Leaking fork seal - Surprising diagnosis

[kesteven]

My '97 R1100RT, bought earlier this year with 72K miles on the clock developed what appeared to be a leaking RH fork seal. I stripped and re-built the fork legs, replacing all the seals. A few miles later the leak was back. Decided the seal must be faulty, so I've just stripped and re-built the leg again. In the process I discovered a minute crack in the otherwise perfect stanchion, located above the seal, with two tiny pin holes through which the fork oil was being forced under pressure. No idea how the crack got there - there is no other damage on the stanchion. With a new stanchion fitted, the problem is solved. Thought that I would share this, in case you also have a mysteriously stubborn leak.

Nick

[freecoder]

I'm sorry if I'm being dense here, but if the crack is above the seal how is oil being forced out since the fork oil is below the seal?

[Tony_K]

Under compression with hydraulic force.

[RFW]

Under compression with hydraulic force.

Above the seal, there is no hydraulic force. The pressure is inside the stanchon, not on the atmospheric side of the seal. There is obviously some part of the story that is missing here.

[ShovelStrokeEd]

There is a change in volume within the fork as it compresses and extends. If the pin holes are connected by a crack within the fork stanchion, it is possible to pump oil through the crack and up to the holes above the seals. No real puzzle here.

[Don_Eilenberger]

Not to spook you - but in this case - I'd be looking the bike over very carefully. I can only imagine this sort of damage being caused by some accident sometime in the past.. so worth looking at the other fork tube also.. It would be unpleasant to have one crack whilst riding.

 

FWIW - I've heard of the same sort of failure on K bikes that bonged something good head-on.

[kesteven]

Thats the odd thing - The bike is otherwise immaculate. No evidence whatsoever of crash damage. The crack is minute, and looks more like a manufacturing flaw than anything else. To explain a little further, oil splashing around within the stanchion was being forced out of the pinholes by the air pressure inside the stanchion which increases each time the leg is compressed, as they are basically a sealed container of variable volume, partially filled with oil. Great to have the bike 100% again - My '99 Busa (I sold my '80 R100RS) is mind-wrenchingly fast - But the RT is such a satisfying ride, and brilliantly practical. I'd really like to try out the new R1200RT - It's made quite an impact in the reviews.