Well I guess the first thing would be some definitions.
FORGED: is a steel billet (or blank, or bar) that has been heated and
forged into a near net shape with final machining into a finished product.
Sizes are restricted to available dies. Due to the expense, dies are only
created for popular demand and volume. Since the greatest volume is for a
stroked anything, I know of no forging dies for de-stroked sizes in any
brand engine. If anybody knows of a de-stroked FORGING, please let me know
because I will buy one.
BILLET: is a bar of steel that has been machined into a finished product.
Billet is used when a forging is NOT available for the stroke you want. You
go to billet only if a forging is not available. If you have a billet crank,
by definition, it is already the finished size you want and welding is not
required. If you found a used, CHEAP, billet in the wrong size for your
application and wanted to de-stroke it by welding, that would be
ill-advised. See below.
Yes, I had a welded de-stroked (2.625") SBC in 1979. That is my disclaimer
and the technology, most assuredly, has improved. So YMMV. The problem with
welding is unknown metallurgy (both the crank and the filler rod) but more
importantly, inclusions, or discontinuities. By that I mean small pit holes
on the ground journal. On my crank I found the fillets on both sides of the
journal to not be perfect and an occasional small pit on the ground surface
of the journal. I was told these imperfections would not matter. Truth be
told, the crank never failed because my poor choice of rods failed before
the crank could. In those days welding was 25% the cost of a billet. Cost
was the deciding factor. With CNC technology today (and its cost savings) I
wouldn't even consider welding. A billet costs the same (or slightly less)
today than it did in 1979. ($1.6K - $2.2k).
When you say "cast" it can be steel or iron. Welding an iron crank is not
done. Welding cast steel is possible but unwise because it is not cost
effective and has the same problems mentioned above.
Generally, common forgings will always have a greater fatigue life and a
marginal strength advantage over a billet. However, not all alloys are
forgeable. Therefore, a billet gives you an unlimited choice in alloys and
heat treatment. Along with CNC machining, some argue that a billet is as
strong as a forging. That might be true if you could pick an esoteric alloy
and had a big budget . . . but you don't. However, the strength difference
between the two technologies is irrelevant for you. De-stroking is bringing
the centerline of the mains and the journals CLOSER TOGETHER. Simple
geometry is going to give you all the strength you need. Buy a billet and
rest in peace.
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