Airhead Tech Pages
Rear Engine Seal Replacement on a R90/6
(This procedure will work on most Rbikes..however, some
of the procedure will be different depending on Model/Year, check you Haynes
or Clymer manual for specifics related to your bike- ed.)
by Matt Parkhouse
I used to be able to R&R a gearbox, in a good shop in an hour.
To do what I've described here, working in front of the house (on a warm day),
I would budget 3-4 hours. I have done this task maybe 100 times over the last
20 years. I certainly would recommend using more than this description to under
take this work. A Haynes/Clymer manual at the least, a friend who has done
it is best.
Parts and Tools
Replacement of Seal and Reassembly
Parts to have on hand...
- Rear main seal, oil pump
- Oil pump cover and 4 bolts (upgraded since /6 series),
- 5 new flywheel bolts
- 4 driveshaft lockwashers
- 4 driveshaft bolts
- 6 clutch bolts
- New driveshaft boot if the old one is cracked or leaking
- Clutch compressing screws (you can use bolts and nuts-less handy but works
OK, or get from Ed Korn)
- Flywheel brake (make out of bar stock or get from Ed Korn)
- Rear main seal installer: from Ed Korn
- 6mm allen drive on 3/8 socket, torque wrench
- 10mm-12 point box end wrench ( I like Snap-on's)
- 1 1/16" socket (turned or ground down on outside)
- Micrometer or calipers
- Slide hammer with hook attachment
- Impact screwdriver with large phillips bit.
- Place bike on center stand ( if a "Ride-off stand", block up
on 2X4s), you want to be able to turn rear wheel freely.
- Drain gearbox if work is planned on it.
- Drain driveshaft as oil will spill when it is disconnected.
- Remove gas tank, set aside out of way.
- Yake carbs off cylinders and hang over front of cylinders, out of way.
- Open seat, tie or prop open, take out tool kit tray and place with gas
- Important! Remove
points cover and place a short piece of metal (cut-down allen key is best)
in allen slot of alternator. Put engine at Top Dead Center (TDC) before
doing this. You want to firmly but gently jam the crankshaft to the rear
by replacing the front cover and putting pressure on the alternator/crank
assembly. Failure to do this can result in the crank moving forward, after
the flywheel is off and pulling an endplay shim off it's posts INSIDE the
engine. You REALLY don't want this happening!
- Clear away air cleaner housing on both sides.
- Remove battery. Remove the 10mm nut that secures the battery box on each
side to the frame, this allows you to tip the battery box back a little.
- Take off the forward hose clamp the secures the driveshaft boot to the
output of the gearbox. This exposes the four bolts that secure the driveshaft
to the gearbox. Rotate wheel so you have a good shot at one of them. Apply
rear brake to lock up everything and undo one bolt. Rotate wheel and repeat
for other three bolts. You want a magnet on a stick tool handy, in case
you lose track of bolts or lockwashers in the driveshaft housing.
- Take out pin that rear brake pivots on and allow foot brake lever and rod
to drop down.
- Use grounddown 1 1/16 socket to loosen lock nuts on swingarm pivots. Use
6mm allen to take out pivot pins.
- You now should be able to pull back driveshaft and rear wheel assembly
about an inch or so. Bag mounts may interfer and have to be removed (mine
- Take off clutch cable ( a long lever helps).
- Take off throwout assembly on back off gearbox (held in with circlip).
Check throwout bearing and two flat thrust pieces for wear and set aside
(this is sort of a weak point in /6 or /7 BMWs).
- Remove two bolts on left side of tranny, one long bolt and nut on right
side, that secure tranny to engine block.You should be able to pull back
on gearbox and lift it up and out on left sideof bike. As you do this, pay
attention to neutral switch wiring on bottom ofthe box.
- Gently pull off the two wires (It's kind of easy to bend and break the
push-on connectors on the switch). You now have the gearbox in your hands.
- Pull out throwout rod from input shaft and set with other throwout pieces.
Setgear box aside.
- To get clutch out, remove every other bolt visible at back of clutch assembly
(3 of 6). Install home-made or purchased compressing tools in their place.
- Tighten down, then pull the other three clutch bolts.
- Loosen the compressing tools equally until the clutch is loose and rattly.
Back all the way out.
- Pull out assembly as one unit, mark with chalk, ink or small scratches;
bothpressure plates and flywheel, so you can reassemble as it was originally
- Check clutch plate with micrometer or calipers: replacement point is at
4.5mm thickness, new plates are at 6mm. Set aside.
- To remove flywheel: Is engine still at TDC?. Install flywheel brake in
two of the clutch bolt holes so that wheel is locked against side of engine
- Remove the 5 bolts and gently lever the flywheel off the end of the crankshaft.
