Topic: Threaded Plugs In Spark Vacuum Ports Not Used In Certain High Performance Carb's

[Dan Case]

Owners, fans, restorers, and authors get the two different designs mixed up frequently.  It does not help that the internal threads in very many carburetors have been damaged in all kinds of ways and currently nothing threads in easily. The pipe threaded part and straight threaded parts are not interchangeable.  It is important to not use a pipe tap to try and chase the threads in a 1964.5-1967 HP289 Ford 4100A carburetor. It is also important not to run a 3/8-24 tap into the ports of C3OF-AB and C3OF-AJ carburetors UNLESS the C3OF-AJ carburetor was produced 4AE or 4ED (service replacement). The pipe threaded part, brass early then plated steel, shows up in selected Holley carburetors and select 1963.5-1964.5 Ford 4100-A models for HP289 engines. The straight threaded part shows up in 4AE and 4ED dated C3OF-AJ carburetors and 1964.5-1967 Ford 4100-A carburetors for HP289 engines.




Note: Starting with the Ford 4100A C4OF-AL assembly, carburetors were manufactured specifically for High Performance 289 (HP289) applications. The main body castings for manual choke models was a different die casting than any of the automatic transmission vehicle carburetor main bodies.  Manual choke carburetor bodies were not modified automatic choke versions, they were just designed and made differently from the start.

[jwc66k]

Just for reference, where is (are) the plug(s) located?
Jim

[Dan Case]

[jwc66k]

Never paid any attention to the plug in the vacuum port as I have a centrifugal advance distributor. So far (with just a cursory look) I have not found a drawing showing it or a part number. It's a task for tomorrow.
Jim

[Dan Case]

Trouble often started when somebody installed a carburetor assembly created for an engine with full mechanical spark advance onto an engine using a vacuum advance distributor. Many people would have had trouble with identifying the thread size and system used. With enough effort you can force just about anything to twist into soft metals; die cast zinc or aluminum in these situations. Subsequently somebody else might have wanted to put the now damaged carburetor on an engine with full mechanical spark timing advance and the plug for that vacuum source in the carburetor was now missing. Missing plugs and often damaged threads in the port create more opportunities for confusion as nothing standard will easily install in a damaged port, even the correct plug.  I have seen many types of items forced into ports to "seal" them off, even long hex cap screws. I once removed a large sheet metal screw and some cured RTV from a carburetor's spark vacuum port.

The other situations I run into often include: 1) Somebody rebuilt / restored a carburetor and just left the part out and 2) somebody robbed a good plug from a used core before selling the core to another person. (Really nice used original plugs are rare. I have purchased cores just to get an excellent condition plug multiple times.)


The easiest units to make a mistake with are the many Ford 4100A family carburetor assemblies for "High Performance 289" engines 1963½-1967; and service only replacements.  The Ford 4100-A family C3OF-AB unit used from start of production the first week of March 1963 to about mid May 1963 and the C3OF-AJ versions for about mid May 1963 unit June 1963 and the revised version used for the 1964 model year at least until 4AB had vacuum ports with 1/8 MNPT internal pipe threads.  The brass plug used was the same part as used in 1963 427 c.i.d. engine powered Galaxie 8V Holley brand carburetor systems. The plug was both a Holley and a Ford part.  The fittings on hard lines for vacuum advance for most engines were straight thread.  At this time the earliest HP289 carburetor I have located with the 3/8-24 plug is a C3OF-AJ model dated 4AE. There was also a service replacement C3OF-AJ model made 4ED that also used the new plug design. When the C4OF-AL carburetor was introduced during late April 1964 they were made with straight thread vacuum ports. Over time owners have tried moving pipe thread "HP289 plugs" to carburetors with straight threads of visa versa. I have come across many incorrect references that one plug design was used in all Ford 4100-A HP289 distributors, which were in error.

[jwc66k]

I just finished going thru the Ford Car Parts (aka MPC), 60-68 edition, Section 95.12F for 4 barrel carburetors (95.12F pertains to the HP version) line by line and there is no plug listed for HP use. The plug is in the carburetor. I've seen it. I assume that the plug is part of the 1.12 carburetor body modification process for use without vacuum advance.
Just in case the plug is included with a replacement distributor (C5OF-12127-E), I looked in the distributor section too. Nothing.
Bottom line is - if you take it out, don't lose it. If you lose it, follow Dan's instructions for a replacement.
Jim

[Dan Case]

You can see illustrations of the domed 1/8" pipe plugs in primary metering blocks in any of the 427 Ford Holley 2-4V Technical Service Bulletins. I have never found a Ford hardware number for the brass pipe plug, steel pipe plug, or the 1964?-1967 straight thread plug. The 1969 service parts table has "PLUG (SPARK HOLE) - 1/8" HEX. SOCKET HEAD 87708-S(P14) listed as a service part. I have seen the hex head pipe plug in some R-3255/R-3255-1 service carburetors.  Ford-Holley units R-2652, R-2652-1, R-2668, R-2804, R-2805, R-2919, R-3259, R-3259-1, R-3300, R-3301, R-3410, and R-3411 Ford Holley all used domed (rounded head) pipe plugs and probably more. Early plugs were brass. Some time during 1964 plated steel followed the brass part.

Holley parts wise:
Holley brass plug = ?
Holley plated steel plug = 7R-5

The manual choke bodies are different castings than ones for automatic transmissions. As unique assemblies from the casting onward they didn't need to be modified, they were just manufactured for HP289 applications with manual chokes.


All the 1963-64 Ford 4100As I looked up only had the 358165-S(II-65) 1/8" pipe connector listed (1/8" external thread pipe on one end for the carburetor, a hard vacuum line fitting connected to the other end.)

Lots of details for C3AF-S (HP390 and HP260), C3OF-AB, most C3OF-AJs, and all 1964-67 versions of Ford 4100A HP289 carburetors are hard to extremely hard to find. 

[CharlesTurner]

Dan, I was able to get a small number of the 4100 hipo ones.  If you need any let me know.

[Dan Case]

Charles, Thanks. I obtained a straight thread plug required. I am still wanting an excellent brass pipe plug for a HP260 carburetor.