Not sure that a dimension between valve stems center lines has ever been published. I have a newer head 1975? with 1.55 exhaust valve and measured it for a project I'm starting. I checked 3 pairs of intake and exhaust valves. There was quite a variation here. 1 set was 1.7705, the next 1.765, and then 1.7525. A high to low difference of .018. This is surprising to me. I would think they could have been more accurate even back then. Then I looked at a bare 1969 head and you can see the valve stem hole is not concentric with the guide OD. favoring one side as much as .020". Like the processes were done at different stations that were not indexed very well. Has anyone else noticed this?
Honestly that’s another issue with the iron heads. When I start a project to port these heads I start with at least 6 pair of heads and the investigation takes time and money. First we clean and magniflux to check for cracks. Then it’s a crapshoot as far as core shift and other issues with the heads. Areas on one set of heads where there is lots of room to port the heads, and then other heads you hit water passages. Same casting numbers and different thicknesses. A few fun facts: 1. the 68-70 heads are the most likely to be cracked. 2. The 73-80 heads never crack. 3. The 74-80 heads flow terribly. 4. I only use 71-72 heads, I find them to be the best for both flow and less cracking issues.
