I was looking at some sbc cranks by lunati and mostly all the cranks had a minium required length of 6.0 in. why is this even the stock stroke cranks said they require 6.0 in rods. Im gonna take a stab at it. because of the short stroke the crank can be made lighter there for to balance the rotating assembly you need to use a longer rod so that you can use a really light piston which more than makes up for the extra weight of the longer rod. so i want to get these premium crower rods that i can get cheap but there 5.7 inch rods they are really light and weigh like 595 grams. will i be able to get a light enough piston coupled with the light rods to be able to balance every thing with a lighter than stock crank.

Its becuase the pistons that go along with a 6.0" rod will clear the counter wieghts. Most likely useing a shorter rod will cause the part of the piston under the pin, the flat part, to smash into the crank counter wieght. Or possibly the skirt.

Umass,
Actually I think it is the other way around. Usually the 6" rods are heavier, so they make the counterweights larger to make it easier to internally balance the crank. If you try to use them with a 5.7 rod, the counterweight will hit the bottom of the piston.

That's not why....

Think about what is happening when the piston is at BDC. At that point, the counterweight, which is directly opposite the rod journal, is at "TDC". In other words, the bottom of the piston is very close to it.

In a stock stroke 400, there isn't enough space between the bottom of the piston and the center line of the crank, to fit a large enough counterweight to completely counterbalance the rod journal and rotating assembly. This is because they had to "flat cut" the top of the counterweight to clear the bottom of the piston. That's why external balance is required.

It's not an issue with most short-stroke engines like 350s.

The weight difference bewtween a long rod/short piston and a short rod/tall piston depends greatly on the particular choice of parts. With some combinations, especially ones with really beefy rods and really lightened pistons, the longer rod setup would be quite a bit heavier. It would be wise to research your particular parts before you jump to a conclusion not based on fact, that turns out to work against you.

so if it says that the crank must be used with a 6.0 inch rod its because they had made the crank weights heavier and larger in diameter so they need a longer rod to clear the piston. how does that make any sense. why not use a lighter 5.7 rod and make the crank weights lighter and of smaller diameter. would the taller piston then be too heavy for the smaller lighter crank weights so that you couldnt internally balance the assembly. maybe they made the crank weights narrower and taller so as to reduce the cross sectional area of the crank weights to reduce oil drag on the leading edge of the crank. i dont know so i should at lest find a crank thats designed for 5.7 rods and takes into account the shorter lighter rod and the heavier piston.

The weight of the rod & piston isn't the only thing that determines the size of the counterweight. The distance that its center is from the center line of the crank changes the balance too. But, increasing the distance from the crank center line to the rod center line, is another way of saying "stroker".

As we all keep saying, the cranks that are set up this way, are typically for longer than stock strokes. The longer stroke requires more counterweight. The only way to fit more counterweight in the motor is to lengthen the rod.

Take a physics course. Even freshman non-physics-major introductory physics, intended for people who don't need alot of "real" physics, like engineeering and pre-med majors, will teach you the basics of balancing.... specifically, the concept of "moment of inertia".

yeah I took physics and I know what moment of inertia is, and I could see the arguement if if in deed I was talking about stroker cranks. take a look here (http://www.lunaticams.com/Crankshafts/ProSerSB.html) and tell me why a 3.0 inch stroke requires a 6.0 inch rod. and here (http://www.lunaticams.com/Crankshafts/RaceSerSB.html) at the bottom where the two largest stroke cranks only require 5.85 rods. i know they require the longer rods so that the piston will clear the crank weights but why is it that most other stock stroke aftermarket cranks will work fine with 5.7 in rods seems kind of a bad descision to limit the use of your cranks to people that have or are going to buy 6.0 in rods or larger.

Adam,
I don't have a clue why they are saying you need a 6" rod with a 3" stroke.
My opinion is someone screwed up the description.

It's kinda like taking everything you read in magazines as "gospel",, not a real good idea :D

The only time I know of you "need" a longer rod is in the case of more counterweight like a stroker so you can balance it internally as has already been stated.
I agree with you there is just no reason to have enough counterweight on the crank with a 3" stroke to need a 6" rod unless they are casting the piston out of lead :D

As RB69SS396Conv said, "It's not an issue with most short-stroke engines like 350s."

The idea is to make the stuff as light as possible for the application without sacrificing strength.