Small Block Ignition System
Small-block Ford ignition systems are straightforward and easy to
221/260/289-2V and 4V engines all used the same basic single-point distributor from 1962-67,
differing only in spark curve, which depended upon vehicle application. Significant changes
came in 1968 with the advent of dual-diaphragm distributors that both advanced and retarded
the spark, depending upon engine load and driving conditions.
The single diaphragm distributor is designed to advance the spark as intake manifold vacuum
increases (under a load with an open throttle). The dual diaphragm distributor works both
ways—advancing the spark during throttle-open conditions, and retarding the spark when the
throttle is closed during deceleration when emissions are normally higher. Retarding the spark
reduces emissions when the throttle is closed. In addition to the vacuum advance, single-point
distributors were equipped with a centrifugal advance designed to give the engine adequate
spark advance at cruise power.
The 289 High Performance dual-point distributor was not equipped with a vacuum advance unit
of any kind. Spark advance for the Hi-Po distributor comes from the centrifugal mechanical
advance inside. Because a 289 High Performance engine does its best work at high revs, the
centrifugal advance is designed to advance the spark at a rate corresponding to rpm range.
Ideally, a Hi-Po engine will experience a total of 36 degrees of advance at 5000-6000rpm, with
12 degrees at idle. This is a target to aim for with any ignition system. Anytime you push total
spark advance beyond 36 degrees, you risk engine damage.
A weak spot in Ford
distributors is the shaft bushings. Bushing wear is immediate and swift
because lubrication and shaft support are poor. Nonetheless, rebuilding a vintage Ford
distributor is easy and requires very few replacement parts. Arm yourself with a set of bushings
(B8HQ-12120-A, top, and C5AZ-12132-A, bottom) and advance springs designed for your
application. Apply liberal doses of assembly lube on the bushings and shaft during
reassembly. Replace the vacuum advance only as necessary.
If you’re a die-hard for genuine Ford parts, upgrading to a stock Dura-Spark ignition is likely
the answer for you. Older 221/260/289/302 engines live happily with the Dura-Spark upgrade
because it is exactly the same size as the original Autolite 12100 distributor fitted prior to 1975.
Aside from the proper positioning of the Dura-Spark ignition module, installation is easy.
To follow is complete information gleaned from the Ford Master Parts Catalog on Ford small-
block distributor identification. We have cleaned up this information for your convenience.
There are literally dozens of original applications that were rooted in emission and
performance requirements and vehicle type.
Truth is, all Autolite and Motorcraft distributors were identical in construction. Where they
differed was in the area of spark advance/retard. Each identification and part number was
based on distributor calibration only.
Calibration isn’t as complicated as it sounds. Calibration was based on rate
advance/retard. You can take any Autolite or Motorcraft distributor and calibrate it to your
application, unless you’re looking for a specific identification number for a concours
restoration. You simply need to know which centrifugal advance springs and weights to use.
Plus you must understand how to adjust in rate of vacuum advance. This means getting the
spark advance in synch with engine rpm and throttle pressure/position.
Another point to remember is both Thermactor and IMCO emissions packages. Thermactor is
the smog pump system. IMCO (short for “Improved Combustion”) is a combination of spark
control and fuel mixture. Thermactor and IMCO are not the same thing.
One other item to keep in mind is the Motorcraft part number, which is shown in parenthesis
where applicable with each Ford part number in the chart.
The humble Ford
didn't change much
throughout its production
life. Aside from the use of a
in 1968 on most engines,
this single-point design
remained the same until
1975 when Dura-Spark
electronic ignition replaced
all point-triggered units.
The Autolite dual-point
used only on the 289 High
Performance engine isn't much
different than the single-point unit,
except for the absence of a
through 1964 had oil wicks like
this one. This is a 289 High Performance dual-point
This is a 1969-70 vintage
Autolite dual-point distributor for the Boss 302 engine. Note the
Previous | Next