I'm working with a guy on a small block engine configuration. He has a set of 3917291 head castings, which according to the NCRS Spec Guide are one year only use on '68 300 and 350 HP engines. I thought they might have accessory bracket bolt holes, but he said no.
So my question is: What's the physical difference, if any, between these ...291 castings and the prior 3890462 castings used on '67 300 and 350 HP engines?
One clue might be that the '68 300 HP advertised CR dropped to 10:1. It was 10.5 thru '66, then dropped to 10.25 for '67 because of the elimination of the small spark plug side quench zone on 462 heads, which increased head chamber volume by about 2 cc to 62-63 cc with the standard 1.94/1.5" valves, and I thought that chamber volume with the standard valve sizes carried over to later big port head castings through '70.
Duke
So my question is: What's the physical difference, if any, between these ...291 castings and the prior 3890462 castings used on '67 300 and 350 HP engines?
One clue might be that the '68 300 HP advertised CR dropped to 10:1. It was 10.5 thru '66, then dropped to 10.25 for '67 because of the elimination of the small spark plug side quench zone on 462 heads, which increased head chamber volume by about 2 cc to 62-63 cc with the standard 1.94/1.5" valves, and I thought that chamber volume with the standard valve sizes carried over to later big port head castings through '70.
Duke
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