302cui 28oz vs. 50oz. different crankshafts?

[CobraV8]

Do the crankshafts for the late model 50oz imbalance engines have heavier counterweigths than the 28oz cranks, or is the extra weight only added to the flywheel and damper?

I'm building a new engine with TRW forged pistons and scat rods. The balancer and flywheel I'm going to use are for a 28oz imbalance rotating assembly.
Can I use a crankshaft from a 5.0 engine that had 50oz imbalance with my parts?
Or is this not a problem at all due to the fact that the complete rotating assembly is going to be balanced anyway?

Thanks,

Simon

[Woodz428]

In most cases the weight added to the flywheel/damper is to compensate for weight that can't be added within the crankcase. So, I would surmise that the later 50oz. components are to compensate for new piston weights or something, possibly less crank weight. In your combination, I would have it professionally balanced prior to assembly, so it doesn't really matter. The balancer would add any needed weight when he balanced. The main difference would be the one piece seal on the later ones, if your block doesn't have a single piece seal, the later crank may not work, the later 5.0 guys will have to give a definite answer on that.

[woody2]

In using the 28oz your motor will rev-up quicker. Good Luck!
PS not sure if the 5.0 crank can be used?

[Woodz428]

Originally Posted by woody2

In using the 28oz your motor will rev-up quicker. Good Luck!
PS not sure if the 5.0 crank can be used?

Since the weight has to be hung on the crankshaft, on either end, to balance...I can't see how that will change the ability to rev? The recip. parts are the same, and to balance out correctly the weight will have to be in the crank or added externally. It would seem regardless of which crank, that you will still be rotating the same "mass"

[CobraV8]

The lighter the rotating assembly is, the quicker it will rev up. That's what lightening pistons and crankshafts, using aluminum rods and flywheels etc. is all about.
As 28oz means a little less counterweight compared to 50oz, it might in fact have some influence.

Can anyone clearify if I can use the 5.0 crank, or if it will cause problems with the rear seal or the balancing act?

[Jim Zagg]

What year is the block? The 28 oz. crank fits into a 2 piece rear seal block. The 50 oz. crank fits into a 1 piece rear seal block. The problem is there are oil slingers on some cranks at the rear seal area and if my memory is correct the one piece rear seal block does not have clearace for one. The 50 oz. crank has less mass in the counterweights and must have the 50oz. balancer and flywheel or it will not be close to being in balance.

[Woodz428]

Originally Posted by CobraV8

The lighter the rotating assembly is, the quicker it will rev up. That's what lightening pistons and crankshafts, using aluminum rods and flywheels etc. is all about.
As 28oz means a little less counterweight compared to 50oz, it might in fact have some influence.

You are correct in the less weight of the recip assembly spinning quicker, just incorrect in your understanding of what the 50oz or 28 oz means. That is not the weight in the crank, that is the weight needed outside the crankcase(on the flywheel/damper) to balance it out. So a 28 oz crank needs less weight added, which means it is heavier than the 50 oz crank. The point that I was making, that applies directly to this, is that the total weight of either recip assembly will be the same... the weights and balance are directy tied to the weight of the rods and pistons. So there will be no advantage in the spin up. Now if you are lightening the rods and pistons, as you discussed, that would help, because it would require less weight be added. But as is pointed out in the post above, the 28oz crank is heavier than the 50oz. The weights stated are not what weight is in the crank, but how much has to be added externally. This, of course, makes the 28oz crank heavier than the 50oz one. From the parts you ar using, I would believe that they are even heavier than stock (heavier duty parts usually are), so you will likely have to add even more weight than the 28 or 50 oz's that the cranks require.

[CobraV8]

I assumed the 28oz and 50oz cranks had about the same weight, and the heavier counterweights on damper and flywheel are necessary due to the use of heavier pistons and rods in the late model engines (i.e. 5.0 HO with forged pistons).

If the weight of the flywheel and damper only was increased to use a lighter crank (for whatever reason) then you are right of course, the total mass will be the same and there's no difference in revving up.

As far as my initial question is concerned, the 50oz crankshaft seems less acceptable in this case. If it is lighter than the 28oz this means more material in form of heavy metal would have to be added to balance the heavier forged pistons and aftermarket rods.