Balancing connecting rods [Archive] - Chevelle Tech

: Balancing connecting rods


Olle
Mar 16th, 07, 11:08 AM
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?

onovakind67
Mar 16th, 07, 11:35 AM
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.

Olle
Mar 16th, 07, 11:46 AM
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. :)

onovakind67
Mar 16th, 07, 11:52 AM
Will the new pistons weigh the same as the old ones?

Olle
Mar 16th, 07, 12:13 PM
I guess I won't know until I buy them. Why?

77 cruiser
Mar 16th, 07, 4:41 PM
Piston weights figure into the balance also.

Tom Mobley
Mar 16th, 07, 5:02 PM
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.

Olle
Mar 16th, 07, 5:36 PM
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?

onovakind67
Mar 16th, 07, 5:52 PM
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.

Olle
Mar 16th, 07, 6:07 PM
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?

BillK
Mar 16th, 07, 10:17 PM
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.

ironhead
Mar 16th, 07, 10:35 PM
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.

Steve340
Mar 16th, 07, 10:39 PM
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


Steve

pdq67
Mar 16th, 07, 11:02 PM
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...

pdq67

Olle
Mar 16th, 07, 11:05 PM
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. ;)

pdq67
Mar 16th, 07, 11:19 PM
Here is how I think it is done!!

You clamp two rods together, (clamp the big ends AND small ends together two different times), on a mandrel 180 degree's from each other and watch them teeter-totter in a jig! Grind until they are balanced and repeat until they are all the same teeter-totter balance is all it does!!

AND try to start first w/ the lightest pair and lighten all the others to them..

pdq67

Wolfplace
Mar 17th, 07, 12:00 AM
Yeah, I will, if someone would just tell me how to do it without the fixture. ;)
=
To balance the rods you need to support one end on the scale & the other end in space without effecting the end that is on the scale.
To do this they have to be exactly level, they should be supported at the center line of the pin & rod.
This is just about impossible without the correct equipment if you want it done right.
Here is the fixture I use & notice the length of the chains to support the pin end.

http://wsm.ezsitedesigner.com/share/scrapbook/19/190264/Steve_s_427-1_balancing_002.jpg


There is a reason I use these long chains, it is for repeatability.
It can be done with shorter ones but I have found that these repeat better than anything I have ever used & I have been balancing engines for over 35 years.
Unless you are going to do this on a regular basis you would be better served to have it done by someone who has the equipment & knows how to use it ;)

Just for your info, here are the bob-weights that Bill referred to that go on the crank to balance it

http://wsm.ezsitedesigner.com/share/scrapbook/19/190264/Steve_s_427-1_balancing_001.jpg

Stalkingbear
Mar 17th, 07, 12:07 AM
And, the whole rotating assembly turns rather slowly when balancing - the electronics tell you where to add/subtract weight. You either drill material out of the counterweights or add mallory metal to increase weight. It is all rather interesting and..... the last time I used a machine was twenty years ago. I like your equipment, Wolfplace. (I hope my further explanation was correct... some of the stuff I have forgotten....)
'bear

Wolfplace
Mar 17th, 07, 12:15 AM
And, the whole rotating assembly turns rather slowly when balancing - the electronics tell you where to add/subtract weight. You either drill material out of the counterweights or add mallory metal to increase weight. It is all rather interesting and..... the last time I used a machine was twenty years ago. I like your equipment, Wolfplace. (I hope my further explanation was correct... some of the stuff I have forgotten....)
'bear
=
Pretty much :thumbsup:
Except I will machine the counterweights down in a lathe if they are very far out & then trim with a couple of holes as necessary.
Takes far more time but sure looks better than a $3000 crank that looks like some someone was using it for drill practice,,,, :pout:

Olle
Mar 17th, 07, 4:24 PM
Thanks, y'all! I love to tinker with things, so I think what I will do is this:

I'll try my best to weigh pistons and rods kinda like what Jeff said, just for the hell of it. Then I'll take it to a shop and have them do it all. I'll still have to pay for a complete balancing, but at least I'll know if my DIY balancing works or not.

pdq67
Mar 18th, 07, 6:38 PM
Mike,

Right......

I figure you know that you can use a teeter-totter jig, BUT it won't weigh them, end for end, but rather just balance them to each other as a set after you go through them to find the lightest first rod to start pairing off against.

After using it, you still would need to weigh each end of at least one of them to get the sets correct bob-weight, imho...

pdq67