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  #1  
Old 05-28-2008, 01:37 AM
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bulldog426 bulldog426 is offline
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Default cast crank vs forged

what are the advantages and disadvantages of each??? isn't a cast crank lighter??
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Old 05-28-2008, 03:39 AM
namvet67a1f namvet67a1f is offline
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Cast is lighter ..and should buzz-UP quicker. I would not shy away from a cast crank combo in any street-strip combo ... but I would want a forged piece in one of my vehicles ...a motorhome.

There could be a max-weight there of almost 20k pounds.
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Old 05-28-2008, 05:32 AM
DartGT66 DartGT66 is offline
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Is the cast crank really lighter? The way I've understood is, that because it's weaker, more material is needed in the rod arms, and that can't be added to the counterweights because tehre is no room. Therefore it requires external balance. So, I think it's rather heavier than a forged crank, than lighter. Otherwise the need for external balance makes no sence; the counterweight sizes are the same.

Forge cranks are supposed to be stronger than cast crank, but how much strength is usually needed? I mean, a billet crank is even stronger than a forged crank, but still very few people insist on using them in combos where the forged or cast crank work just fine. I have broken two 440 forged cranks, but not a single cast crank. But what is the advantage of a cast crank? I think for the manufacturer it's the price. It's cheaper to build casta than forged cranks, and the customer will never know or care about the difference. The biggest disadvantage is, that they usually have to be externally balanced, and nowadays that you can have even new forged cranks for under 600$, there may not be a lot of sence in grinding and balancing a cast crank combo. Otherwise the mopar cast cranks should work just fine up to about 600 hp in BB's, and over 500 in SB's.
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Old 05-28-2008, 04:58 PM
John Kunkel John Kunkel is offline
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Quote:
Originally Posted by DartGT66 View Post
So, I think it's rather heavier than a forged crank, than lighter. Otherwise the need for external balance makes no sence; the counterweight sizes are the same.
For balancing purposes, the overall weight of the crank is irrelevant.

Cast iron is less dense than forged steel, so a cast counterweight is lighter than a forged counterweight of the same size; therefore the lighter cast counterweight (sometimes) can't counterbalance the weight of the rotating/reciprocating assembly without external help.
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Old 05-30-2008, 01:20 AM
DartGT66 DartGT66 is offline
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Less dense? Can you relly compress metal or liquid? I have never seen different weights for a quart of cast or forged iron. I really don't know for sure, but would like to see eomone actually weigh a cast nad a forged 440 cranks, and publish the results.
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Old 05-30-2008, 01:28 AM
namvet67a1f namvet67a1f is offline
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The numbers ...8 to 10 lbs HEAVIER on the forged unit ....sticks in my mind.
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Old 05-30-2008, 02:26 PM
thatwasfunny thatwasfunny is offline
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Ok, hughes book says...

383 forged=66 lbs
400 cast=64 lbs
Now this is funny...
440 cast or forged=69 lbs
And for u small block guy's ...
318 cast=54 lbs
340 forged=56 lbs
360 cast=58 lbs
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Old 05-30-2008, 03:23 PM
valiant64 valiant64 is offline
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Not a huge difference, especially if one decides to have a forged crank lightened. Cast crank- less expensive, somewhat lighter, but if a bearing failure occurs less likelihood of being able to save the journal, requires external balancing w/ corresponding damper/flywheel/converter.
Forged crank-more expensive, sometimes heavier than cast, stronger than cast, easily repaired if damaged, internal balancing (except 6pak engine) uses unweighted converter/flywheel/damper.
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Old 05-30-2008, 03:27 PM
thatwasfunny thatwasfunny is offline
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Yea, and really if your rebuilding u can balance how ever you want internal or external.
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Old 05-30-2008, 03:34 PM
valiant64 valiant64 is offline
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The thing that sux about internal balance w/ cast crank is having to add Mallory metal to the crank ($) so at that point, the weight advantage of the cast crank is pretty much nil. But if what you have is a cast crank in your engine, & you have no intentions of a serious build/big nitrous set up/ridiculous rpm's, I wouldn't worry about running one.
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Old 05-30-2008, 03:39 PM
thatwasfunny thatwasfunny is offline
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Yea I agree, and that mallory metal whoowee gets pricey.
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Old 05-30-2008, 04:36 PM
John Kunkel John Kunkel is offline
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Quote:
Originally Posted by DartGT66 View Post
Less dense? Can you relly compress metal or liquid?
Have you ever seen a red hot steel crank in a forging press? It gets smaller (compresses) with each stroke of the press.
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Old 05-30-2008, 07:11 PM
Walkercolt Walkercolt is offline
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If you want to spin 7000+ RPM's, you should have a forged crank. It will hold-up better. For us 5500 guys, cast or forged doesn't make much difference. There are actually some metilurigical advantages to cast cranks, but that's really pretty far out there for the real world. Oiling is critical for cast cranks. They MUST have a solid supply of oil to survive. Not an issue for Mopars usually.
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Old 06-02-2008, 01:40 AM
DartGT66 DartGT66 is offline
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Quote:
Originally Posted by John Kunkel View Post
Have you ever seen a red hot steel crank in a forging press? It gets smaller (compresses) with each stroke of the press.
No I haven't. But I have read some physics instead. How much do you have to compress or forge steel to make it as heavy as lead?
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Old 06-02-2008, 06:05 PM
John Kunkel John Kunkel is offline
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Quote:
Originally Posted by DartGT66 View Post
No I haven't. But I have read some physics instead. How much do you have to compress or forge steel to make it as heavy as lead?
Sarcasm noted. I wonder why you think steel is forged.

