I've got an offenhauser 360 manifold with 4 threaded studs to hold the carb down. I can hand turn an M8 nut onto them for a couple of turns before it binds up. I assume as it is a US manifold, it's an imperial thread, but which one?

5/16 unf

adrian

5/16 UNC

Oops stand corrected just read it properly it will be UNF where the nuts go and UNC where the studs go into the ally

[Edited on 28-6-09 by v8kid]

hi
almost certainly 5/16 unf if american.
graham

hi
almost certainly 5/16 unf if american.
graham

Thanks all

Sounds like UNC if an M8 will start on the stud - UNF is very fine and wont .

It's 5/16", that we know! Just count how many TPI and we can tell you exactly! I would say UNC.

For the sake of a couple of quid I've bought both UNC and UNF from eBay. I'll post up which ones actually fit

The thread in the ally body will almost certainly be UNC.
You usually have a coarse thread in the ally body to give max strength.

UNC

And the winner is......



UNF

UNC will be closest to standed M8.

M8 std pitch is 1.25mm

5/16 UNF pitch is 1/24" or 1.058mm
5/16 UNC pitch is 1/18" or 1.411mm

Therefore UNC is closer, but only slightly.

IMO it could be either...quite possibly the studs are coarse where they screw into the ali and fine for the nuts...that can happen.