TECH TIP
Harley-Davidson®
Big Twin
Primary Chain
Adjustment
By The Pharoah
If you are experiencing an increasingly excessive vibration in
your Harley, or a “clunking” sound that seems to come from an
area low in your engine usually at low RPMs, and/or your Harley
“bucks” (sometimes dangerously) at extremely low RPMs when
in gear, and your model is equipped with a steel primary chain –
you may need to adjust the primary chain to its proper tension,
or maybe even replace it. This Tech Tip article is about properly
adjusting it.
When science came up with non-metallic fibers that are
stronger than steel, such as Kevlar and carbon fiber, simple logic
convinced the motorcycle manufacturing industry that belts made
from these fibers were a perfect replacement for steel chains –
and Gates Rubber jumped on the idea. Ducati quickly replaced
their valve timing chains with belts and Harley went to primary and
secondary belt drive systems. Generally speaking, primary drive
system consists of 3 major components, 2 sprockets or pulleys
and a chain or belt, which transfers power from the engine to
the transmission. The secondary drive system consists of three
similar components but transfers power from the transmission to
the back wheel. However, many Harleys still run a steel chain as
a primary and/or secondary drive member.
Whether you run chains and/or belts on your Harley, they still
need to be inspected every 10 thousand miles and adjusted
if necessary. This Tech Tip will focus on properly adjusting the
chain tension on Big-Twin Harleys that employ a steel chain
primary drive system. However, there are a couple of variations
which this Tech Tip does not cover. One variation involves 1980
through 1984 models with belt primary drive systems and the
other involves Panhead primary chain drive systems.
1966 - UP BIG TWINS
Chain tension adjustment is a straightforward process. Simply
stated, loosen the bolt that holds the chain tension adjustment
mechanism in place so it can slide up or down. Slide the
adjustment mechanism upward to tighten the chain. When
satisfactory adjustment is achieved, tighten the mechanism’s bolt
and it’s done. Now, I’ll describe the process in detail.
The primary chain is enclosed inside the primary chain housing
located on the left side of the motorcycle. Midway between the
engine and transmission you will find an access cover plate held
to the outer primary cover by four screws. Removing this cover
14 Thunder Roads Magazine® Colorado
plate provides access
to the mechanism that
tightens or loosens
the primary chain. A
gasket between the
access plate and the
outer primary cover
keeps the chain
Chain
Housing
lubricant from leaking
out. The access cover plate comes in three styles: If your model
has a forward shifter and brake controls, the plate is flat and plain
with 4 bolt-holes around its edge. Full dressers sometimes have
a foot peg mount welded to the access plate. If you have a model
with mid-shifting and braking controls, a shaft on which the shift
arm and peg are mounted comes out through the center of the
access plate. On these models, you must first remove the shifter
arm from the shaft. The access plate will then slide off the shaft
and reveal the primary adjusting mechanism. All access cover
plates come with four bolt holes and use the gasket.
Important Note: It is always the best and safest practice to
disconnect the ground cable from the battery before you begin
this procedure or before you stick your fingers into the chain
housing to check the chain tension.
After the access plate is removed, shine
a light into the access hole and you will
see the adjustment mechanism and two
strands of the primary chain. The lower
strand runs through the chain adjusting
mechanism. Bolted into the mechanism
you will see that the chain travels over a
white or slightly yellow Teflon slipper shoe
shaped like a cross-section of an airplane
wing. Closely examine the area where
the chain makes contact with the shoe for
wear. Over time the chain will wear into
the shoe partially hiding the link plates. If
Adjustment
Mechanism
the shoe is excessively worn it will need to
be replaced before you finish.
Above the tension adjusting mechanism is another strand of
the primary chain. This section is where you determine whether
the chain is too loose or too tight. You should be able to move the
upper chain strand up and down with your fingers without applying
a lot of force. Harley’s service specs say the chain should move
up and down between 5/8” to 7/8” while cold and 3/8” to 5/8”
while hot. If the distance is outside these measurement limits
you must go on to the next step in order to bring that distance into
specification.
Just above the Teflon shoe you will find the head of a bolt on
which a 9/16” socket usually fits. Sometimes you will encounter
a stud with a Nylock nut onto which a 5/8” or 11/16” socket will
fit. Either way, use the appropriate deep socket on a ratcheting
wrench along with a long extension. Loosen the bolt (or nut) no
more than a couple of turns, or until the adjuster will freely slide
up and down. Do not loosen the fastener completely because the
threaded unit it screws into will suddenly drop out of position, you
can no longer see it, and you will pay Hell trying to fish it back into
position again.
Chain adjustment is accomplished by sliding the adjustment
mechanism up or down. Moving the adjustment mechanism
upwards tightens the chain. Lowering the mechanism loosens
March 2016
www.thunderroadscolorado.com