The non concentric section of the Holden / Buick V6 cylinder head runs into and contacts the 70 degree bottom cut. The non concentric cut has not been cut using a cutter which revolves around the valve guide pilot and centreline. It's been cut prior to the seats with a "centre-less cutter" as a roughing cut to remove large amounts of casting dags prior to final machining and seat cutting. This is why the junction of the bottom edge of the 70 degree cut and the top of the 78 and 80 degree non concentric cuts will vary in height and not form an equal width 70 degree facet all the way around the throat. The non concentric face on the inlet is reportedly 80 degrees, and on the exhaust 78 degrees.