EVO: Primary Drive & Clutch

Primary Drive Design

The Primary Drive is responsible to transfer the crankshaft rotations (generated from the pistons, rods & flywheel) thru the Clutch to the Transmisison. The engine sprocket transfers engine power into the primary chain which turns the clutch assembly. The clutch assembly, when engaged, transfers engine power into the transmission on the mainshaft. (The transmission gears, along with final drive sprockets and chain or belt, transfers that engine power to the rear wheel.)

On the 1986-1990 models, the clutch assembly, at the transmission end of the primary chain, incorporates the rotor/stator assembly, while on the 1991-later models, the rotor/stator assembly was moved to be incorporated with the engine sprocket at the crankshaft end of the primary chain. This upgraded design allows the rotor to spin faster (at the same engine rpm) than it could previously when it was incorporated with the clutch at the transmission end of the primary chain.

The tension on the primary chain, rotating between the engine sprocket & the clutch sprocket, is controlled by an adjustable 'shoe'. This nylon shoe rides against the bottom length of the chain and reduces the slack as the upper length of the chain transfers engine power. Too little slack in the chain and it will bind in its rotation. Too much slack and the chain will slap as the engine decelerates and then accelerates.

Primary Chain - Manual Tension Adjustment

Illustration created by IXL2Relax at the XLForum FSM Recommended Procedure

  • Place the bike on a lift
  • Remove the Primary Chain inspection cover
  • Remove the spark plugs
  • Put the bike in 5th gear
  • Rotate the rear tire to cause the Primary Chain
    to rotate past the inspection hole

Rotate the chain fully around its length to find the spot that has the tightest tension. This is the point at which you must set the minimum play (or looseness). The initial primary chain vertical free play may be slightly tight from the factory since they use precision measurements.

On a cold engine (left overnight), the overall movement of the chain from pressed down to pressed up must be 3/8“ to 1/2”. On a hot engine, the recommendation is 1/4“ to 3/8”. By finding & using the tightest tension point and using the looser specification, you will avoid overtightening, which might otherwise occur. An overtightened primary chain will cause whining and excessive wear of the sprocket bearings.

Play in Primary Chain Upper Length
Year Cold Adj Hot Adj
1986-later 3/8“ to 1/2” 1/4“ to 3/8”

Although the 1986-1990 models have a rotational mount for the primary chain shoe and the 1991-later models use a fixed T-mount, the tension adjustment for all models accomplishes the same goal and is performed in the same way.

The adjustment of the chain tension is made using the Primary Chain Adjuster, mounted to the bottom of the Primary Cover. The adjuster mounting bolt extends through the Primary Cover to allow adjusting the tension without removing the cover. The adjuster bolt has a lock nut to keep the adjustment from changing. The adjuster has a nylon shoe atop it that rides against the loose side of the primary chain to take up excess slack. By loose side is meant the bottom path of the chain because the upper chain path is the one that transmits power to the clutch/transmission when accelerating. When decelerating in gear, the bottom side chain path is tight against the shoe because the engine is braking the bike motion thru the clutch/transmission.

To make the adjustment, hold the bolt steady and loosen the lock nut. Turn the bolt clockwise (viewed from below) to reduce (or tighten) the amount of chain free play or CCW to increase the free play. Once the adjustment is correct, hold the bolt steady & tighten the lock nut.

Primary Chain Tensioner

Primary Chain Adjuster - 1986-1990 - 39966-80 PAD Only
Primary Chain Adjuster - 1991-2003 - 39975-90A (upgrade in 2001+)
Primary Chain Adjuster - 2004-up — 40039-02C
84 XLH Primary Chain Tensioner & Spring (40019-86) 1)
91-later XLH Primary Chain Tensioner - Manually Adjustable 2)
Upgrade 39975-90A should be used in place of 1991-2000 stock Chain Tensioner.
2001-later models used -90A part as stock. 3)

Sprocket Torquing

When disassembling or re-installing the Engine Sprocket (Rotor), Primary Chain and Clutch Basket, use the following information:

Nut DescriptionNut SizeTightening TorqueWhereNotes
Engine Sprocket Nut
1991-1997 Models1-1/8“ Nut 150-165ft-lbs at Rotor Assy *HD Tech Tip 211 – September 16, 2005
Flywheel Sprocket Shaft Nut Torque Update
The flywheel (engine) sprocket shaft nut torque
for 2004 & later Sportsters and 2003 & later
Buell XB models has been changed.
The torque has been increased from 190-210
to 240-260 ft lbs. Remember to clean the
threads and generously apply LOCTITE
Threadlocker 262 (red) onto the threads of
the engine sprocket shaft.
1998-2003 Models1-1/8” Nut 190-210ft-lbs at Rotor Assy
2004+ Models1-1/8“ Nut 240-260ft-lbs* at Rotor Assy
Transmission Main Shaft Nut (Clutch Hub) (1991-later)
Left-hand Threads!!1-3/16” Nut 75ft-lbs at Clutch HubUse only 2 or 3 drops of RED threadlocker
on this nut

Use regular six-sided sockets which are less likely to slip and round off the edges of the nuts. Impact-type sockets are likely too thick to fit.

When lossening or tightening these nuts, you will need to lock the hub & sprocket - Do not wedge the chain.

Buy a primary locking tool (bar) or use a closed 4“ BRASS hinge as a wedge. (Place the wedge so as not to put pressure on the shifter shaft)
There are also several 4 and 5 speed primary locking tool examples in the tool section.


Removing the Primary Drive

91 and later models.

  • After you've removed both nuts, first pull the clutch out about halfway and then use 2 hands to pull firmly on the rotor.
    The magnets on the rotor are very strong. Once you get past the magnet resistance it will come of quite easily. 5).
2006 primary sprockets 6)


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