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The most important thing to know about a set of heads is condition and authenticity. Basically heads come in two flavors rectangular port high performance and oval port standard performance. Some later date replacement heads are found occasionally on earlier dated blocks. Which brings up the subject of authenticity. While the replacement heads are genuine they are not original to the vehicle. Sometimes, but not often, casting numbers have been changed on heads, intake manifolds and blocks.
How could this be possible one might ask. The answer is a technology known as spray welding.

As an example, around May of 1967 GM began using a casting number of 3919840 these heads were continued through June of 1969. There was no difference in heads ending in the number 840 other than the casting date. If someone had a set of heads dated January 1968 and needed a set dated January 1969, all that is required is to remove the eight and replace it with a nine using the spray welding technique.

  In GM heads this phenomena is most often found on the hardest to find heads. Cast iron big block Chevy heads numbered 3856208, 3873858, 3904391, 3919840, 3964291, and 3994026.
Aluminum Chevrolet big block heads numbered 3904392, 3919842, and 3946074.
Small block Chevy heads numbered 3782461, 3890462, 3917291, 3927186, 3927187, and 3947041.

Although we havenít seen it yet, as they become harder to find we predict the following oval port heads will start turning up with changed numbers. Chevrolet big block cast iron numbered 3856206, 3872702, 3904390, 3909802, 3917215, 3931063, 3964290, 3993820, and 3999241.

What devalues a head? If the head has been ported or polished, or extensively milled on the head surface. If the stock valve size has been opened up or enlarged the head has been devalued. If replacement seats are installed the head is devalued. Having bigger return springs installed devalues the head if any metal removed to facilitate. In short any cutting or altering of the head as stated above will to some degree devaluate the head.

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Learning About Cylinder Heads