The 848 casting number late 235 head gives the 261 a good compression bump, and there's a tech tip here on how to drill the steam holes correctly. They're angled slightly to intersect the coolant passages, not absolutely necessary, but it's a very good idea to do it the way the factory did. The main difference in the 261 is the bore size, 3 3/4 inches, which is way more than a 235 block will tolerate. 235 and 261 crankshafts are a direct swap- - -no difference in dimensions or stroke. Some 261's had heavier connecting rods, IIRC. The 261 camshaft is the same grind as the stovebolt-engine early Corvettes used, but there are much more radical cam profiles available aftermarket if you want a hot rod.
Jerry
Last edited by Hotrod Lincoln; 01/02/2014 7:00 PM.
