Category: BMW maintenance

Replacing Fork Seal (oilhead)

I’ve busted another (it’s the 2.nd in 4 months) fork seal and I intend to replace it myself. I’ve searched the wisdom here and it looks simple. Just to make sure, can the wrench gods confirm the following steps:

a) With a 15 mm and a 19 mm loose the stanchion from the top triple clamp

b) once loose, the stanchion will move down inside the slider.

b) remove the plastic black band at the top of the stanchion.

c) unscrew the cup (is it a sort of dust seal ? is it screwn in?)

d) remove a retaining clip

e) pry the busted seal off. (anyone knows standard seal dimensions?)

f) Install new seal, reverse the rest of the operation

g) take front wheel and axle out, take air purge plug at top of stanchion out and drain fork leg at bottom drain plug.

h) fill 0.47 l of SAE 5 susp. oil.

Is it something like this or am I totally wrong? Why do these shites blow if there’s no susp internals inside? Are we supposed to purge the air regularly like on real forks?

TIA

Don’t remove the slider at all. Just remove the fork brace bolts, the axle, and the brake caliper, and the fork lower drops right onto the garage floor. At least, that’s how my memory remembers it. I do know that there is nothing positively attaching the slider to the lower fork leg.

The rest of your procedure sounds fine, although I wouldn’t even bother changing oil.

Jon

John,

Thanks.

Sorry, it may be my english. I am calling stanchion to the inner fork tube (attached to the top triple clamp) and slider to the outer tube (attached to the bottom triple clamp / telelever arm).

So do I need to loosen the lower (outer) fork leg? I though that, by releasing the inner from the top triple clamp, this inner would move down inside the (outer) bottom fork leg, allowing me to take the seal out by the top part. Hum… maybe a drawing…

I just redid my left fork seal, and the upper (shiny part) of the left fork, today for the 2nd time since doing both in Achorage early this May, I tried to do it with out taking the entire fork out of the mounts. But it will save you time and knuck skin if you just take the bloody hold thing out. Rememer to re-aline the front axel before putting the tire back on. i tighten the bolts that #4 on preivioius pict. withthe axel in then put the tire back on.

as for replacing the fork seal. by taking the entire lower fork off you will have the ability to remove the bad seal easier. they are really tigith fit. also be very careful on putting the new one back in not ot damage the inner seal ring part of the seal. I used a large socket for to evenly get the seal to sit flat on the metal o-ring that goes below it. I had BMW Anchorage show me how to do it with their BMW speciality tool, I used a large socket that just fit into the lower fork section. If you have a friend with mechanic shop they should have a large enough socket. .

in addition, i put a bit of extra grease on the top cap to help keep dust getting into the seal and wearing it out quicker. To get old seal out is a pain and you need a L shaped hook type of tool to pull it out. that is why i take the lower section, so i can use my feet and legs to get a good hold on the fork to pull the seal out.

thing to check while doing seal replacement
take the tiem to clean out any grime in the bottom of the fork lower section, i use a little but of desiel fuel as a solvent and clean inner part of the fork of and grime fork oil that did not drain out.

the uper part of the fork (shiny part) will slide in easily if the seal is sitting flat on the lower metal o-ring. also take out the tiny hex bolt on the top of the upper part of the fork to let air in as you re-insert the lower part back onto the upper part. and then when everything is at the right place tighten the tiny screw back to seal the fork.

In addition, you should check you upper fork section to see if there are any fine scratches… i had some bad ones from all the dirt roads i ride in Colorado, that could be the reason you seals are wearing out so quickly. And buy a pair of fork covers.

Remove front wheel and brake callipers:

Loosen the inner top from the top triple clamp:

Loosen the pinch bolts on the bottom triple clamp:

Lower the fork tube in the lower triple clamp, let the cup(dust seal ) rest agaisnt it and push down to extract the dust seal.

how it looks once out:

Now remove the air purging little bolt from the top of the inner tube. Then pull the inner from inside the outer. Beware oil spillage!

