Originally Posted by PatM
68 NovaSS and BigDog7373
Thanks for enlightening me. When I bought my parts (back 10-12 years ago) the market was more limited, and the vast majority of internal balanced 383 Stroker cranks required the 6.0 rods. This was for the reason that CNC Northeast mentioned, or at least I thought and continue to believe. The Mallory Metal is, of course, an answer for almost anything remotely within reason. But, it is very expensive. And the more you need, the more expensive it becomes. As a result of your posts I went to the Eagle and Scat sites and found a broad assortment of cranks and kits set up for internal balance with 5.7 rods. I have to presume this requires the use of their specified (or somewhat akin) pistons, to clear the counterweights.
Anyway, again, thanks for enlightening me . . And CNC Northeast -- I still agree that for an internally balanced 383 it probably makes most sense to use 6 inch rods . . but it seems there are ways to use those 5.7 inch rods now in an internally balanced 383, without the need for Mallory Metal. I'm sure you knew that already, but I (for one) feel enlightened.
You are right; this is a place where you need to pay attention to the crankshaft manufacturer's recommendations as to rods and pistons. After-all the counterweight is there to offset a complex sum of weights and partial weights of the material hanging on the rod throw portion of the crank. This is divided into rotation and reciprocating weights.
The crankshaft rod throws are rotating weights to these are added the weight of the big end of the connecting rods. So the big ends need to be made to weigh equally. One can see that the rod throws are not equaled and will contribute to some degree of off balance of any one compared to the others when the crank is spun.
The connecting rods not only need to the big ends of equal weight but the entire rod also has to be of identical weight of any one to all others, this usually means adjusting the small end after the big ends have been equaled. This would include the weight of typical rod bearing set. The total weight of the rod along with equaling the weight of the pistons each to all others plus a ring set are added together where in the case of a 90 degree V8 one half of the sum of the connecting rod and piston weights and all of the weight of the big end are summed, This result becomes the bob-weight that is attached to the crankshaft for the spin balance cycle.
Given there is only so much mass that can be stuffed into a counterweight because of space limitations to clear the piston at BDC and the crankcase, if the use of Mallory Metals is to be avoided one needs to be cautious about part weight selection.
I use the term Mallory Metal somewhat loosely as simply meaning heavy metals rather than Mallory as manufacturer and seller of heavy metal alloys. These alloys contain large amounts of metals like tungsten which weigh about twice that of steel or cast iron and are rather expensive so you don't want to create a situation where you have to use much of this stuff. There are plenty of pictures on the web and in the popular press of huge multiple applications of Mallory metals in some poor crankshaft where the situation has gone wrong for improper parts selection among other things that can happen of huge multiple applications of Mallory metals.
Always try to use a crank that has the all of rod throws drilled. This saves a lot weight in the rod throw making these cranks a lot easier to balance.