This plug fits the intake manifold, exhaust crossover on the bottom of a 1959-66 Buick 401/425 nailhead engine with a 2 or 4 barrel stock intake manifold.
For an easy installation, insert the plug into the bottom of the manifold and hit the center with a hammer, this will flatten and expand the plug for a secure fit.
If you have a Buick 364 engine and require the 3" plug please click this link here.