Engine Balance

[Tophipe]

Hey guys. So I recently posted about finding a crankshaft for my AMC 258 i6 / 4.2l . Between the cost of actually buying a used one, having the machine shop work on it, and actually driving to get it I decided it would be about the same to just buy a new one. I talked to one of the techs at scat and he said their cast crank would be a direct fit. He also said that because it's an inline engine I won't have to worry about balance and that I can just swap i directly in. Is he right about this? It doesn't seem completely kosher to me. Thanks for the help as always.

 

[Torxhead]

I am sure that the crankshaft might be perfectly balanced to itself right out of the box, but when you add a variety of parts from different sources to it. Odds are that assembly will no longer be balanced.

 

[Tophipe]

I am sure that the crankshaft might be perfectly balanced to itself right out of the box, but when you add a variety of parts from different sources to it. Odds are that assembly will no longer be balanced.

Right. I plan on just using the old rods, but I've got a new flywheel. Not sure if this will be an issue.

 

[GatorBate]

I just rebuilt my AMC 304 and balancing it only cost an extra $125.00. He matched the weight of the pistons, rods and balanced the crank with the flywheel. It is as smooth at 5000 rpm as it is at idle.

I don't know why an I6 would be any different than a V8 as far as balancing it.

I would recommend having it balanced.

 

[IOPort51]

With the six , the components can be balanced, the 8 needs to have the crank, flywheel, rods, pistons and balancer done as one part. The difference between an internally and externally balanced motor. A SBC can be balanced in parts while a ford small block 8 has to be balanced as a unit.

You now know everything I know about this subject.

 

[Tophipe]

With the six , the components can be balanced, the 8 needs to have the crank, flywheel, rods, pistons and balancer done as one part. The difference between an internally and externally balanced motor. A SBC can be balanced in parts while a ford small block 8 has to be balanced as a unit.

You now know everything I know about this subject.

So basically, because I am keeping the pistons and rods, theoretically they should already be balanced together and so I shouldn't have to have them balanced with the crank? Or did I misunderstand?

 

[IOPort51]

with the six, you will be able to assemble the crank assy. and be good to go. you should be able to replace a couple of pistons and/or rods as well and still be good.

 

[mstar]

I recommend you consult a trusted engine/balancing shop in your area.
They can tell you what you will need to have done. mike

 

[IOPort51]

I recommend you consult a trusted engine/balancing shop in your area.
They can tell you what you will need to have done. mike

and let us know what you find out.

 

[tarry99]

So basically, because I am keeping the pistons and rods, theoretically they should already be balanced together and so I shouldn't have to have them balanced with the crank? Or did I misunderstand?

Lets clear this up for everyone.................The crank can be balanced at zero and perhaps that is what Scat is telling you. But that does not make any of the rest of the rotating assembly like the rods.pistons, rings and bearings all the same weight or balanced. Any replacement parts have to be weighed or balanced to match the others irregardless if it is a inline 6 V6 or V8.

The components are called the "Bob Weight" and all the balanced components should all be weighed individually , number stamped and kept together. The final adjustment on a rod is normally the bearing cap end where the engine balancer will grind off a small amount to adjust the weight. The Pistons will also be weighed individually and small amounts may be removed or adjusted under the pin boss.

Anything that is rotating becomes part of the rotating assembly and is subject to balance....

Now the difference between internal and external balance is this. And keep in mind that on some motors both process's can be used. Normally a motor that has large counterweights on the crankshaft will be internally balanced......and in some case's as I mentioned it is not unusual to see the front internal and the back half externally balanced. In order to understand this better if an engine manufacture because of design cannot have heavy or large counterweights , they just add the weight to the flywheel in the rear IE: external balanced......the same can be done at the front by adding a weighted front hub.

Any motor that has a zero balanced crankshaft ( more so a racing crankshaft) can have a zero balanced flywheel added , or a zero vibration dampner added with no issue.
On our race motors we always employ and over balance to the assembly to compensate for the effect of parasitic drag of the oil hanging and falling off parts we even employ that method with a dry sump oil system.

Simply speaking if you want it right.......take all your components to a professional in the trade and get it balanced.

 

[IOPort51]

so replacing a couple of rods or pistons would be a very bad thing but you could get away with replacing the balancer,crank or the flywheel if it was zero balanced??

