Can I run my cast iron heads (originally designed for leaded fuel) on unleaded gas?
Maybe. Cast iron exhaust seat surfaces tend to work-harden with use. Heads that were broken-in and run for many thousands of miles on leaded fuel may very well survive a long time on a diet of unleaded fuel, IF valve temperatures are kept normal, and the engine is not run too lean. Some manufacturers used more nickel in their cast iron, which makes it stronger/harder. But when a valve job is performed, new metal is exposed. Even with a high nickel content cast iron, this newly exposed metal lacks hardening and needs to be work-hardened again with a mixture of leaded race fuel or a good lead additive for several thousand miles. The best solution is to install hardened valve seat inserts. This is only needed on the exhaust seats.