Now remove circlip:

Then pry the seal out with a big flat blade screwdriver. Protect the top of the outer tube with some plastic piece or something:

Bellow the seal there’s a large washer (rusty in my case):

Where it all seats (once cleaned):

All in pieces:

Now, when I took the inner from the outer I saw that the seal didn’t have any visible damage. The inner is also in good aparent shape without scratches… humm.. I begin to suspect crash is right. Let’s put the inners in the truing stand:

Shit. Crash is Right. About both inner tubes.

I didn’t have my feeler gauges handy. But I had a book 1 cm thick and a 100 pages :): . I can slip 2 pages but not three bellow the middle of the inner. So I guess it has a runnout of 2 to 3/10 mm. Is this acceptable? I can turn the inners inside the outers with no effort…

The Telelever A-arm looks OK. (Apart from the fact that it hits the Hepcko & Becker crash bars :splat.

I can’t see the dreaded cracks in the paint.

The grease under the dust cup is common practice on dirt bikes (real ones, that is). But the manual of my KTM says to put new grease in every 15 riding hours! So, I guess it is a good thing to do if we do ride off-road, but it should be at least inspected on a regular basis (like monthly?)

I borrowed a friend’s micrometer and built a pennytech measuring stand:

As expected, the tubes are more or less inside spec (0.016″)regarding runout. (one is at 0.015″ and the other at 0.018″).

Well then why does the shop manual say to do so?

BMW Shop Manual Page 31.9]
• Install fixed tube.
• Install washer (6).
• Push the lightly oiled shaft sealing ring fully up to
the stop on the slider tube, then press home by
tapping lightly and using threaded bush,
BMW No. 31 5 611, expander,
BMW No. 31 5 612, and reducing adapter,
BMW No. 31 5 613.
• Install retaining ring and dust wiper.
• Insert bleed screw (1).
• Bleed telescopic forks under zero load.

Timing Adjustment – No Tools Needed

Well ok, not really no tools, but only two wrenches.

NO electronic tools needed, and NO electrical knowledge.

AND no need to remove the fuel tank.

Setting the timing uses a ‘feature’ of the 1150 Motronic ECU. It works on ’01 and ’02. But I do not know how widespread the ‘feature’ is across other model years.

Here’s the deal. You know how the crank pulley trips the hall-effect sensors right?

Well, if the ignition key is on, and the crank is turned slowly by hand, at the moment the crank pulley trips the hall-effect sensor, the Motronic will switch the fuel pump on for a moment.

Roughly the process is:

Ignition key off
Unplug the headlight
Remove the flywheel viewing plug in the bell housing
Remove the alternator belt cover
Rotate the crank by hand to the ‘ S ‘ flywheel mark
Turn the ignition key on – the fuel pump will cycle normally
Rotate the crank very s-l-o-w-l-y by hand until the fuel pump cycles again
(approx 5° of crank rotation)
The crank is now at the hall sensor trip point
Look at the flywheel marking to see where the timing is set
Adjust if necessary

Pictures and details to follow.

– Jim

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Tools needed

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Turn the ignition off.

Reach up under the instrument panel and unplug the headlight. This step is optional, at your discretion. I prefer to unplug the headlight. If a timing adjustment is performed, the ignition key will be on a while.

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Remove the black plastic front engine cover. Behind the cover are the crankshaft pulley, hall effect sensor adjustment, and alternator belt tension adjustment.

TIP: If an aftermarket shock is fitted, the shock may be too close to the front cover. If so, the lower mounting bolt may be removed to tilt the shock out of the way of the front cover. If a stock shock is fitted, there is enough clearance to remove the front cover.

IMPORTANT: Think about this. If removing the lower mounting bolt of the front shock, be sure the rear of the bike is weighted so that the bike will not tip forward. And, it helps to have another person to lift up on the front wheel to remove the shear load from the shock bolt.

After the front engine cover is removed, replace the shock bolt for safety.

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Rotate the crank until you see the ‘ S ‘ flywheel mark in the bell housing hole. For clarity, this picture is staged with the technician on the left side of the bike. You may find it easier to do this from the right hand side of the bike.

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Rotate the crank until you see the ‘ S ‘ flywheel mark. The ‘ S ‘ mark is 5� Before Top Dead Center (BTDC).

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Turn the ignition key on. The fuel pump will cycle normally. And if you unplugged the headlight, the headlight will not be on.

