You mean Plumstead -- Norton had been part of AMC since 1954! However, the factories didn't produce castings themselves - they were cast by BMC, and from 1963 on, by BIRCO. How did casting quality deteriorate? The alloys used for cylinder heads at the time were standardized, and the process was gravity fed sand casting. As casting was performed by dedicated foundries, I much doubt quality suffered, rather the opposite, due to much larger production figures throughout the 60's. You would have to examine hundreds of heads and perform metallurgical testing of many heads to reach a conclusion like that.
S/N 116372 Feb 1966 6-Start Oiling System, This includes the pump, positive pressure lubrication of the head, plain spindles. (enlarged oilways in the head?), redesigned oilways in the timing cover, oilways in the engine case (enlarged), modified oil junction block, and hoses to/from the oil tank. I believe the rationale was improved lubrication and cooling due to experiences made when tuning the engines for racing.
It's sometimes claimed that the 6-start pump capacity was wasted, since only 10-15% is spent on the crankshaft and ca. 20% goes to the top end. However, 65-70% is left for heat absorption and transport - an important and sometimes underrated task in a combustion engine.
- Knut
