[Chapman-era] Steel Twin Cam Crank

[Erik Berg]

Howdy all,

The time has (finally) come for me to start collecting parts for an
all-steel twin cam.  Initially this will go in my '65 Elan (the former Fred
Miranda car).  Later this engine will go into my Brabham BT-15 for vintage
FB.  It will need to tolerate 8500 RPM.

I'm seeking advice on what to use for a crankshaft.  I think it ought to be
fully counterweighted, with 12 bolt flywheel mounting, wide journals, and
lipseal configuration.  It will have a standard twin cam stroke (72.75 if I
remember right).

Poking around the web turned up UK suppliers such as Farndon, Arrow, and
Wilcox that offer premium EN40B twin cam cranks.  Pricy too, at #1150 to
#1500.  QED has something that sounds similar at #1200, material
unconfirmed. QED also has a more reasonable priced EN19 crank that sounded
like it might have been an alternative, until they told me it has only 4
counterweights. Stateside sources such as Scat and Moldex make similar
cranks out of 4340.  I believe these are cut from a billet, not forged.

Questions:

Can I get away with a fully counterweighted crank made from billet 4340, for
a 72.75mm stroke engine that will see at most 8500 RPM and 190 HP?

What would be the appropriate surface hardening treatment for 4340?  I am
aware that 4340 has good hardenability, but nitriding is generally the
treatment of choice for race cranks and I don't know if 4340 is as well
optimized for nitriding as EN40B is.

Thanks,
Erik Berg
Southern Cal