Balancing connecting rods - Chevelle Tech
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post #1 of 21 (permalink) Old Mar 16th, 07, 11:08 AM Thread Starter
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Balancing connecting rods

I'm going to do a quick overhaul on a 350, and thought it might be a good idea to weigh the pistons and balance the connecting rods. There are special fixtures you can buy where you set them up to weigh both ends, but I don't want to buy one of those just for a cheap rebuild like this. I figure that the process is quite simple, as long as you can set up the rods and weigh them in a consistent way. Is it possible to do this without special tools?

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post #2 of 21 (permalink) Old Mar 16th, 07, 11:35 AM
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Re: Balancing connecting rods

Unless you suspect a real balancing problem, I'd leave it alone. Even if you had a scale setup that would work, it costs money to have the pins pressed out and re-installed. If you're going to balance the rods, you'll want to resize them first.
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post #3 of 21 (permalink) Old Mar 16th, 07, 11:46 AM Thread Starter
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Re: Balancing connecting rods

I'm planning on changing the pistons (I suspect that I have the dished low-comp type in it now) so the pins will have to come out anyway. I'm not going for a full balance job, blueprinting etc, this will just be kind of an exersise in how much you can improve an engine by doing as much work as possible yourself. Got the engine for free and just want to tinker with it.

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post #4 of 21 (permalink) Old Mar 16th, 07, 11:52 AM
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Re: Balancing connecting rods

Will the new pistons weigh the same as the old ones?
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post #5 of 21 (permalink) Old Mar 16th, 07, 12:13 PM Thread Starter
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Re: Balancing connecting rods

I guess I won't know until I buy them. Why?

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post #6 of 21 (permalink) Old Mar 16th, 07, 4:41 PM
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Re: Balancing connecting rods

Piston weights figure into the balance also.

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post #7 of 21 (permalink) Old Mar 16th, 07, 5:02 PM
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Re: Balancing connecting rods

there won't be any improvement at all if you just equalize the big and little end weight of the rods. If you're not going to have the assembly balanced in a balancing machine it's a complete waste of time jacking around with stuff like this. You could make the balance situation worse by taking weight off parts, maybe it needs heavier big ends to make the bob weight come out right?

If it were a flat four like a VW you could equalize all the rod and piston weights and then send the crank alone out to be spun. V-8's don't work like that at all.

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post #8 of 21 (permalink) Old Mar 16th, 07, 5:36 PM Thread Starter
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Re: Balancing connecting rods

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Originally Posted by Tom Mobley View Post
If it were a flat four like a VW you could equalize all the rod and piston weights and then send the crank alone out to be spun. V-8's don't work like that at all.
I don't know much about balancing, so are you saying that factory V8 cranks are balanced as an assembly in the plant? In that case you can't even change pistons without re-balancing the whole assembly. Is that correct?

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post #9 of 21 (permalink) Old Mar 16th, 07, 5:52 PM
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Re: Balancing connecting rods

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Originally Posted by Olle View Post
I don't know much about balancing, so are you saying that factory V8 cranks are balanced as an assembly in the plant? In that case you can't even change pistons without re-balancing the whole assembly. Is that correct?
If the new pistons are close to the same weight as the old ones you won't need to re-balance for an average driver motor.
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post #10 of 21 (permalink) Old Mar 16th, 07, 6:07 PM Thread Starter
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Re: Balancing connecting rods

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Originally Posted by onovakind67 View Post
If the new pistons are close to the same weight as the old ones you won't need to re-balance for an average driver motor.
The engine I have is from an old farm truck, and I know that it has been rebuilt a couple of times by "the local experts" so I'm afraid to use the weight of the old pistons as a reference. Let's put it this way: I probably won't be surprised by anything I'll find when I take it apart, and the project can very well change from a quick overhaul to a complete rebuild, depending on what I find.

As I understand it, a complete balancing job includes weight balancing of the pistons and the rods, then balancing the complete assembly. I do want a cheap engine (alright, fairly cheap then), but would like to balance it anyway. This brings up my original question: If I can weight balance the pistons and the rods, I can leave the rest to a shop and save a few bucks, right? In that case: Are there any ingenious ways to do it myself without that fixture?

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post #11 of 21 (permalink) Old Mar 16th, 07, 10:17 PM
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Re: Balancing connecting rods

Olle,
This is the short version ..... the crankshaft itself is balanced to the weight of the parts that are attached to it. All the parts are weighed, then a formula is used to compute a "bob weight" then, special weights are made up that attach to the crankshaft. Then the crankshaft is balanced in a special machine similar to a tire balancer.

So just making all the parts the same weight will not help much, especially if the pistons are a different weight than the original ones. If you want to try and balance the rods yourself, go ahead, but you will probably not save enough to buy the balancing fixture.

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post #12 of 21 (permalink) Old Mar 16th, 07, 10:35 PM
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Re: Balancing connecting rods

My pistons were equalized.The rods were resized..then the same.The bob-weight was calculated and the crank was polished and balanced for $300.
This is cheap insurance and a reasonable cost...considering the expense the machine shop has invested in the equipment and the operator.
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post #13 of 21 (permalink) Old Mar 16th, 07, 10:39 PM
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Re: Balancing connecting rods

The cool thing is, have you ever been in a car, with a well built balanced engine?

Have to use crank, rods, pistons, flywheel ( for ext balanced ) Harmonic balancer Smooth as all get out man


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post #14 of 21 (permalink) Old Mar 16th, 07, 11:02 PM
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Re: Balancing connecting rods

Right!! SMOOTH!!

My old junk301 UNBALANCED AS ALL get-out had at least three harmonic's it went through going up to 7,000 rpm and higher!!

I could pretty much tell my rpm by them at speed!

AND it had a bad habit of throwing a damper going down the highway running 55 to 60 mph or so SO BALANCE THE SOB AND DO IT A FAVOR!!

It will last a long time...

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post #15 of 21 (permalink) Old Mar 16th, 07, 11:05 PM Thread Starter
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Re: Balancing connecting rods

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Originally Posted by BillK View Post
If you want to try and balance the rods yourself, go ahead, but you will probably not save enough to buy the balancing fixture.
Yeah, I will, if someone would just tell me how to do it without the fixture.

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