USA produced O.E. four barrel mounting flanges

“Square” | “AFB” | “Holley” | “Spread-bore”

General Information

This article contains information concerning flange sizes and carburetors using them as ORIGINAL EQUIPMENT on US-produced vehicles from 1952 through 1974. There are four different patterns. While never named as such by any manufacturer, enthusiasts often refer to these patterns as: (1) “square”, (2) “AFB”, (3) “Holley”, and (4) “spread-bore”. Some aftermarket manifolds will use one of these patterns, but many will use two of the patterns. In fact, many Offenhauser aftermarket manifolds were shipped drilled for the “square” pattern, but with bolt-on adapters that would adapt to the “AFB” or “Holley” patterns. A number of aftermarket carburetors (possibly some O.E. carburetors) were produced with dual patterns (AFB, Holley). A number of adapters exist to allow the use of a specific carburetor on a different manifold than for which the carburetor was designed. GENERALLY, adapters from any square-bore design to any other square-bore design may be used with a minimum of airflow disruption and CFM loss (assuming the CFM of the carburetor is not larger than the CFM of the manifold). Adapters to allow use of a square-bore carb on an O.E. spread-bore manifold or vice versa should NOT be used. Why? Do the math. Assume a 750 CFM carb. The square-bore carb (or manifold) is rated 375 CFM both primary and secondary; while the spread-bore carb (or manifold) is rated 150 CFM (200 CFM some models) primary, and 600 CFM secondary. If trying to use a spread-bore carb on a square-bore manifold, we have 150 (limit of carb) primary plus 375 limit of manifold secondary; so our 750 CFM carb is now effectively a 525 CFM carb! The reverse (trying to use a square-bore carb on a spread-bore manifold) is also true. Note that this math was prefaced by the term O.E. manifold. Some aftermarket manifolds are supposedly designed to use either style of carburetor.

“Square”

The “square” pattern is not really square; rather the center-to-center mounting bolt pattern is 3-¾ inch by 3 7/8 inch. This pattern first appeared in 1952, and is of the “square-bore” design. That is, both the primary and secondary throttle bores are either the same size, or very close to the same size. The pattern was discontinued for automobile use over a period from 1955 through 1958, but was continued for truck use on some models as late as the mid-1970’s. While generally used for smaller 4-barrel carburetors and engines, I.H.C. used this pattern on some 549 CID engines in the 1970’s. Examples of carburetors utilizing this pattern:

Carter – WCFB (early production)

Holley – 2140 series

Holley - 4000 series

Rochester – 4G series

Stromberg – 4A

“AFB”

The Carter AFB, the Autolite 4100 and the Holley 4150 were introduced for the 1957 model year. The Carter used the “AFB” mounting pattern, while both the Autolite and the Holley used the “Holley” pattern. The “AFB” pattern measures 4-¼ inch by 5 5/8 inch. This pattern first appeared in the 1957, and continued at least through the 1974 model year for some square-bore carburetors. One issue that MAY be confusing for many enthusiasts is the fact that ON CERTAIN DESIGNS, the front to rear bore centerlines were changed about 1960, with the centerlines being moved further apart by ¼ inch (1/8 inch in each direction from the true front-to-rear centerline). This change was necessary to facilitate the use of larger CFM carburetor designs. For this reason, the throttle plates of some carburetors, even though the mounting flange pattern is the same, will hit the edge of the manifold bores when the plate is fully opened. When installing a later carb on an early O.E. manifold, ALWAYS check for this possibility. Generally, this condition may be corrected through the use of a thicker mounting gasket. Aftermarket manifold manufacturers, aware of the issue, generally produced manifolds with slightly larger diameter throttle bores. Examples of carburetors using the “AFB” pattern:

Carter – WCFB (later production)

Carter – AFB (most)

Carter – AVS (some)

Rochester – 4G series

“HOLLEY”

As mentioned above, the Autolite 4100 series and the Holley 4150 series carbs were introduced in 1957, but used a slightly larger mounting pattern than the “AFB” pattern mentioned above. The “Holley” pattern is 5 1/8 inch by 5 5/8 inch. Comparing this pattern to the AFB pattern, one will note the front-to-rear dimension is the same 5 5/8 inch, but the side-to-side dimension 5 1/8 inch is wider. This pattern was used from 1957 at least through the 1974 model year for some square bore carburetors. Examples of carburetors utilizing this pattern:

Autolite – 4100

Carter – AFB (some)

Carter – AVS (most)

Holley – 3150 (REQUIRES “notched” manifold to allow secondary operation)

Holley – 4150 series

Holley – 4160 series

“Spread-bore”

The spread-bore design was introduced by Chevrolet in 1965, and was continued through at least the 1974 model year to accommodate spread-bore carburetors. The bolt-mounting pattern is exactly the same 4 ¼ inch by 5 5/8 inch but the actual throttle bore sizes and spacing have changed. As mentioned above, the primary and secondary throttle bore sizes on the square bore design are either equal or close to equal, but the spread-bore design has small (1 3/8 or 1 ½) inch primary throttle bores, and 2 ¼ inch secondary throttle bores. Examples of original equipment carburetors that utilize this pattern:

Carter – TQ (thermoquad)

Rochester – 4M series (normally called Q-jets or quadrajets)