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1985 YZ250N, With the Fork off, Cap off, Spring out, take a clear school plastic ruler pore in 5W synthetic fork oil till it's 135mm from the top with the fork tube all the way down, till it just touches the ruler and the 135mm mark. Reinstall the spring, pacer, washer, Damping rod to cap, and install back in the clamps. I raced this bike for 18months in 1985/1986. Change the seals when they leak and toss the 38Kg Fork springs for 41Kg + springs to your riding weight. Over all was a good fork for it's time and Race tech can rebuild the fork and shock to your riding style and you're riding weight. I Miss the Fork flex on hard landings to my other New bikes. I still ride and Race the 85,YZ490 with the same fork.
its not an impossible job, but there are a couple of special tools that make it easier. and a couple of areas to stuff it all up.
A manual would be needed if you havent done it before. The manual takes 4-5 pages of explanation.
A good bike mechanic should do both seals in less than 1 hour, if you remove the forks.
If they arnt leaking oil, they may be OK, The seal that is visible and looks cracked may just be the dust seal, this just pushes into the top of the fork leg above the fork oil seal
I was told by many people it was a hard job. I didn't think so.
What you need is: New Seals. 8mm Allen Key. Half Inch Stocket bar AND Extender. Extra Fine Sand Paper/Polishing Device. Fork Oil + Measuring Jug. Threaded Bar with a Two 19mm Nuts Locked to each other on one end. Spanners. Screw Drivers.
How to do:
First Get the Front end off the Ground (ALOT, Center Stand on Bricks for extra height.)
Remove Front Wheel and Brake Calipers.
Open Drain Philips Screw on Base of Fork (Both Sides). Dont Lose Rubber washer under screw. Watch Out Also for Oil Being under Pressure.
Remove Nut from TOP of Fork (Alowing Handles and mirrors to come loose).
From Top you can see down tube has Nut/Cap with a Square Half Inch center. Use Socket Bar (and Extender if needed) without Bit to Remove reach in and remove. Watchout for Spring Pressure under the Cap/Nut.
Use Screw Driver or Hook of some kind to remove Preload Spacers, Washers AND Spring from Fork.
Slide your Threaded Bar with19mm Nuts (Make sure they are Locked Tight together at one end) down into the Fork and let it catch on the Oil Damper at the base. You will know when it's cought because you wont be able to spin the threaded bar anymore. This will stop the Damper from spinning while you undo the Allen Bolt at the very base under the fork (Outside).
Use Allen Key to remove Bolt from Underside of Fork While having someone or somthing hold the Threaded Bar form spinning.
Now look at the Old bust/leaking Oil Seal. There will be a Clip/Locking Spring in a Groove holding it in place.
Remove Clip without Damaging any part of clip or fork.
If Everything above was Done, you can now give the slider( Bottom Part) a big tug and it will all come lose.
One Part of the Oil damper will either fall off, or be left in the slider(Bottom Part). It slides over the Damper in the base of the tube.
Clean Everything.
Clean Again.
Polish Tube.
Clean Again.
Remove Two Split Washers from base of Tube and Copper Washer. Remove Old Oil Seal.
Place New Oil Seal on Gently.
Replace back on Copper washer, followed by two split washers.
Use Grease as a kind of glue to hold the part of damper that fell off back on.
Slide Slider back onto Tube.
Screw Allen Bolt back into base of slider as per originally was (Remeber to hold Threaded Bar if it starts spinning)
Push Gently new Oil Seal into Fork Slider for Snug Fit. Be Gental.
Replace Clip/Spring over top of Oil Seal Locking it in place.
Remove your threaded bar from top of Fork.
Replace Spring into Fork.
Replace Philips Oil Drain Screw into base of fork (Dont forget Rubber washer).
Now: For a GT550 (I am told also KZ550 but can confirm) you need 300ml of 15 weight Fork Oil. EACH Fork. So a Total of 600ml whole Job. IN EACH SIDE YOU WILL ADD ONLY 300ml of FORK OIL. (15w)
After this. Replace any washers, Preload Spacers and so back into for.
Replace Cap/Nut into Fork Tube with Socket Wrench and Extender.
Replace Handles and Moirrors and Nut holding both of these.
Replace back onto bike the Front Wheel
Check Brake Calipers for leaks and Pad Wear. It's Very Cheap to get new Pads. Not cheap to have months off work after a crash.
Replace back onto bike Calipers.
Pushing Bike (Not ride) test Brakes and Suspension.
At This point I am thinking your done. You may need to Add Air Preload into the Fork via the Air Valve. 4PSI is a Starting Point. I have run upto 15psi. Without Air in the Preload you may find it too soft. Upto you.
To replace the fork seal you must open the top of the fork and remove the spring. Then drop the fork tube out of the clamp. Once the fork tube is removed replacing the seal is simply pulling the old one off and putting the new one on. You will need to remove your front wheel.
It's not difficult, but is messy and needs a special tool or a bit of ingenuity.
You could save yourself a bunch of cash by stripping them off, cleaning them, emptying the existing oil out and cleaning out the fork - then whip it into your local shop for a seal replacement and top up.
Remove front wheel. Loosen fork tube clamps at top and bottom of triple clamps. Slide fork tubes out and remove. Access to seals is from top of tube. Remove cover and unscrew cap. Shop manual for specific bike is $24-$30. Will show detailed instructions.
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