Just picked up a 351 Cleveland and noticed that the heads have "GAS" stamped on them. Whats the differences between these and standard heads, and also the pros/cons of these heads. Apparently the engine was running on petrol not gas before i got it.

gas heads have hardened valves and seats on at least the exhaust valves and the heads have bronze guides and a poofteenth more clearance in the guides. This is done because there is no lead to line protect and lubricate the valve and stem. It prevents sticking valves when there is prolonged use of gas. The real benfit is that you can safely use unleaded fuel with these heads without concern for the heads as they are virtually an unleaded head. There is no need for aftermarket products to protect the valves. if you run gas all the time then have you advance restriced in the dizzy so it cant advance to more than 30-32 degrees instead of 40 or 40+ as most do and then advance it at idle from 10 degrees to around eighteen degrees. I ran this setup on my clevo with 11.3:1 compression and it hammered, absolutely loved it. Only draw back is when you run petrol to get to a servo you cant use more than half throttle on petrol or it pings but on gas you cant beat it and nothing ever did.

Cheers Paul , great info! Everything I need to know .