Talking primarily about stock Mopar carbs.
I have been seeing a lot of different definitions of the terms
"Spread-Bore"
and
"Square-Bore".
I was under the impression that it is the size of the primary bore compared to the secondary bore holes in the base of the Carb.
I have recently read, only the Thermo-quad was spread bore because the rear butterfly shaft is longer on them. (?)
That kinda still goes along with my first impression.
It just eliminates some other carbs with a differing size bore pattern.
I used to think the AFB was square and the AVS was spread bore.
Looking at my stash of carb.s, I have both in the AVS variety.
Still guessing the older AFBs were square bore.
Which, which, is wich?
One fella even told me it was the bolt pattern.
That makes absolutely no sense to me.
Thanks in advance for clearing this up.

easiest might be to pull up some pics of squarebore and spreadbore intakes, the mounting flange patterns. EDIT Go to mancini & type in "intakes" then scroll down to a pic of an Eddy "victor" which is a squarebore & further down to an Eddy "performer" which is a spreadbore.

Spread bore = Thermo-Quads, Quadrajets, some Holleys
Square Bores = AFB, AVS, WCFB's etc.

Not all square bore carbs have equal size primary and secondary bores (some do), but the difference is small as compared to the more dramatic difference with a Spread bore design.

First Pic is a square bore intake, second pic is a spread bore intake.

AVS square bore carb & a Thermoquad spread bore

Robert,
I have what I am thinking is a '66 square bore, and a '68 spread bore stock Chrysler manifolds. Looking at them bears out my thought of the bore size being the difference. I believe a square bore could be used on the spread bore intake, but not visa-versa.
My 66 AFB came with, what I consider, a square bore pattern and the 68 AVS sits upon, what looks to me, to be the spread bore pattern.
This is where I get this idea from.
I have 3 AVS carbs and one of them has larger secondaries.
Does this mean the AVS could be either?
What about the AFB?
Maybe depending on year, usage, or make of car?
I am by no means any type of expert.
Basically, is it the shape or pattern of the bores or is it more complicated than that?

All of your AVS and AFB's are square bore designs.

Thanks Scott.
That helps a lot.
Still a little confusing though.
Am I reading this correct?
The actual bore size is irrelevant and the ones with a considerably larger secondary is the spread bore?
Is the Quadra-jet the only true spread bore?
All AVS and AFBs are considered square bore?
Does Holley have each or just spread bore?
Talking factory spec. only here.

So in essence, looking at an assembled set-up, it is the bolt pattern that differentiates the two?
OK this is what I was looking for.
Thanks a ton.

Also consider that not all square bore carburetors will fit all square bore intakes, the intake hole sizes need to be large enough to accomodate the chosen carb.

Thanx, great info.