Buick 231 manifold bolt/stud hole size

[blalp!]

The outer four of the six manifold bolt holes on the new to me manifold are 1/2” diameter, while the middle two are 3/8”. Any idea what hardware is needed? The manifold was off of a ‘81 Regal.

 

[Rich M.]

My experience everything in the head is 3/8-16. I don't know if they ever used studs that may have been larger on the manifold side.

 

[sterlclan]

they probably had a funky washer over the oversize hole while still using a 3/8 bolt

 

[garage gnome]

The bigger bolt holes are to allow for expansion/contraction of the manifold and to not put shear stress on the bolt. The middle ones can be a tighter fit because in theory, the middle won’t move.

 

[nickmil]

I have never seen from the factory larger than 3/8-16. At least all the 231’s and 252’s had a bolt through the manifold with a washer built in and a stud out the top.