Am in the process of stripping engine from 70 240Z - approx 150,000 original miles - engine was rebuilt at 80,000. I took head off last night. No visible problems with head. Anyway propose to have it reconditioned now while everything is apart. The timing chain cam sprocket was at number 2 position. While the chain and sprocket look fine I am wondering if I should change it now while I am rebuilding everything else. So far the mechanical condition of drive train is excellent as compared to the rusted-out condition of suspension, frame and floor.