Fork Oil Viscosity, Seal Leakage

[brickme]

Changed out the fork oil as a matter of maintenance, using Bel Ray 10w as a starting point before I knew of the differences between oil brands.  While this allowed the K1100LT to corner like on rails, it is too stiff for my back.  My local dealer didn't have lighter Bel Ray so after looking at a cross-brand viscosity chart I replaced one side only with Torco 7w, which is quite a bit lower viscosity.  This provides a great compromise between comfort and handling but I noticed today the seal on the higher viscosity side is now starting to leak.  Coincidence or cause-and-effect?  If there is a significant difference in viscosity between the oil in the two forks, could the result be too much pressure in the high viscosity side for the seals?  Or would it have started to leak anyway?  Thanks for your collective wisdom on this.

[Rick G]

The pressure should not change with a change viscosity of the oil.
Would be worth changing to the Torco to see if it leaks but in my experience it is time to put new seals in.
Frankenduck has described a method of cleaning the seal lips with a feeler gauge and a few have reported good results although I havent done it myself it works and is worth a try. You insert a very thin feeler gauge in between the leg and seal and run it round and it removes the parts of long dead insects that have committed suicide and been draged down into the cavity between the seal lip and wiper lip.

[DRxBMW]

The pressure should not change with a change viscosity of the oil.
Would be worth changing to the Torco to see if it leaks but in my experience it is time to put new seals in.
Frankenduck has described a method of cleaning the seal lips with a feeler gauge and a few have reported good results although I havent done it myself it works and is worth a try. You insert a very thin feeler gauge in between the leg and seal and run it round and it removes the parts of long dead insects that have committed suicide and been draged down into the cavity between the seal lip and wiper lip.

I've experimented with different weights of fork oil.

dunno, NO real change in handling per say.

FWIW, volume is critical ! Wrenched on a K 75S that was low on fluid, the proper amount did wonders after R&R. 

Tuned springs are better avenue to explore.

[brickme]

The pressure should not change with a change viscosity of the oil.
Would be worth changing to the Torco to see if it leaks but in my experience it is time to put new seals in.
Frankenduck has described a method of cleaning the seal lips with a feeler gauge and a few have reported good results although I havent done it myself it works and is worth a try. You insert a very thin feeler gauge in between the leg and seal and run it round and it removes the parts of long dead insects that have committed suicide and been draged down into the cavity between the seal lip and wiper lip.

I've experimented with different weights of fork oil.

dunno, NO real change in handling per say.

FWIW, volume is critical ! Wrenched on a K 75S that was low on fluid, the proper amount did wonders after R&R. 

Tuned springs are better avenue to explore.

Yeah, I didn't really start out trying to tune the front suspension with the oil but it was surprisingly stiff after changing it out.  I really had no idea where to start on the viscosity and looking at the chart went too high initially.  Your comment on volume is well taken though.  As for the leakage past the seal I now wonder if it may have to do with exercising the forks trying to get the last bit out of the one side I drained the second time, while the side with the now-leaking seal was still full.  Normally both drain plugs would be open so this wouldn't be an issue.  Doesn't seem to be continuing to leak.   Keeping the feeler gauge trick in mind if it does though.  Thanks guys!

[Motorandy123]

Bel Ray has seal swellers in it. I have heard it recommended to fix slight leaks.
Maybe the lighter stuff doesn't?

[frankenduck]

You can also purchase 3rd party seal sweller additives.  (Made for transmissions and stuff but would probably work in a fork too.)

[brickme]

Bel Ray has seal swellers in it. I have heard it recommended to fix slight leaks.
Maybe the lighter stuff doesn't?

It was the heavier, Bel Ray side that leaked.  Since it stopped now I think it was my pushing up and down on the fork with one side open/draining and the other side sealed/full that caused oil to blow by the seal temporarily.  Or perhaps the Bel Ray lived up to its billing by stopping the leak!?  Next time I will be able to change out both at the same time with the same oil so there shouldn't be a problem.  Might try some Bel Ray 5w next year if I can find it locally.  Or some custom mix of what I have left over.  Anyone know who makes BMW fork oil or what it is equivalent to on the viscosity charts?  I have a liter of 10w but would like to know where it falls rather than going through "trial-and-error" again.

[healthy]

The post in the lieberry has Bel-Ray 5W as the recommend weight.

http://www.motobrick.com/index.php/topic,982.0.html

you can always mix down the 10W with a 5W to get something in between / not waste what you have left.

but as others have said, adjusting the volumes (within reason) can also greatly affect your response.