Doing a ground up build with a 1 pc seal block. I understand that all of these engines were externally balanced.Anyone know why? I suspect that accountants were involved but I really dont know.
I need to procure the entire rotating assembly anyway, so is there anything about this block that should prevent me from having my parts internally balanced?
Also, will it matter if I use a 3.75" vs 3.48" crank? Shouldnt my machine shop be able to balance either?
My thanks to anyone who can clear this up for me. -tim

The two-piece seal cranks have a counterweight built into the flywheel mounting flange. The one-piece seal cranks slip the seal over the rear of the crank, so they cannot have a big counterweight.
Any crank can be internally balanced, it's just a matter of cost and material.

You can use a 2pc seal crank with one of these for $15 http://www.catpep.com/searchengine/searchengine.asp

This is one of those "The chicken or the egg" dilemas.

I’ve been told by a “professional” that even though the 1 pc rear main seals have a counter weight on the flexplate/flywheel, they’re not truly externally balanced. Why? Because they can still be internally balanced with a little extra work (or rather, could have been if the General wasn’t such a cheap ass).

Therefore, some people claim they really aren’t externally balanced. A “true” externally balanced engine has no physical means to be internally balanced due to size limitations.

Again, this comes from a “pro” that I’ve talked to, contrary to what other pros have said, such as the late John Lingenfelter, Smokey Yunik, David Vizard, etc.

MY thoughts are, if it some kind of weight has to be “added on” (i.e. bolted on) after the crank is installed, then it’s externally balanced.

Anyway, there is no reason to have to internally balance a 1 pc rear main seal crank. There is no “good” or “bad” to either method (internal or external). As long as everything is balanced, that’s all that matters.

I always thought that an internally balanced engine is easier on the mains & will therefore last longer. Not true?

Theoretically, (and I figure too), that any balance that is not actually right next to the journals and in their counterweight(s) at each jounal pair can cause a bending moment at speed!

B/c there will be a static balance weight(s) that are out on the end(s) of the crank opposite the weight of the pistons to make up for it not being where it should be due to usually space limitations unless Heavi-Metal is used....

AND this/these balance weights out on the ends in the damper and flywheel/flexplate can cause a bending moment at speed that will cause the crank to bow, ever so slightly b/c of the distance they are away from the point of needed balance.

This bowing due to the bending moments at speed are harmful to both the crank b/c of fatigue over time and bearing wear!

I hope this poor explanation helps b/c this is really a complex subject.

I do want to say that I figure that GM designed both the SB and the BB cranks to account for stuff like this so really they aren't that much of a cencern unless you are producing tremendous power AND at high rpm! Otherwise, stock GM balances work just fime for most applications!

I had my 496 BB externally balanced so I could run a new OEM 454 damper and an aftermarket 454 stick flywheel b/c I was going to make a 475" motor first! (.100" over, 454)... Then the dreaded, "more power, Scotty" bug bit me and I went 496, but had a new 454 damper and flywheel.

I am not going to rpm my 496" er much above say 5800 rpm due running rebuilt truck rods that have 3/8" aftermarket bolts and nuts.

Therefore I really don't see the need to internally balance my "tow-truck", grunt motor..

pdq67

[This message has been edited by pdq67 (edited September 26, 2005).]