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Small Block Chevrolet Distributors and Ignition Systems
This is an area of automotive development that has seen an avalanche of new products and
applications in recent years and will see continued expansion of your available choices. The number
of ignition devices that increase engine power and flexibility is growing. The computer and electronics
age has caught up with ignition systems and made many things possible. It is impossible for me to
cover everything that is currently being offered by aftermarket companies. Keep your eyes open for
new developments and materials that are finding their way into the ignition product marketplace with
every passing year both from GM and aftermarket suppliers.
Distributor with mechanical tach drive Dual point distributor
Mechanical, cable-driven tachometers were
driven off a gear mounted in some distributors.
These “tach drive” distributors were found in
Corvettes through 1974.
Dual point distributors were offered by
Chevrolet and are still available from
aftermarket manufacturers. This one is from
Chevrolet distributor casting number and casting date Springs and weights for mechanical distributor advance
The springs and weights for the mechanical
ignition advance are seen in this non-computer
HEI. They help control the amount of total ignition
advance and the advance curve.
Chevy distributors have casting numbers and
casting dates, which can be useful while doing
restorations or if you are trying to find out the
original application the distributor was used in.
Smallblock Chevy V-8 distributors are interchangeable with distributors found on Chevy bigblock,
short deck engines and that opens a lot of interchange possibilities because so many different types
of ignitions and distributors were offered for the small block and bigblock Chevy V-8. Single-point,
dual-point, High Energy Ignition (HEI), magnetic pulse, transistor, capacitive discharge (CD),
OptiSpark, and multiple spark discharge ignitions with rev limiters were made for Chevrolet for V-8
engines. Gen. III LS1 motors use a distributorless ignition that has eight individual ignition coils and is
computer controlled. Corvette distributors through 1974, had a mechanical cable-driven tach drive
incorporated into the distributor shaft housing. Distributor housings were made of cast-iron, but most
were made with an aluminum housing.

If you are doing a restoration, the housings are date coded and stamped with casting numbers that
coincide with their respective engine/vehicle applications. The numbers were either stamped into an
aluminum ring that was wrapped around the upper part of the distributor shaft housing, had a
stamped plate riveted to the housing or was stamped into the housing itself. The date codes are in a
slightly different order than other casting date codes. You might see a distributor date code that looks
like this “9B13,” which translates into the ninth year of the decade (9), February (B), 13th day (13). If
you are building a stock engine that will be used for everyday driving, a stock production single point
or HEI electronic, non-computer distributor is more than adequate and will give trouble-free service for
many miles. The HEI distributors will work well up to 5,500rpm and the HEI has no points to wear out or
replace. If you are restoring a “numbers matching” classic, that originally had a point style distributor,
then use the correct point distributor.
Single point distributor
Single point distributors were
used extensively and early
models had cast-iron bodies
while most later styles used
an aluminum body.
Distributor Advance
Stock, non-computer controlled distributors regulate ignition advance two ways. One is by the use of
springs and weights that are located under the rotor. These weights are spun out centrifically as
engine rpm go up. The spring tension regulates how fast the weights spin out. Another part of the
advance system is a vacuum diaphragm canister that is hooked up to the point plate. The canister on
the distributor is connected by a vacuum hose that is hooked up to a carb port. As engine vacuum
goes up or down or as the engine comes under a load, the amount of vacuum acting on the
diaphragm changes ignition advance appropriately. The amount of distributor advance given by the
vacuum canister is marked on the canister itself. The last two digits of the number is the amount of
vacuum advance in crank degrees. These two advance systems, vacuum and mechanical, are
separate and are in addition to whatever amount of initial advance is put on the front balancer at the
timing pointer when you set initial ignition timing.

Be aware that factory stock non-computer distributors came with different mechanical advances and
vacuum advances depending on what vehicle application the distributor was going into. This created
different ignition advance curves for different motors in different vehicles. If you have a used
distributor, which came out of a two-barrel, smallblock V-8, station wagon application, it may not have
the correct advance curve for a high-performance smallblock in a lighter car. The distributor will drop
right in and the car will probably run with it, but the incorrect ignition curve may prevent the motor from
achieving its best power. Later changes were also made in advance curves to help control emissions.
Computer controlled engines have an advance curve that resides in, and is controlled by, the black
box itself.

The point of this is that whatever application you intend for your motor, the ignition advance must be
appropriate for that application. In addition, the vacuum canister diaphragms do sometimes fail. This
will make the engine feel like it lacks power when you step on the throttle. The canister can be tested
by applying suction to the canister nipple and seeing if the arm on the other side of the diaphragm
moves the advance plate. The mechanical advance weights and springs sometimes fail also. The
weights can rust or be covered with grime so that they can’t move as they should. The springs can
also break or the little plastic bushings that the weights pivot on can wear out and get very sloppy.
Side play in the distributor shaft itself can develop when the shaft bearings wear out. If you are being
driven crazy by a problem causing a lack of engine power or a low speed hesitation, don’t overlook
the possibility that the ignition advance systems may not be working correctly.
Some performance distributors have no provision for a vacuum advance. In this case, usually you can
adjust the mechanical advance to provide the type of ignition advance curve and total ignition
advance you need. On some race engines, the desired total ignition advance is determined and then
locked into the distributor. Since the motor is used for short duration, full throttle running only, no
advance curve is used. The motor is started with the ignition timing totally advanced or some other
device is used to retard total timing when the motor is first started. Once the motor fires up the ignition
is switched to full advance timing. Now, this is something that is done with a race motor that always
operates over 3,000rpm. If you are driving on the street or the engine will be operated at part throttle,
then you need some type of ignition advance curve.

Stock HEI
High Energy Ignitions (HEI) were introduced on some vehicles in the 1974 model year. It is very
popular to install a stock, non-computer HEI distributor with vacuum advance into an early vehicle that
originally had a stock point distributor.
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This has been a sample page from

Chevrolet Small Block Parts Interchange Manual Chevrolet Small Block
Parts Interchange Manual
by Ed Staffel
Beginning with the earliest small block and carrying through the
very latest "Gen III" models, CHEVROLET SMALL BLOCK PARTS
INTERCHANGE MANUAL provides complete factory parts
interchange information, allowing the hot rodder to custom-build
his own high performance version of the famous Chevy "Mouse"
motor from off-the-shelf parts. Includes factory numbers, casting
marks, production histories, suppliers, component performance
capabilities, etc.

Temporarily Out of Stock - More On their way!

This is a great book and one that any
enthusiast will love!
View Sample Pages
Engine Blocks
2) Crankshafts
3) Oil & Lubrication System
4) Timing Chains & Covers
5) Cylinder Heads
6) Intake Manifolds
7) Ignition Systems
8) Gaskets
9) Exhaust Manifolds
Condition: NEW
8.5 x 11-inches
144 Pages
300 B/W Photos
Item: SA55P
Price: $22.95
Click here to buy now!

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