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Originally Posted by STEVE POTTS
Carnut,
Your math is off. To increase stroke, you must move the centroid of the rod journal down. So, when offset grinding, you would leave the bottom of the journal at its present location...cause you cant go any lower...and cut at the the top of the journal. The new journal centroid for the BBC journal would be at 1.1" from bottom. The original stock centroid was at 1.2192" from journal bottom. So, the new effective centroid moved .1192" down. Hence, your new stroke would be 3.78" + .1192" = 3.899" This is max stroke for this combo.
However, some shops can weld more material to the bottom of each journal to increase stroke even further...but this is expensive...call Kieth Craft. I sold kieth all 4 of my 391 cranks...he machines them and sells them to racers who like to spin them at 7500 rpm. Forged steel units are great cranks, but for $500 bucks...you cant beat the cast iron scat cranks in 4.125 or 4.25" strokes.
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I had a 3.78 (Cast) crank taken out to 3.90. I used Ford Cleveland bearings. This kept everything FORD, and didn't require widening. When all done, the rotating assembly was zero-balanced with no mallory.
Cross-drilling is a waste (just my opinion) and actually a potential trouble-spot.
Nothing wrong with cast iron cranks. 391 cranks ARE nice, because they're steel and NOT cross-drilled. If you need all that. Most can get along with a cast crank.