MSD Pro-Billet Flat-Top
Chevrolet V8 Distributor
PN 84891
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Important:
Read these instructions before attempting the installation.
Parts Included:
1 - Pro-Billet Distributor
1 - Rotor, PN 84673
1 - Distributor Cap, PN 8531
1-
1-
1-
2-
Advance Kit
Gasket
Tube of Gear Lubricant
O-Rings
WARNING:
Before installing the MSD Distributor, disconnect the battery cables. When discon-
necting the battery cables, always remove the Negative (-) cable first and install it last.
Note:
The terminals of this Distributor require spark plug style terminals. You may need to change the
terminals and boots of your wires. MSD offers two kits, PN 8849 or PN 8848 that are supplied with
nine boots and terminals.
Note:
If the gear is ever replaced, MSD Gear (PN 8531) is required for replacement due to the .500"
diameter shaft.
Note:
For a replacement rotor, contact MSD Customer Support at 915-855-7123.
TIMING FUNCTIONS
Before continuing with the installation, here are a few definitions you should be aware of:
Initial Timing:
This is the base timing (also referred to as idle timing) of the engine before the
centrifugal advance begins.
Centrifugal Advance:
The centrifugal (or mechanical) advance mechanism is made up of weights,
springs, advance cams, and an advance stop bushing. The amount and rate of advance that your
distributor is capable of is determined by the centrifugal timing. If you ever wish to lock out the
centrifugal advance, refer to the centrifugal advance section.
Total Timing:
This is the total of the initial timing plus the centrifugal advance added together.
Example: 10° Initial + 25° centrifugal = 35° Total Timing. (When checking Total timing, disconnect
the vacuum canister and plug the vacuum source.)
Note:
MSD Distributors are supplied with the heavy (slow) advance springs installed. This is to prevent
detonation in certain applications. Review the information on pages 2-4 to determine the best
advance curve for your application.
MSD
•
WWW.MSDPERFORMANCE.COM
• (915) 857-5200 • FAX (915) 857-3344
2
INSTALLATION INSTRUCTIONS
CHOOSING AN ADVANCE CURVE
The function of the advance curve is to match the ignition timing to the burning rate of the fuel and
speed (rpm) of the engine. Any factor that changes the burning rate of the fuel or the engine speed
can cause a need for an ignition timing change. Figure 1 shows some of the factors that will affect
engine timing.
FACTOR
Cylinder Pressure
Vacuum
Energy of Ignition
Fuel Octane
Mixture (Air/Fuel)
Temperature
Combustion Chamber Shape
Spark Plug Location
Combustion Turbulence
Load
Advance Timing
For
Low
High
Low
High
Rich
Cool
Open
Offset
Low
Light
Figure 1 Ignition Timing Factors.
Retard Timing
For
High
Low
High
Low
Lean
Hot
Compact
Center
High
Heavy
As you can see from the chart, most factors will change throughout the range of the engine operation.
The timing mechanism of the distributor must make timing changes based on these factors.
Example:
An engine has 11:1 compression with a high energy ignition. With the specifications given,
you will have to retard the timing for the high compression and high energy ignition. By comparing
the engine’s specifications against the chart, a usable timing guideline can be found. Engines with
a combination of items from both columns will require a timing that is set in the mid range.
Obviously a full technical explanation of correct ignition timing would be very complicated. The best
way to arrive at a suitable ignition curve for your engine is to use the Ignition Timing Factors Chart
as a guide and compare it to the Advance Graphs in Figure 4 until a suitable curve is found. When
selecting your advance curve, use detonation (engine ping) as an indicator of too much advance,
and a decrease in power as an indicator of too little advance.
TIPS ON SELECTING AN ADVANCE CURVE
• Use as much initial advance as possible without encountering excessive starter load.
• Start the centrifugal advance just above the idle rpm.
• The starting point of the centrifugal advance curve is controlled by the installed length and
tension of the spring.
• How quickly the centrifugal advance (slope) comes in is controlled by the spring stiffness. The
stiffer the spring, the slower the advance curve.
• The amount of advance is controlled by the advance bushing. The bigger the bushing, the
smaller the amount of advance.
CENTRIFUGAL ADVANCE CURVE
SELECTING THE ADVANCE SPRINGS
The rate, or how quick the advance comes in is determined by the type of springs which are installed
on the distributor. The MSD distributors are equipped with two Heavy Silver springs installed. These
will give you the slowest advance curve possible (Figure 2). The parts kit contains two additional sets
of springs which can be used to match the advance curve to your particular application. Refer to the
Spring Combination Chart (Figure 3) for combinations that can be achieved.
MSD
•
WWW.MSDPERFORMANCE.COM
• (915) 857-5200 • FAX (915) 857-3344
INSTALLATION INSTRUCTIONS
3
To change the springs, remove the cap and rotor and use needlenose pliers to remove the springs.
