From ozone.breath at gmail.com Wed Jul 29 20:50:43 2009 From: ozone.breath at gmail.com (Erik Berg) Date: Wed, 29 Jul 2009 19:50:43 -0700 Subject: [Chapman-era] Steel Twin Cam Crank Message-ID: 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