Re: 318 - 360 interchange; was Cat question

From: Nicholas McKinney (nickmckinn@mindspring.com)
Date: Tue Feb 24 1998 - 21:57:18 EST


At 02:01 PM 2/23/98 -0800, you wrote:
>>Actually in some situations removing material from a forging helps. Look
>...
>>surface. This is a good place for cracks to form on connecting rods and
>>the polishing lowers the chance. Also forged cranks when prepped at a good
>
>I think the theory says that it removes a "stress riser" -- basically, by
>removing extra material, you let the whole part flex at the same rate,
>instead of concentrating the flex at the point where the dimension drops,
>ie, the end of the flash.

Well the stress riser is due to the rough surface at the parting line of
the forging. Any sharp corner is a no-no in machining when high strength
is needed.

>A billet crank would be the ultimate - finely machined over the whole
>surface. I'm sure billet costs more due to the machining (much more
>expensive to cut away metal than to cast or forge the part so the metal
>wasn't there to begin with). I really don't know whether billet are
>stronger or weaker than forged. I doubt billet crankshafts are actually
>made from a single huge chunk of metal the diameter of the crank throws.

Yes they are, that helps to explain both the metal grain issues and the
extravagant cost.

>>The manufacturers are going to external balance (Chevy went this route in
>>1986) because they can make the crank with much much smaller
>>counterweights, and adding mass to the dampner and the rear plate is much
>...
>>Blocks do not care about internal and external. And forged are better to
>
>Isn't it a lot more demanding of the bearings to have an assembly with
>out-of-balance parts strung out along the crank?

Well any sort of serious imbalance with respect to the parts other than the
crank is bad. My original intent was to state that blocks are not made
differently with respect to internal and external balanced assemblies.

>I suspect the real reason is that the manufacturing tolerances are closing
>up so that the basic balance is much closer to begin with. The powder-
>metal con rods in the LT1 (Chebby) engine are forged with carefully
>measured amounts of material, and all come out of the factory weighing
>within a gram or two of one another. Supposedly, there's no longer any

I do not have any facts on the new LT1, but I remember the older chevy
parts had a 10 gram tolerance with was considered pretty tight back in the
day.

Regards

Nicholas



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