Hi All,
Yesterday I took a 25 mile ride on a new to me 1992 K100RS.  After the ride, the final drive was quite warm, almost hot, but I could hold my hand on it.  Brake rotor was cool, swingarm was cool, oil level in final drive was up to bottom of threads of fill plug hole.  
Is this normal?  I’m not that familiar with the K bikes.  If not normal, what might be a cause?  Thank you.

The whirring ring & pinion with a coupla hundred cc's of (GL5, hypoid 75/ or 80W90) gear oil do make quite a party. I'm sure the dance floor was hot. Unless you're seeing chunks in the oil or hearing an odd rythmn I'd not worry, hands on means it's near normal. 

An infrared thermometre gun aimed at the casting after a ride will allow you to obtain accurate readings for records purposes.

Back in an earlier life situation engineering hydraulic systems an old timer told me that your hand is a great temperature gauge for hydraulic and lubricating oil and the general happiness of mechanisms.

The maximum surface temperature that the typical person wants to touch is about 135 degrees F.  That is the temperature of a surface that is uncomfortable, but you can touch it for as long as necessary.  Above 135F the reflex is to pull your hand away.  Around 110-115F the surface feels pleasantly warm and you can touch that surface as long as you want.  That is also the temperature range where oil and the machinery are the happiest as well.

135F corresponds to about the upper range of temperatures that general purpose hydraulic and machinery gear oils like to operate at.  So, at 135F your oil will work happily for as long as required with no problem.  Synthetic and heavy duty gear lubes will run at higher temperatures before molecular breakdown.  That usually occurs at temperatures well above 250F.  Viscosity and film strength usually are good to well over 190F, so up to that temperature as long as there is enough oil to create a film, wear will be minimal.

Hope this helps.

Wow!  That’s is a tremendous help!  I really appreciate your wisdom.  Based on your info, final drive housing temp must be about 130-135 degrees.  There is GL-5, full synthetic, 75w90 Motul gear oil in it.  So it sounds like it’s fine.  Thank you!

Wirelic wrote:Hi All,
Yesterday I took a 25 mile ride on a new to me 1992 K100RS.  After the ride, the final drive was quite warm, almost hot, but I could hold my hand on it.  Brake rotor was cool, swingarm was cool, oil level in final drive was up to bottom of threads of fill plug hole.  
Is this normal?  I’m not that familiar with the K bikes.  If not normal, what might be a cause?  Thank you.

My good friend who passed away over a year ago was a local BMW tech. Whenh e retired he still bebuilt final drives for them.

His avice always was keep good oil in them, but also keep an eye on the temperature. They do get warm as a matter of routine BUT getting hot quickly was a sign of the big bearing on the way out. He could rebuild them at that point very quickly. The next step is the seal fails. 

My first K1100 did a specatcular fail, came back from France, on the way back from the ferry, an 8 km journey, I stopped at a supermarket of supplies, then 2km home. All was good at the supermarket, but in those 2km home the seal gave out. I parked it and took another bike instead. So I rode it another 15km to a friends place and in that 15km the bearing race spat out the dummy but I got there.

For now, change out the oil, have a good look at what comes out and keep a very close eye on the temperature. 

You may be ok. My RT monolever did what ours did and ran another few months until the seal leaked and bearing was gone.