Remember that in a 4 stroke motor only 1 in 4 strokes is a power stroke, the other 3 the crankshaft has to push or pull the piston. If all of the pistons are a little bit different weight it sets up vibrations in the crankshaft that over time are detrimental to the engine.
