The rod length ratings have to do with the OD of the counterweights. A crank for a 6" rod is going to have larger counterweights and will have more material for an entirely internal balance. A crank for a 5.7" rod would have smaller counterweights and may require a split internal/external balance like stock (with the weight on the flywheel). Using a 5.7" rod on a crankshaft that requires 6" rods will probably result in the counterweights hitting the piston skirts at BDC.

You should reconsider a stroker crank unless you're building a solid roller motor with a boatload of boost making power deep into 4 digits, see the Grubbworm motor. The justification for destroking is more about reducing rod angularity and taking load off of the cylinder walls than reducing piston acceleration. For a street car, a little extra displacement will help spool a turbo faster, especially with a low CR boost motor.

 

1-piece rear seal will need a counterweight on the rear. They are always internal front and external balanced rear. I asked the machinist I trust that did my 383 crank about full internal balance. Not possible as the 1-piece seal cranks are missing a chunk of counterweight that the 2 piece cranks have outside the engine.

I have a 6" rod version of a 1-piece seal 3.75" stroke crank in my 383. Took minimal work for the machinist to balance it.

 

Not necessarily, there are 1pc flywheels that are neutral balanced and have no rear counterweight. If there's enough mass in the counterweight and the bobweight of the rod/piston/pin combo is small enough, there's no reason a 1pc crank can't be fully internally balanced.

 

Thanks for getting back to me about the differences between the two types of crank shafts for different rod lengths. I will consider stroking out the lt1 when the time comes. When it comes to the 383, I am more concerned about the side loading rather than the piston speed. If you would like to talk more about the future of my set up then feel free to pm me. I wouldnt want to get too off topic with the thread. c:

 

My 396 is internally balanced.

 

My 383 is Internal/External balanced. Makes peak power at 5,600, the PCM is programmed for the 4L85E to upshift at 6,000, and the fuel kill is at 6,200.

 

I had mine internally balanced for the sake of ever having to replace/re machine the flywheel for any reason, it can be done so by simply zero balancing a new flywheel instead of hoping the old one can be used as reference.

 

If you are worried about piston speed, then you want to stick the longest rod possible in the engine.

 

Being a member of the Eagle 2 piece crank club, it was cast, I won't use either of their cast or forged stuff. Scat is "OK" for forged cranks. It was around 2007 my cast Eagle crank broke just driving about 30 mph one day. between #1 * 2 journal. Took out the block. Golen replaced the motor promptly, new block and all with a Scat forged

383/6" rods, internal balance....45k mi on it now

 

While this is true, changing stroke has a much greater effect compared to changing rod length. It's also acceleration that kills parts, not speed.

All of this is kinda moot tbh unless OP is revving to the moon. Speaking practically, building a stock or lower displacement motor to be used in street RPM ranges is a waste of time and money if you're already shelling out for a 4340 crank. The only time that's not the case is if OP is one of the 1% of people shooting for four digit power numbers, building a FI + solid roller setup with inferior parts, or building a motor for road racing.

 

While a cast crank is fine for street use and as others have stated; 500hp. I’d like peace of mind as well as being able to swap a forged crank I already have into another engine built for more power.

Ive read that a used crank is stronger due to it being through so many heat cycles over the years however I’m not sure how true that really is...

Thoughts on nitrated cranks ?

 

I have a stock crank in my 12 to 1. 355. I run 11.0 na and deep tens on nitrous. Not really worried about it.

 

The stock crank and even stock rods have always been the bombfuckingdiggity.

 

You need to be more specific with what it is you're building and why. Help us help you. Are you saying that you're trying to make 500hp? At the wheels or crank? And you're running a turbo? And you're buying a stock stroke forged crank to use in a 350, but are going to build another 350 later? I'm confused by all of this. You can't be selecting parts solely on vague public opinion.

The sentiment that I'm trying to get across is that if you're going to drop thousands of dollars on a rotating assembly, a different rod/piston combo to allow more stroke and paying your builder to clearance the block costs peanuts in comparison.