 

[tarry99]

so replacing a couple of rods or pistons would be a very bad thing but you could get away with replacing the balancer,crank or the flywheel if it was zero balanced??

Yep...........If the components added are already balanced to Zero and your rotating assembly is internally balanced you can add those with no effect.

If you add a rod or piston to any balanced assembly it must be weighed and matched to the bob weight of the other like components.

When the assembly has both internal and external balances used one has to understand the correct weight and match what is being replaced.

Sounds confusing but it really is not.

 

[mstar]

Tarry, thanks for explaining that in a way I never could.That is the way I always heard it was and have read in many publications. Clears up some
points on balancing and brings credibility to this forum. mike s.

 

[Tophipe]

Yep...........If the components added are already balanced to Zero and your rotating assembly is internally balanced you can add those with no effect.

If you add a rod or piston to any balanced assembly it must be weighed and matched to the bob weight of the other like components.

When the assembly has both internal and external balances used one has to understand the correct weight and match what is being replaced.

Sounds confusing but it really is not.

Thanks for the info that clears a lot of things up. A little off topic, but I got on the phone with scat and the snout length on their cranks is 54mm. I have heard that 81-86 AMC 258 i6 / 4.2l 's had a 64mm snout. Would this be a problem as a stock replacement for a 1983? I also have a serpentine belt (California cj) and have heard that all serpetined cj's had the 54mm cranks. If someone could clear this up it would be greatly appreciated.

 

[tarry99]

Thanks for the info that clears a lot of things up. A little off topic, but I got on the phone with scat and the snout length on their cranks is 54mm. I have heard that 81-86 AMC 258 i6 / 4.2l 's had a 64mm snout. Would this be a problem as a stock replacement for a 1983? I also have a serpentine belt (California cj) and have heard that all serpetined cj's had the 54mm cranks. If someone could clear this up it would be greatly appreciated.

As far as direct replacement 54mm versus the 64 mm snout I'm not going to be able to answer that.........But any good machine shop that has been around for a while should be able to tell you.........The slight difference in snout length does not sound like and issue as long as all your accessories drive pulleys will still line up.........I'm sure someone more versed on those motors has a solution with perhaps a spacer if needed.

 

[Tophipe]

As far as direct replacement 54mm versus the 64 mm snout I'm not going to be able to answer that.........But any good machine shop that has been around for a while should be able to tell you.........The slight difference in snout length does not sound like and issue as long as all your accessories drive pulleys will still line up.........I'm sure someone more versed on those motors has a solution with perhaps a spacer if needed.

Sweet, thanks . I will give them a call tomorrow and see what they say about those 10mm.

 

[Bent]

The crank is balanced at the factory, as is the flywheel. They are not balanced as a unit. Some motors need a weighted flywheel to balance out the crank. But the I6 is internally balanced. You can swap flywheels as long as you don't try and use one off a externally balanced motor.

 

[Tophipe]

The crank is balanced at the factory, as is the flywheel. They are not balanced as a unit. Some motors need a weighted flywheel to balance out the crank. But the I6 is internally balanced. You can swap flywheels as long as you don't try and use one off a externally balanced motor.

That makes sense. Now that the balancing issue is sorted out, I'm just trying to figure out the snouth length issues. I'm getting pretty frustrated with this thing.

 

[Bent]

That makes sense. Now that the balancing issue is sorted out, I'm just trying to figure out the snouth length issues. I'm getting pretty frustrated with this thing.

Check the stroker forums. those guys use a bushing to push the harmonic balancer into the correct position when using the long snout AMC 258 i6 / 4.2l crank for a stroker build. A 12cw crank is short snout.

 

[Tophipe]

Check the stroker forums. those guys use a bushing to push the harmonic balancer into the correct position when using the long snout AMC 258 i6 / 4.2l crank for a stroker build. A 12cw crank is short snout.

Really? I was under the impression that the 71-86 AMC 258 i6 / 4.2l all had the same 64mm snout. From what I can tell, since they are stroking 4.0s which have the short snout, they use a spacer to take up the extra 10mm. For me the issue is the difference between a AMC 258 i6 / 4.2l with a long snout vs a short snout. Are the cranks interchangeable between the two?