On ebay there is a set of Oldsmobile D heads up for sale.
One head has a large heat riser opening the other a small heat riser opening.
Why would there be a difference in heat riser openings on D heads?
I understand that they are most likely not a matched set of D heads and from different production runs.
Could the one D head be a early 68 production run while the other is a 69 head?
What was the last year of the small heat riser opening ?

http://www.ebay.com/itm/Oldsmobile-4...hYd8hS&vxp=mtr

 

Good question. My D heads have the small heat riser with a 110 date code. Not original set to my car, so I'm not sure the original application. I've wondered the same but never looked into it.

 

I have four sets of C heads the ones that came off a 1967 425, had small heat riser openings. While all the others that cam off 69 cars have had the larger heat riser.
I personnel like the small heat riser heads as it is a smaller area to close off.