by kitabel

The difference in journal diameter between the 80 (UL, not shovelhead) crankpin and the OHV 74 (not 61) crankpin is 1/8": 1.125" vs. 1.250".

Error # 1:
Using the full 1/8" difference only moves the centerline of the journal (the crank throw distance) by 1/2 the diametric difference, or 1/16".
This increase is both up and down, so the total change to the stroke is not 1/4", but 1/8".
(BTW: even with all the parts brand-new, the maximum offset isn't exactly the .125" difference, but most frequently .120" to .123" to allow the grinder to "spark out", to make a complete pass and remove material all around the circumference).

Error #2:
Even if 1/4" were added to the stroke, the new stroke length would be 4.53125" (4-9/32" + 1/4" = 4-17/32"), giving 84.1" (not 86") displacement with FL standard bore.

Correctly using 1/8" as the increase, the new stroke is only 4.40625" (4-9/32" + 1/8" = 4-13/32"), giving 81.8" displacement with FL standard bore.

The offset is of course in the tapered hole, not the journal. The dimensions are correct, just brain-faded into automotive territory.