Heating helps sometimes. Put it aside with brake still attached.
- Now you see the oil pump cover. If you see bolts with 6 point heads in
flat recesses in the cover: it has been up-dated. If you have phillips head
bolts, you have not been updated, an impact driver helps in the latter case.
If there is no sign of leaking, just check the bolts for tightness (8 ft.lbs.).
They are often loose. If you are replacing a leaking rear maim seal, yank
out the old one using a slidehammer with a hook. Even though you have braced
the crankshaft from the front, avoid putting a lot of force on it as it
is nowable to move forward. The bike is now torn down all the way for these
- Now is a good time to clean all this stuff. Part 2 will cover putting it
Replacing Seal and Reassembly
More specialized tools:
- BMW swingarm grease
- Fiitted grease gun or chain sawnose-wheel greaser,
We left the bike torn down to the rear crankshaft. I like to use spray carb
cleaner to clean parts and the usually grungy clutch compartment. Not unusual
to go through 2 or 3 cans on this sort of job.
- The new rear main seal goes in, using the installing tool. BMW makes a
nice one, so does Ed Korn. Hopefully
you noted exactly how far in the block the old seal was; try and get the
new seal to the same level. Since we have already tightened the oil pump
cover or replaced the O-ring there,
- Replace the flywheel. I like to clean up, again, with carb cleaner, and
then touch up the markings ("F", "OT" and the dot for
full advance) with a white paint stick, while I have the flywheel in my
hands. The flywheel goes back EXACTLY as it came off. Is the OT mark where
it was when you took it off? New bolts are in order here, but some
folks do reuse the much heaver /7 bolts (I have). Bolts are clean and dry.
Tighten to the recommended torque.
- Remove the flywheel brake and release the brace at the front of the engine
and make sure the crank turns easily.
- The clutch goes back next, with cleaned up parts and maybe a new clutch
plate. I reuse these bolts without out problem, others like to replace them.
Torque to 16 ft. lbs.. There are clutch centering tools, I use them but
don't have one, get it "pretty close" to centered. The first time
the clutch is used, the plate will line up anyway.
- The gear box is next, with nicely cleaned up splines on the input shaft
(the part that goes into the clutch plate). Put the throwout rod back into
the hollow input shaft, tapered end toward the clutch, with a bit (small
bit) of antiseize on the end, with a bit of grease or gear oil on the shaft
of the rod. Coat the splines with a thin layer of antiseize ( a do-every-20K
task) and install the gear box in the reverse of the movement you used to
take it out. Do pause part way into it and hook up the neutral wires to
the switch on the bottom. The two bolts on the left and the long bolt with
nut on the right reattach the box to the engine block. The throwout assembly
goes back to the rear of the gearbox. Loosen and back out the adjuster bolt
a little. Be sure to coat the pivot pin with grease and put a little grease
in the cup where the throwout rod contacts the throwout lever. Do you need
to replace the little rubber boot that covers the cup? When you have properly
adjusted the throwout lever and clutch hand lever, the throw out lever should
be, or close to, parallel to the back cover of the gear box.
- The driveshaft goes back now. If you are putting on a new boot, now's the
time. The "OBEN" marking goes on top. The pivot pins go back into
the frame and swingarm mounts. Some folks use calipers to measure the equal
find eyeballing is fine. Back off the lock nuts a turn or two. Using the
6mm allen tool in a torque wrench, tighten both to 14 ft. lbs; then back
off and retighten to 8 ft. lbs. This is the preload for the tapered roller
bearing of the swingarm. Tighten the locknuts, using the 1 1/16 socket,
to 75 ft.lbs..Give them a generous shot of grease (until it appears, emerging
from the seals). Reassemble the rear brake set-up first, as you will need
this to lock up the driveline for tightening of the bolts. Then, with new
bolts and lock washers, reattach the driveshaft to the output of the gear
box. Get all four bolts pretty well in by hand, then one by one, rotate
the assembly by turning the wheel as you use the 12point 10mm box end to
tighten the bolts. There are devices for measuring torque, I just give a
good grunt to the wrench. In a
pinch, I have reused the bolts, it's REALLY a good idea to use new lockwashers
- Reinstall the hose clamp that anchors the drive shaft boot to the gearbox.
- The battery is replaced,
- Replace the aircleaner housing halves and the airfilter. A little silicone
spray and some needle nose pliers really help to
pull the crankcase vent hose into the right hand air intake.
- Reattach the battery box to the frame with the two 10mm nuts.
- The carbs go back on the heads and aircleaner housing. If you were gentle
with them and didn't pull on
them or bang them around, you shouldn't need to rebalance them.
- Fuel tank back on,
- tool tray back in,
- replace any drained oil and you are all set.