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Author Topic: Sport Fork Damper Overhaul  (Read 5268 times)

Offline TimTyler

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Sport Fork Damper Overhaul
« on: May 18, 2016, 04:36:31 PM »
Trying to figure out why my front end is bouncy. I have the Sport forks and guessed that maybe the O-rings in one of the dampers was shot.

I disassembled the forks and found that one of the O-rings was in pieces. There are two. Ordered replacements from the dealer, due tomorrow. Part # 31421452011 $0.89 each.

I'm curious if the "valve" part #31421454620 is good, so since it's just $4 I ordered one of those too.

Anybody know how to get the cap off the end of the rod so that I can slide on the new parts?
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Offline TimTyler

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Re: Sport Fork Damper Inspection
« Reply #1 on: May 18, 2016, 07:16:06 PM »
I followed the Clymer instructions and took a blowtorch to the piston ring. It came off easily after about a minute of heat.
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Offline The Dude

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  • Posts: 509
Re: Sport Fork Damper Inspection
« Reply #2 on: May 18, 2016, 07:33:37 PM »
Quote from: TimTyler on May 18, 2016, 04:36:31 PM
Trying to figure out why my front end is bouncy. I have the Sport forks and guessed that maybe the O-rings in one of the dampers was shot.


Interesting,for me.I got an '86 K75 S(port).
Back then...only one leg had damping.I learnt,over my years,to incrementally increase the fork oil viscosity to compensate for decline in damping through,presumably,wear in the damping components.Worked for mine.
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  • Auckland, New Zealand
  • K75s
Current.
TR6R 1973 from new.
Moto guzzi 850 III .1983 from,almost, new.
K75S 1986 from new.vin-0103141
On my second millionth km.give or take a hundred k Kay's.
"The Dude abides. I don't know about you but I take comfort in that. It's good knowin' he's out there."
All the best!

Offline The Dude

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  • Posts: 509
Re: Sport Fork Damper Inspection
« Reply #3 on: May 18, 2016, 07:42:52 PM »
On that note of finding things in pieces.
I remember removing my original fork spring from the damping leg in seven pieces!The spring in the other leg was fine(just one piece)We got lots of shit roads, here and I didn't used to slow down for them.
Replaced them with progressive springs,all good for years.
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  • Auckland, New Zealand
  • K75s
Current.
TR6R 1973 from new.
Moto guzzi 850 III .1983 from,almost, new.
K75S 1986 from new.vin-0103141
On my second millionth km.give or take a hundred k Kay's.
"The Dude abides. I don't know about you but I take comfort in that. It's good knowin' he's out there."
All the best!

Offline TimTyler

  • Adrninistrator
  • ^ Quintessential Motobricker
  • Posts: 1884
Re: Sport Fork Damper Overhaul
« Reply #4 on: May 20, 2016, 08:36:37 PM »
Finally got the new O-rings from the dealer and installed them; one on the rod and the other on the "valve" that slides on the rod.

Used a drop of Loctite on the piston threads as per Clymer's. Need to let that set overnight before adding the fork oil.

I've decided to try the Bellray 5W fork oil instead of the "motobrick-popular" Honda SS8 10W after doing a little research. The Bellray has a higher viscosity index and it's cSt numbers are closer to the old school BMW specs.

Cst@40°C (100°F)Cst@100°C (210°F)Viscosity Index
Honda Showa SS8 Fork Oil  10W35.487.38200
Belray fork oil 5W20.56.2280
Military Mil-H-5606E / Old BMW 4WMIN  13.2MIN   4.9??

Reinstalled the fluidless forks on the bike. I need to work tomorrow but I can refill the forks on Sunday and go for a ride.  :riding:
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