Bobweight for Mopar-Perf partnr. P5007256 4-inch stroker crank

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???...

what is it you want to know...how heavy you can go with the parts before you have to add...
 
There isn't any info like that on a cast crank...every one of those cranks is different
Keep the parts as light as you can**little more $$$ parts** because those heavy part cost you money in the end because you'll need 200-300 worth of mallory to get it to balance...there's no way around a balance job with that crank...
 
NorthernSwede said:
Well I want to know how much the assembled piston and rod should weigh to get the crank balanced right.
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Eagle cast cranks in a 4.00 or stock are in the neighborhood of a 2050g boboweight. so if you can get your formula for rotating mass - big end and bearings- and reciprocating mass (everything else) and get it very close to 2050, you should be within 1 slug of mallory or even lightening the crank. They say cast are for external balancing but there is nothin gto say that they cant be internally balanced if the bobweight is less than the throw weight. Forged cranks are heavier by nature so they usually need to be lightened, cast are lighter so more mass needs to be added to deal with beefy rods and old OEM mopar lead like pistons.
 
I agree with the post above it would be nice to get the bw at 2000 but pretty hard to do...piston and pin 600...a 6.1 rod 700 that that's 2600 grams and no rings ..brgs.. and oil added yet...
if it were me building a tq stroker on the cheap I wouldn't bal it and use a 205@50 cam...as long as the rods are matched to +-1...just my take
 
I had my cast nodular crank balanced int.

nothing wrong with it at all.

as for bob weight....weigh the the parts, thats your bob weight, lighten/heavy the crank to match/counter.
 
Everybody is kind of saying but then not really. There is no set bob weight. You have to weigh all the parts involved, rods, pistons, pins, rings, pin locks if equipped, and add for oil. If a shop is asking you for the bob weight they don't know what they are doing, get your stuff out of there now.
 
There are a bunch of SB 4" strokers being built with 1700-1800 bobweights. New I beams and light pistons.

My 360's are all in the 2000-2100 range and that's with stock 760 gram rods/KB107 pistons.

Most every crank out there has a target bobweight range, never seen one that didn't, it's part of the design.

(Big end rod and bearing) x 2
(Small end rod, Piston, pin, rings) x 1
Throw in a little oil 5-10 grams

That's the bobweight calculation
 
Yes there is a target bob weight that the crank was designed for, that doesn't mean it can't be changed. Lighter is better, less stress on all the parts involved. I would never choose rods and pistons based on the target bob weight. It would probably require less work to balance but so what, it's not that expensive to start with.
 
Guitar Jones said:
Yes there is a target bob weight that the crank was designed for, that doesn't mean it can't be changed. Lighter is better, less stress on all the parts involved. I would never choose rods and pistons based on the target bob weight. It would probably require less work to balance but so what, it's not that expensive to start with.
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I agree, lighter is better. I would not buy rods/pistons based on the target bob. More of a statement that it's designed into the crank. I wouldn't buy stuff that made for a heavier bob, that's for sure.

You are going to pay for a balance job, so if they have to take material from the crank, have at it.
 
the bob weight for my 408 ended up being 1640 with scat i beams and diamond pistons i am using a scat forged crank and my machineist said they had to take nearly 400 grams out of the counter weights it should rev to the moon i hope
 
Well I'm sorry if I was unclear but I wondered how much the stuff should weigh to be as close as possible, I mean someone must have the measurements of an unbalanced crank even if they differ somewhat atleast I know where to aim.
 
just aim at good light parts it doesent cost more to take more weigth out anyway but it cost alot more to add, and with the precision of anything that can be bought in a box it doesent matter since some numbers are always of anyway :( i get you question its just one of those situations where its very hard to get close
 
scat i beams weight around 600 grams...and kb 847 weight 430 grams...

combine those two you and you would lighten a crank alot...
 
1qwikScamp said:
the bob weight for my 408 ended up being 1640 with scat i beams and diamond pistons i am using a scat forged crank and my machineist said they had to take nearly 400 grams out of the counter weights it should rev to the moon i hope
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My Scat 4" Cast Crank, Scat I-Beams and KB Icon Flat-tops internally balanced required 1-small slug at the front, I forget exactly how many grams, but wasn't much.

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Rickster
 
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