Does Crank Stroke Affect External Balance

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72DMag

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So I bought the How to Build Big Inch Mopar Small Blocks Book by Jim Szilagyi and in the back of the book there are like 6 engine builds listed with all the details. One engine build uses a cast 4" stroke crank and a stock 360 external dampner to build a 408.

If this is the case then does the stroke of the crank not affect the external balance?
 
Yes. Any time components are changed to something different it needs to be rebalanced.
 
You can buy balanced stroker kits but I don't know how close they are to being correct.
 
Not referring to internal balance stroker. Talking about an external balanced stroker. Weights on the dampner and flexplate.
 
72DMag said:
Not referring to internal balance stroker. Talking about an external balanced stroker. Weights on the dampner and flexplate.
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I have never heard of a external balanced stroker crank. Not saying there isn't such a thing.
 
I have that same book and have read it cover to cover many times and never noticed.
 
Your big end weight is farther away from the axis which makes it greater (centripetal mass) so your balance will most likely have to be modded, added. Most cast stroker crank list as 'external balanced' as they have less mass than a forged crank. Look at a SCAT cast crank and look how light it is, requiring lots of tungsten @ $75 a slug. Then look at a forged cranks bobweight, its heavier so you dont need to add as much. Crank manufacturers know this and offer better selections now but a cast crank is usually 5-8% lighter than a forged.
 
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They make new stroker cranks with longer throws so of course the balance would be effected.
 
The build is on page 127 its the 414ci best of production build. @pishta Exactly being further away from the axis makes sense to me and thats why i question the build in the book. Not sure how you can external balance a 4" stroke crank with a factory 360 dampner.
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@toolmanmike I attached pic of the page. Take a look at the crank and dampner and he even explains in the description above to use these for external balance. Doesn't make sense to me since stroke is 4" not stock 360 3.58" stroke.
 
72DMag said:
@toolmanmike I attached pic of the page. Take a look at the crank and dampner and he even explains in the description above to use these for external balance. Doesn't make sense to me since stroke is 4" not stock 360 3.58" stroke.
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Interesting. Not a real engine he built though. A 4" stroke crank with lighter 273/318 connecting rods and no balance? I'm not buying it.
 
He says use the 360 balancer and associated 360 parts (already added mass in the right areas) as it wont require any mallory metal to balance, read that ADD any mallory but you'll still have to remove some metal to balance the assembly. Its no charge to remove more metal when balancing, but adds up when you have to add to crank as it has to be a metal that has more mass than cast iron and this is $$ tungsten. Hes also using lighter 318 rods so he's already plus side of a stock 360 balance. Being on the plus side means no added tungsten.

>>PS try finding that P5007731 piston anywhere.....
 
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I don't know how or if this applies, but when a local machine shop balanced my small block 273 back in the '70's, the guy welded a small weight onto the clutch pressure plate. I have no idea what that was all about. (When I sent the pressure plate to McLeod for a "rebuild", they said if I did that again they would void my warranty).
 
Here is a balance sheet of a cast 360 crank that is internally balanced...this sheet was for rebalancing the crank due to using lighter rods.....Look at the balance sheet...NO where is there any calculation for the length of the stroke...everything is based on the weights of the pistons..rods...rod bearing...spiro locks...rings...

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There is your external balance. Welding anything to a cast pressure plate will most likely warp it being those are as flat as can be. He should have attached it to a bolt if possible. I zapped 2g of MIG wire into the center of a floater pin to balance a light piston and just that amount of localized heat caused the pin to bind in the piston.
 
@70aarcuda Thanks for sharing. I guess that confirms it then. Machine shops don't even look at the stroke for balancing. Thus 4" stroke cast cranks can be balanced with stock 360 weight dampner, rods, pistons, rings, and wrist pins etc as long as weight is the same.
 
