The Poly-V belt drives the alternator. It is a small, inexpensive part, but it is not something to ignore on an older R1100S.
A cracked, glazed or unknown-age belt can leave you without charging while riding. BMW specified replacement at 60,000 km / 36,000 miles.
This is a straightforward job on the earlier, conventional-belt R1100S. The important part is identifying which belt system your bike actually has before ordering parts or setting tension.
First: Identify the Belt Correctly
There are two broad R1100S belt arrangements.
Earlier bikes: conventional Poly-V belt
Most R1100S models built up to approximately July 2003 use a conventional adjustable belt, commonly identified as:
- Continental 4PK611
- BMW part number commonly listed as 12 31 1 341 779
This is the belt shown in this guide.
It is a normal, non-elastic belt and is tensioned using the alternator adjuster.
Later bikes: ELAST belt
Later R1100S models use BMW’s maintenance-free ELAST belt and matching pulley arrangement.
BMW commonly identifies this belt as 4PK592, while Continental may label the equivalent belt 4PK611 ELAST (592). This difference is confusing, but it refers to the same later elastic-belt family.
The important point is this:
Do not choose the belt based on whether the bike has ABS, a Bosch alternator or a Denso alternator.
That shortcut is unreliable.
Check the designation printed on the old belt, confirm against your VIN or BMW parts fiche, and order the same system. A conventional 4PK611 and an ELAST 4PK592 are not installed or tensioned in the same way.
This Guide Applies to the Conventional Adjustable Belt
The photos and procedure below follow the earlier, conventional Poly-V belt system.
On this arrangement, BMW specifies an 8 Nm preload on the alternator adjusting screw. That is not “8 Nm of belt tension”; it is the torque applied to the adjuster while tensioning the belt.
The alternator mounting bolts are then tightened while the torque wrench remains loaded at 8 Nm. BMW specifies 20 Nm for the alternator-to-support fasteners.
For an ELAST belt, do not follow the conventional adjustment method. BMW describes the later system as maintenance-free after installation and specifies that it should not be re-tensioned in normal service.
What You Need
- Correct replacement Poly-V belt
- Torque wrench capable of accurately setting 8 Nm
- Socket set or spanners for the alternator fasteners
- Basic hand tools for the front cover
- Good light
- Gloves
BMW used special tools for access and belt work in the workshop manual, but the tensioning procedure itself can be completed with a normal torque wrench.
Before You Start
Work with the engine cold.
Switch off the ignition and remove the key.
Take a photo of the existing belt routing and alternator-adjuster position before loosening anything. It is simple, but there is no reason to rely on memory.
Inspect the old belt before removing it. Cracks, missing ribs, glazing or frayed edges confirm that replacement was overdue.
Step 1: Remove the Front Cover
Remove the front engine cover to expose the alternator belt.
On the R1100S, BMW’s workshop procedure also involves removing the spark plugs before removing the front cover. Follow the access sequence appropriate to your bike and take care around the ignition components.
Once the cover is removed, you can see the belt running between the crankshaft pulley and alternator pulley.
Step 2: Loosen the Alternator Fasteners
Loosen the three alternator retaining points.
Do not remove them completely. You only need enough movement for the alternator to slide and release tension from the old belt.
The alternator should now be able to move inward enough for the belt to come free.
Step 3: Remove the Old Belt
Remove the old belt from the pulleys.
Check both pulleys before fitting the new one:
- No damaged or chipped ribs
- No oil contamination
- No embedded debris
- No obvious pulley misalignment
A new belt will not solve a charging problem caused by a contaminated or damaged pulley.
Step 4: Fit the New Belt
Fit the replacement belt around the crankshaft pulley and alternator pulley.
Make sure all ribs sit fully and squarely in the pulley grooves. A belt sitting one rib out of line can be damaged quickly.
Do not use screwdrivers or sharp tools to force the belt into place.
Step 5: Set the Belt Preload
For the conventional belt system, use the alternator adjuster.
- Hand-tighten the adjuster nut.
- Apply 8 Nm to the adjusting screw with the torque wrench.
- Keep the torque wrench loaded at 8 Nm.
- While maintaining that preload, tighten the two alternator mounting fasteners.
- Fit and tighten the final retaining fastener.
BMW specifies 20 Nm for the alternator mounting fasteners.
The important part is holding the adjuster at 8 Nm while tightening the alternator bolts. If you release the torque wrench first, the belt may end up under-tensioned.
Step 6: Turn the Engine and Recheck
Turn the engine over by hand once, where practical, to allow the new belt to seat correctly.
Then check that:
- The belt remains centred on both pulleys
- All ribs are seated correctly
- The alternator fasteners are tight
- Nothing touches the belt or pulleys
- No tools or loose items remain behind the front cover
Step 7: Refit the Front Cover
Refit the front cover and reinstall any components removed for access.
Start the bike and confirm that:
- The battery warning light goes out normally
- The belt runs smoothly
- There is no squeal
- There is no visible belt wander
- No unusual charging-system warning appears
Common Mistakes
Ordering by ABS specification
Do not assume ABS determines belt type.
The more reliable method is to inspect the old belt, check the production date and confirm with VIN or parts fiche.
Treating 8 Nm as belt tension
The 8 Nm figure applies to the adjusting screw preload, not directly to the belt itself.
Tensioning an ELAST belt like an early conventional belt
The later elastic system is different. It should not be routinely re-tensioned using the early procedure.
Over-tightening the belt
An over-tight belt can place unnecessary load on alternator bearings and related components.
Leaving oil on the pulley or belt
Oil contamination shortens belt life and can cause squeal or slip. Find the source before fitting a new belt.
When Should You Replace It?
BMW specified replacement every 60,000 km / 36,000 miles.
On a twenty-year-old R1100S with incomplete history, the sensible rule is simpler:
If you do not know when the belt was replaced, replace it.
It is cheap insurance and easy to inspect while doing other front-engine work.
Bottom Line
The Poly-V belt is not a glamorous upgrade, but it is proper baseline maintenance.
Use the correct belt for your bike, do not rely on ABS or alternator assumptions, and distinguish clearly between the early adjustable system and the later ELAST system.
For the conventional belt shown here, BMW’s procedure is straightforward: preload the adjuster to 8 Nm, hold it there, tighten the alternator fasteners to 20 Nm, then recheck belt alignment before closing the front cover.