Scat 4.250" cast crank and adding Mallory?

[Dumpling]

Just a random synaptic connection. Just wondered why external balancing was alright for 100's of thousands of factory Fords but seems to be looked down upon by machine shops charging for mallory today.

Aren't you marrying a specific set of rods and pistons to a specific slug-filled crank?

[GerryP]

Sure you are.  Any engine that is precision balanced is now unique in its entirety.  But if the flywheel is uncommon and I scorch it, I have to get another flywheel and have it precisely balanced to my engine assembly.  If I blow the engine, then it really doesn't matter about crankshaft balance since I'm starting from new again.  I'm not saying external balance is a bad thing.  It's just not the one you'd bring to the dance if you actually had a choice.  The factory did external balances because they are cheaper and faster in assembly.  A choice might be if you went to the machinist and he said he could do an internal or external balance and the cost would be the same, which would you choose?

[Barry_R]

Internal balancing is "better" because it keeps all the load concentrated within the main bearing span.  External balancing - although functional - has offset weights outside of the mains in a comparatively unsupported area.  As for engineering on the parts, they do have to work within limits on counterweight size and position.  As the counterweight width and diameter goes up you can run into interference with the bottoms of pistons and with the block itself.  We have run into problems there when the initial counterweight diameter operation is not set correctly on stroker FE cranks - so they are already out toward the edges of "what will fit".  After that you simply have no option in engineering or application; you simply add heavy metal.  I think we had one crank with nearly a dozen on a really unusual application, and I have seen an older F1 crank that had a ring of mallory slugs on each counterweight in an effort to reduce diameter and thus windage and inertia.

[pbf777]

Internal balancing is "better" because it keeps all the load concentrated within the main bearing span. 

     This would generally be the ideal engineering intention, but unfortunately, although the often accepted practice, the concentration of the counterweight offsetting loads created with the addition of significant sums of heavy metal at the opposing ends, particularly if we will assume for simplicity of the argument, a properly engineered crankshaft otherwise just lacking in required counterweight masses, accentuates the end to end twisting and bending forces and may not be within the original engineering capacities of the unit.  In other words, the counterweight value corrections should be spread over a greater area rather than just crammed into one counterweight mass at each end.  And even the external weight, on the same plane as the, in the case of the typical American V8, the large internal crankshaft counter weight, on opposite sides of the last main bearing position, at either end, does aid in countering the bending force imparted to the crank from its' mass which is at times quite great, thus aiding in squaring the crankshaft journal in the main bearing bore.   

     This consideration may lead one to conclude that although as stated previously that one would wish to contain this loading between the crankshaft main bearing supporting structure of the block, sometimes, most often due to unavailable space with in this area, one finds that reasonable counterweight sums can be located outside, if just not cantilevered out an excessive distance from the main bearing support.  This consideration for example may be why F.M.C. engineers chose to incorporate the additional front external counterweight value for the 428's on the spacer-sleeve behind the damper vs. incorporating it in the damper as was the established practice with the S.B.F. at the time, as this would have placed the mass an even greater distance from the main bearing support than that of the S.B.F..     

     Back in the day, it was often, as we did also, practiced in the example of the S.B.F. to "neutral" or "internal" balance the nose of the crankshaft but allow the external counterweight value on the flywheel  to remain (for monetary cost reasons     ), as this offsetting mass carried on the flywheel was a shorter distance from the main support and didn't demonstrate the crankshaft deflection imparted as was seen on the nose.   

     Just food for thought!     

    Scott.