S-L-O-W-L-Y rotate the crank. After about 5� of rotation the fuel pump will cycle again.

You can see that 5� is not very far. If you ‘feel’ that you over-rotated the crank by some amount, just reverse the crank 5 degrees and try again. This is a light-handed maneuver.

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Take a look at the flywheel mark again. If the timing is correct, the ‘ OT ‘ mark will be lined up in the center of the viewing hole. The ‘ OT ‘ mark is Top Dead Center (TDC).

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Ok. Repeat the last two steps a few times. The fuel pump will cycle each time the crank pulley crosses the sensor trip point.

Each time the fuel pump cycles, stop and look at the location of the flywheel mark. Slowly repeat the process of crossing the sensor trip point, and checking the flywheel mark. After some number of tries, the final location of the mark will be consistent.

The flywheel mark will be in one of three places. Exactly on TDC, somewhat ahead of TDC, or somewhat behind TDC.

PARALLAX ERROR: It is easy enough to position your eye exactly centered above the bell housing hole. The hole in the bell housing has side walls or ‘draft’. And it is easy to spot whether your perspective is lined up.

From a look at the pictures below, it is possible to determine that the camera lens is not lined up with the centerline of the hole. The camera lens is a little above the hole centerline.

What gives the high lens location away is the ‘draft’ visible along the bottom edge of the hole. Eyes will see the same thing if their perspective is a little above the hole centerline. Get your reading glasses, and a good light.

When your eye is lined up with the hole, it is natural to draw an imaginary line through the center of the circle. Take a look at the green line, is very natural to see where the ‘ OT ‘ mark is, relative to that imaginary green line.

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Spark advance looks like this. Some amount ahead of TDC.

For example, this is what 2� advance looks like.

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Spark retard looks like this. Some amount behind TDC.

For example, this is what 2� retard looks like.

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The hall sensors are hidden from view. They are behind the crank pulley. The hall sensors are mounted to a circular plate. The circular plate can be seen around the perimeter of the crank pulley.

Spark timing is controlled by the hall sensors. Spark timing adjustment is done by changing the position of the hall sensors, relative to the crank pulley. Changing the position of the hall sensors is done by adjusting the circular plate.

The timing adjustment range is limited to about �5�. And about the middle of the adjustment range sets the timing to about TDC.

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Loosen the THREE hex head fasteners that hold the circular hall sensor plate. Fully loosen the fasteners so that the circular sensor plate will slide freely.

People have reported that the plate on their bike was stuck in place or almost impossible to turn. If that is the case the fasteners can be removed fully and the plate can be freed by lifting it away from the surface. Some canned air and spray lube should remove any grit or debris, and keep the plate moving freely when loosened next time.

It is much easier to make small adjustments to the plate if the fasteners are very loose. If the fasteners are even slightly snug, too much force must be applied to the plate to move it. And when it does move it will overshoot the intended mark.

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PAUSE: This just seemed worth stopping for. I did not really notice until working with these pictures. This is the UNDERSIDE of the front control arm. I am glad my friend lets me use his bike for these photos. My hands (and the camera) stay clean.

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Adjusting the timing.

First, turn off the ignition key.

The timing is adjusted by moving the hall sensor plate. The full range of adjustment is limited to about �5�. The sensor plate can be moved to either end of the 10� range and the engine will start and the bike is plenty rideable.

Make a pencil mark across the plate and front cover and experiment. I am running mine at about 3� advance timing. The idle is smoother and the pinging is about the same. Though as the weather gets warmer the engine may ping less at 0� timing.

To advance the timing, move the bottom of the hall sensor plate to the right. A small flat blade screwdriver is used to slide the plate.

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To retard the timing, move the bottom of the hall sensor plate to the left.

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After moving the hall sensor plate where you wish, snug one of the fasteners to hold it in place.

Turn on the ignition key and repeat the previous steps of rotating the crank pulley until the fuel pump cycles. And check to see how the timing changed.

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After all of this smooth sailing, there is one pesky gotcha here. When reattaching the plastic cover, it can sometimes be impossible to get the screws on the left side of the cover started.

Here is why. The oil return tube has two brackets that are held in place with two of the front cover screws. You can see the problem you will have if the brackets are not lined up with the screw holes.