BMW K-bike Mechanical History

The following information is compiled from the Haynes reference manual and other sources.  If you have any additions to make, please contact me using the link at the bottom of the page.  1984 refers to calendar year 1984, and '84 refers to model year 1984.

1983 - '84 K100 models introduced as the K100 and K100RS, followed later by the K100RT in 1984.

Early '84 UK K100 models...

• had a 3-roller starter clutch while all other models had a 14-element sprag clutch
• had a drilled rear brake disc (recalled due to cracking)

'84 and '85 K100s...

• had rubber-mounted footpegs, replaced with rigid footpegs with the K75 (warranty replacement on older bikes).
• had a 75mm frame/bellhousing mounting bracket, all others were 100mm.  Engine mounting procedures are different.
• had different side-panel mounting systems than the '86 models had.  A third system was used on K75s, but later changed to the later K100 type.
• had seven screws holding the pod together, '86 -> had nine plus the gore-tex vents
• had the twin fuel warning lamps  (4 liter and 7 liter) and a resistive sensor.  Later ones had float sensors and one light, and could drive a fuel gauge.
• were 90hp european spec for the 49 states, and later ones were all 82hp California models.

Early 1985 models got...

• ignition control units with reinforced covers and better sealing.
• a revised starter clutch which allowed more oil to pass through it. Freewheel with more holes.

Fuel gauge system: The 84-85 K-bikes had self-heating thermistors that are used to activate the low, and reserve indicators. In practice the low light started to come on and off when the tank was just less than full, which was quite distracting. In 1986 the low light was removed.  BMW also switched to a potentiometer that allowed for some hysteresis in the circuit to keep the light from blinking when the fuel sloshed. The potentiometer also allowed the fitment of a fuel level gage.

'85 models...

• got a revised final drive breather vent
• had the spring removed from the fuel tank check valve (due to noise)

Mid-1985 models got:

• a flange on the end of the engine output shaft, while earlier models had locating circlips.
• springloaded fuel cap instead of pushbutton style
• revised anti-backlash assembly

Early K100s... (no date provided)

• had a vacuum switch on the intake manifold which changed the advance curve at medium throttle (deleted in late 1985).
• had a single rear fuel tank mounting which was prone to cracking.  Subsequent models had two-point mounting.
• had no integral fork brace (one-part front fender?)
• had shorter final drive mounting bolts and holes
• had oil drain plugs in the valve cover and lockable cam chain tensioners.
• had an inferior swingarm boot, compared to later ones
• had a gasket on the master cylinder cover (removed before the K75 models)
• had exhaust valve seats which were not hardened so leaded fuel was required.

1985 - K75C introduced as '86 model.  All K-bikes got...

• solid-mounted footpegs
• two mounting points at the rear of the tank, instead of the one which vibrated and was prone to cracking
• a revised rear cowling and seat configuration, and revised exhaust pipe mounting.
• an improved fuel pump

Later in 1985 K75 models got a revised steering head bearing setup (locknut instead of top bolt) and the Fluidbloc damper.

Starting with the K75S in mid-1986, the  fuel tank check valve was replaced by a standpipe on all models.

At some point, fork gaiters were replaced by small air scoops, and the tank was insulated on the bottom.  This was to prevent excessive buildup of heat in the tank.

During 1986...

• K75s went from 11.0:1 to 10.5:1 compression.  Power went from 75@8500 to 70@8200, and torque went from 50@6750 to 48@6500
• New rear brake switch and footpeg plate introduced

6/86 (MY87?)

• throttle cable revised to move adjuster from middle to handgrip.
• All bikes got rubber tire valve stems

May 1987, all clutch splines have been nickel plated as a precaution against seizure (Walker&Dobson:BMW K-series motorcycles,1989)

Late 1987 K75s got revised anti-backlash gears.  Didn't help :)

Early K100RS mirrors were held in the housings by friction, and later ones by a clip.

Late 1987 modified fork seals used, distinguishable by two external rings

1988 model changes:

• K100 fuel tank capacity  from 22 to 21 liters
• K75 and K100 got the low seat
• K75S got black drivetrain and wider handlebars
• K100 Std. got K75 Std. tank and low seat.
• K100RS Special Edition (blue/white) introduced with ABS
• Different radiator cover on K100 models.
• Rear brake reservoir moved on all models (needed for ABS).  The older, exposed location is prone to deterioration in sunlight.
• Front brake line routed outside the steering head (earlier models went through it) although fiche shows this as '92

During 1988, K100 aux. shaft seal was modified.

In late 1988...

• the K100 exhaust heat shield was modified to reduce rattling
• The master cylinder was reduced from 13 to 12mm

1989 model year

• K75 Std. introduced
• Second accessory socket added on some models
• Fresh air louvers added to LT/RT fairings (removed after 1.5 years)
• Sintered metal front brake pads used, with different calipers.  Not backwards-compatible.

During 1989...

• pinned piston rings were introduced 6/88
• during 1989 the coolant reservoir was modified to eliminate the sight tube.
• Gore-Tex inserts were added to the instrument pods (have three-segment ventilation holes for ID, and nine screws not seven)

1/90 K75 water pump was revised.

03/91 K100LT production ended. K1100LT production began 03/92 as '93 models.

01/92 K100RS 4-valve production ended. K1100RS production started 11/92.

Forks:

• Late 1984 - modification to the Original forks (Brembo?)
• 1986 - heavy revision to the K100 fork, no more changes for 2-valve K100s.
• 1985 - Original K75 fork introduced on the K75C. I think Fichtel and Sachs was the supplier
• 1990? - "S" forks introduced with K75S (available on others)
• 1992 - Showa forks used on all K75 versions from what I can tell.
• Marzocchi forks were used on the 4V K100RS, K1, and K1100RS/LT without changes.

Electrical:

• original models had a 460W alternator
• K11 models got 700W alternator in 9/92 (production from 3/92 was also considered '93)
• K75 models got 700W alternator in '94

K11 models got ABS2 and Motronic 2.2 for '94. For '93 they had 2.0, ABS1 and could fit the larger battery.

At some point...

• the "choke" light was discontinued (only the switch and emblem were removed)
• splines were hardened ('89 I think)
• water pump impeller went from bolt to nut or vice versa (around '86?)
• Shift drums were changed to a harder material at some point  (around '91?)
• The centerstand of the K100 was revised (warranty fix)
• Extra ground wire was added to the switchgear to prevent current from flowing through the throttle cable.  The throttle adjuster was moved from mid-cable to the throttle assembly in 6/86, then an insulator was added.

From Joe Dille: "I believe it was the lower left bag mount. They added some additional vibration reducing piece down where the muffler mounts"

K model history