Check the fourth paragraph down on this page:

http://www.popularhotrodding.com/tec...ech/index.html
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Old 06-02-2008, 10:51 PM
dave5711 dave5711 is offline
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That's a great article John.
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Old 06-03-2008, 12:38 PM
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i've got a 77 rv 440 that i'm putting in a charger, and to get rid of the crappy compression from that year i ordered a set of KB hyperutectic pistons, and from my understanding they weigh differently than the stock pistons so i have to get the bottom end balanced, guy at the machine shop said to bring in pistons, rods, crank, torque converter, damper, to get it balanced, i'm pretty sure that in 77 this isn't a forged crank, torque converter had balances on it, which means, externally balanced which means cast crank right??? and it also has the tapered harmonic balancer on it. to get this balanced being a cast piece, what is there to this??
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Old 06-03-2008, 02:05 PM
hotroddave40 hotroddave40 is offline
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318s have cast cranks and are internally balanced.

The Forging process eliminates air or other gases in the steel that a cast crank has in it. That is part of why they are heavier. Metal and solid can not be compressed just rechaped.

Most pistons and rods are lighter now days than they used to be, instead of buying mallory metal invest in some light pistons and rods to go along with a light crank and you should not need mallory to get internally balanced. If you are not going over 6000 rpm cast is fine.
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Old 06-04-2008, 03:57 PM
John Kunkel John Kunkel is offline
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Quote:
Originally Posted by hotroddave40 View Post
318s have cast cranks and are internally balanced.

Simply because the counterweights, cast or forged, have sufficient mass to counterbalance the rotating/reciprocating weight. When the 340 went from forged to cast they needed external weight even though the piston got lighter.


Quote:
The Forging process eliminates air or other gases in the steel that a cast crank has in it. That is part of why they are heavier. Metal and solid can not be compressed just rechaped.
So, you're saying the metal can't be compressed but you state that the forging process eliminates air and gasses? What replaces the space taken up by those gasses?

If the gasses are driven out and replaced by steel there is a term used for that.......let's see.........it's called........."compresssing".
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Old 06-05-2008, 12:00 AM
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Quote:
Originally Posted by John Kunkel View Post
Simply because the counterweights, cast or forged, have sufficient mass to counterbalance the rotating/reciprocating weight. When the 340 went from forged to cast they needed external weight even though the piston got lighter.




So, you're saying the metal can't be compressed but you state that the forging process eliminates air and gasses? What replaces the space taken up by those gasses?

If the gasses are driven out and replaced by steel there is a term used for that.......let's see.........it's called........."compresssing".
No, he is saying that metal cant be compressed. If you squeeze the air and gasses out of a forging, you are doing exactly that. The metal is not being compressed, it is just being purged of voids. You still have the same amount of metal. Cast Pontiac rods, bad..Forged Chryco rods, good!
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Old 06-05-2008, 12:25 AM
Walkercolt Walkercolt is offline
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Well, even vacumm re-melt forged steel can be compressed, but an infintesimal amount and then it springs back to it's original size. That's known as the "modulus of elasticity". It's why a spring tries to return to it's orginial shape and size. And the "springiness" is why a crank has to be balanced, then have a flywheel on one end and a harmonic balancer on the other end. Join me in Physics III class later, and we'll discuss that a rotating mass can be too ridiged, causing stress points leading to fractures. Ask an owner of an old Harley about his motor mounts cracking the frame. Short story is, a well designed cast crank can be nearly as strong as a well designed forged crank. All depends on the foundry techniques and the intended use. The forged crank can do without full lubrication for a very short time, while a cast crank MUST have a constant supply of oil for the journals to "surf" on the bearing surfaces, and cast cranks don't like very high RPM applications as well. Metalurgy 101 will convene at 2:30 PM.
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Old 06-05-2008, 08:36 AM
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Didn't the forgin process also have something to do with the molecular structure and the coal atoms? I have some memory from metallurgy classes, that the process reduces / removes excessive free coal atoms, especially from the surface area. This again reduces the risk of crack development.

In the old proses of making swords, blades etc, the forging process was the way to control the coal and the amount of coal will affect how brittle the steel gets when hardened.
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Old 06-05-2008, 10:14 AM
Dr. Righteous Dr. Righteous is offline
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Quote:
Originally Posted by DartGT66 View Post
Less dense? Can you relly compress metal or liquid? I have never seen different weights for a quart of cast or forged iron. I really don't know for sure, but would like to see eomone actually weigh a cast nad a forged 440 cranks, and publish the results.
Look up steel FORGING and Steel Casting.

You will see the difference.

another difference that comes to mind is that cast steel is HARDER than forged. That can be a benefit and a weakness. You are more likely to get a crack in a cast crank, they are less able to absorb a shock load (detonation?) than a forged one.
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Old 06-05-2008, 06:01 PM
John Kunkel John Kunkel is offline
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Quote:
Originally Posted by pishta View Post
The metal is not being compressed, it is just being purged of voids. You still have the same amount of metal.
If you take 10 cubic feet of air and compress it down to 1 cubic foot you still have the same number of molecules of air.
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Old 06-05-2008, 06:53 PM
Walkercolt Walkercolt is offline
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Since at least the Magnum engines came out, Mopar has been using nodular iron which is a process that controlls the carbon content and molecular structure of the metal, making it stronger and more stable, and less likely to form stress points, and makes their cast cranks superior to many forged cranks are. Just an aside on this topic, Toyota, Honda and Nissan use cast cranks but then dip them in liquid nitrogen to make them stronger and harder, almost the equal to the best forged metal.Moving the molecules closer together with cold instead of heat and force. Super cold forging anyone?
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