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While the casting number is visible from outside, you need to pull the valve cover and remove the rockers to see the actual manufacturer as there are multiple companies that made the heads, Castech being one of them. They each have different logos in the castings.Looking at a '03 Tahoe. My question is will the VIN tell me if the motor has the Castech 706 heads? Is the 706 tab on the head visible with the valve cover gasket on?
Thanks!