Be sure the new springs seat in the groove on the pin.
Timing Curve From Factory
SPRING COMBINATION
2- Heavy Silver
1- Heavy Silver
1- Light Blue
1-Heavy Silver
1-Light Silver
2- Light Blue
1- Light Silver
1- Light Blue
2- Light Silver
RATE OF ADVANCE
SLOWEST
FIGURE 4
A
B
C
D
E
FASTEST
F
Figure 2 The Factory Equipped Curve.
ADVANCED
STOP BUSHING
RED
SILVER
GREEN
BLUE
PURPLE
BLACK
Figure 3 Spring Combination Chart.
ADVANCED
STOP BUSHING
RED
SILVER
GREEN
BLUE
PURPLE
BLACK
ADVANCED
STOP BUSHING
RED
SILVER
GREEN
BLUE
PURPLE
BLACK
ADVANCED
STOP BUSHING
RED
SILVER
GREEN
BLUE
PURPLE
BLACK
ADVANCED
STOP BUSHING
RED
SILVER
GREEN
BLUE
PURPLE
BLACK
ADVANCED
STOP BUSHING
RED
SILVER
GREEN
BLUE
PURPLE
BLACK
Figure 4 Advance Curves.
MSD
•
WWW.MSDPERFORMANCE.COM
• (915) 857-5200 • FAX (915) 857-3344
4
INSTALLATION INSTRUCTIONS
SELECTING THE ADVANCE STOP
BUSHING
Five different advance stop bushings are supplied
in the distributor kit. The distributor comes with a
Blue (21°) bushing already installed. If a different
amount of centrifugal advance is desired, follow
the next procedure to change the bushings. The
chart in Figure 5 gives the size and approximate
degrees for the corresponding bushings.
BUSHING SIZE
Red-Smallest
Silver
Green
Blue
Purple
Black-Largest
APPROXIMATE
CRANKSHAFT
DEGREES
28
25
23
21
19
18
Figure 5 Advance Stop Bushing Chart.
CHANGING THE ADVANCE STOP
BUSHINGS
1. Remove the distributor cap and rotor.
2. Remove the locknut and washer on the bottom
of the advance assembly (Figure 6).
3. Remove the bushing and install the new one.
Install the washer and locknut. Do not
overtighten.
LOCKING OUT THE CENTRIFUGAL
ADVANCE
1. Remove the advance components including the
springs, weights and the advance stop bushing
from the advance assembly.
2. Remove the roll-pin from the drive gear and
remove the gear from the shaft.
3. Slide the shaft two inches out of the housing.
4. Rotate the shaft 180° and insert the advance stop
bushing pin into the small hole on the advance
plate (Figure 7).
5. Install the locknut and washer to the advance
stop bushing pin. This locks the advance in
place. Do not overtighten.
6. Install the drive gear and roll-pin.
Figure 6 Changing the Advance Stop Bushing.
Figure 6 Locking Out the Centrifugal Advance.
Figure 7 Locking Out the Advance.
MSD
•
WWW.MSDPERFORMANCE.COM
• (915) 857-5200 • FAX (915) 857-3344
INSTALLATION INSTRUCTIONS
5
INSTALLING THE DISTRIBUTOR
1. Remove the existing distributor cap without
disconnecting any of the spark plug
wires.
2. With the cap off, crank the engine until the
rotor is aimed at a fixed point on the engine
or firewall. Note this position by making a
mark (Figure 8).
3. Place the distributor cap back on and
note which plug wire the rotor is pointing
to. MARK THE SPARK PLUG WIRES and
remove the distributor cap.
4. D i s c o n n e c t t h e w i r i n g f r o m t h e
distributor.
5. Loosen the distributor hold down clamp
and slide the clamp out of the way.
6. Lift the distributor out of the engine.
Note that the rotor rotates as you lift
Figure 8 Marking the Rotor Location.
the distributor out. This is due to the
NOTE: BLOCK MUST BE MODIFIED TO USE O-RINGS.
helical cut gear and should be taken into
consideration when installing the new
distributor.
7. Install the gasket and apply a liberal
amount of the supplied lubricant to the
distributor gear. (The supplied O-rings
can
only
be used if the block has been
modified as shown in Figure 9.)
8. Install the distributor making sure that the
rotor comes to rest pointing at the fixed
mark. If the distributor will not fully seat with
the rotor pointing to the marked position,
you may need to rotate the oil pump shaft
until the rotor lines up and the distributor
fully seats.
Figure 9 Modified Block for use with O-Rings.
9. Position and tighten the hold down clamp
onto the distributor.
10. Install the distributor cap and spark plug wires one at a time to ensure correct location.
MSD
•
WWW.MSDPERFORMANCE.COM
• (915) 857-5200 • FAX (915) 857-3344
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