I wouldn't go that far, the rotating masses are weighed by the balance machine, at a certain RPM and are not assigned values. When its 'bobbed up" (matching bob weights attached to crank and spun up) the machine will tell you where to add/subtract what amount of balance as needed in degrees. ie. -12g at 240 and at axial distance (front middle or back of crank). The machine operator turns the crank so 240 is straight up and then he drills the crank (he has a drill press on a sliding arm over the crank) x amount of mm's to reduce that point by 12g. He then spins the crank up again and he gets his next point. If he needs to add, he drills a .750 bore and presses in a slug of Tungsten (W) on the side of the throw (or on top but it will require a weld to set it) then has to counter balance that fixed mass by drilling somewhere else. They will balance to a certain +-g as you can balance that thing all day and never get it absolutely 0. The '4g of oil' is an estimate anyway and changes at different RPMs so your balance tolerance is probably +-5g easily. The 360 external motors were built then spun as a unit and then dynamically balanced by the balancer (maybe). This allowed a lighter cheaper cast crank to be used with the other forged components. The weight had to be added and the throws were not available as they were sealed under the pan already. Not sure of they were able to zap the weights to the convertor on the line or they came 'pre-balanced'. Heck, they may have been pre calculated off the line and just assembled 'as is' to accept a pretty close balance. Anyone have a butter smooth 360 at idle? Have to ask an assembly line worker how the dynamic balance was performed at the engine plant as the 360 balancers looked 'as cast' for their imbalance, no additional holes in the front or backside ala 318.
 
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I had a stroke and it definitely affected my internal balancing............don't know if I'm cast or forged, have to check my birth certificate!!!!!!:rofl::poke:
 
They can balance internal usually with heavy metal (Mallory) or external, or internal front and external rear as a 400 Chevy I did, It just depends on the weight of the parts.
 
dibbons said:
I don't know how or if this applies, but when a local machine shop balanced my small block 273 back in the '70's, the guy welded a small weight onto the clutch pressure plate. I have no idea what that was all about. (When I sent the pressure plate to McLeod for a "rebuild", they said if I did that again they would void my warranty).
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McLeod clutch covers are heat treated. Welding on them causes them to fracture at the weld.

Doing that is a piss poor way to balance a crank assembly.
 
72DMag said:
@70aarcuda Thanks for sharing. I guess that confirms it then. Machine shops don't even look at the stroke for balancing. Thus 4" stroke cast cranks can be balanced with stock 360 weight dampner, rods, pistons, rings, and wrist pins etc as long as weight is the same.
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You are missing the whole issue.


When the mass of the counterweights is not enough to counteract the weight of the reciprocating and rotating weights, you either need to add mass to the counterweights (Mallory metal) or you need to add that mass at both ends of the crank, externally, like adding weight to the damper and flywheel.


I’ve had to add Mallory to more steel cranks than I care to remember. The cast crank, with the same size counterweight doesn’t have the same mass as a forged steel crank.
 
yellow rose said:
You are missing the whole issue.


When the mass of the counterweights is not enough to counteract the weight of the reciprocating and rotating weights, you either need to add mass to the counterweights (Mallory metal) or you need to add that mass at both ends of the crank, externally, like adding weight to the damper and flywheel.


I’ve had to add Mallory to more steel cranks than I care to remember. The cast crank, with the same size counterweight doesn’t have the same mass as a forged steel crank.
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well said
 
@yellow rose I follow with you but thats what I am saying. Based on the article from the book if you keep the weights the same as what the external weights were designed for then the stroke doesn't affect that. So for an easier example to explain my point. If you take a 360 la and change nothing but the crank and pistons but those pistons weighed the same just shorter compression height. You can slap the 360 la damper back on and go driving. At least according to the article that's whats being said.

I believe he lists this particular crank cause weight on cranks are designed for la 360. IF you look at last paragraph he states no mallory needed for this crank.
 
Think of a teeter totter, put 100#s on each side equal distance from the center, and then move one out a ft. What happens? Same with crank throws.
Yes, you can use a 360 balancer, on a 408, but corrections will have to be made, but the external balance saves on the bill verses tungsten.
When you balance, you don't tell the machine the stroke, you tell it the radius of the counterweight since that is where you correct it.
 
72DMag said:
@70aarcuda Thanks for sharing. I guess that confirms it then. Machine shops don't even look at the stroke for balancing. Thus 4" stroke cast cranks can be balanced with stock 360 weight dampner, rods, pistons, rings, and wrist pins etc as long as weight is the same.
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But the stroker pistons are different than the stock ones. The block would probably be overbored which adds to piston weight. Anybody that thinks that a longer stroke crank won't weigh more and effect the balance is a fool. The bob weight is added to the journal regardless of stroke length and weight is added or removed as needed. I would never trust a stock balancer and flywheel to balance out anything other than the stock crank that the parts came off of. Even if you switch from a cast crank to a steel crank there is a difference in balance.
 
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