Here is a head story. Years ago I took a set of 383 commando heads to a well-known, reputable speed shop. This guy had a 8.90 car in the late 80's. I had him install new hardened seats and guides. I still did my home inspection and found one head to be perfect and the other head would pour light or alcohol through the exhaust valves (all 4) like it was a funnel. Every one of those stink'n exhaust seat were not aligned to the new guides. I spent hours and hours at home lapping those to seal. Do not think just because you pay money $$$ at a machine shop that you don't need to still inspect them at home...… and you may still find JUNK. I sure have.