I have 2 1957 Corvette 4BBL intake manifolds…P/N 3731398…both are identical except for the mount hole diameter where they mate to the heads….one marked No.1 next to the casting date (a Jan 57 cast date) has 3/8 dia. holes…the other marked No.2 next to casting no. (June 57 casting date) has 1/2 inch dia. holes….don't see any other obvious differences…what change was made over 6 months to require bigger intake holes and why? Anyone? thx…Billu38

By the way, they are both cast iron manifolds, not aluminum…Billu38

There was an early and late 4 barrel intake in 1957. The early had casting #3731398 and the late 3746829. They fits cars and Corvettes.

Gotcha Gene…I understand...but these have the same casting number but different hole size….Bill

i have seen this before. probably the one with the 1/2 holes was used on a car with milled heads. when the heads are milled the intake manifold is supposed to be milled also. i had a 324 with .040 milled off the heads and lo and behold the intake would not line up. the solution was to open up the holes in the manifold. this allows the bolts to sort of line up with the heads, altho it is not perfect it works.

That makes some sense scottmacintosh…thx…Billu38