Front fork rebuild

Common faults.

1. Worn and leaking fork seal.
2. damaged bush causing scoring to chrome
3. Pitting, scratches or damage to chrome

Seal dimensions:

• Fork seal (oil seal): 41mm X 54mm X 11mm
• Dust seal: 41mm X 54mm
  

Note: Only use genuine honda seals,
Even reasonable quality pattern seals don't work properly.
One fork seal + one dust seal: Honda P/N: 51490-MN8-305

The fork tube bushing is also known as: Lower bush, inner bush, black teflon on outside.
The slider bushing is also known as: Upper bush, outter bush, black teflon on inside.

If only one fork leg is leaking and the other
leg was recently done and is in good condition then
I would only repair the leaking leg, hopefully you
will avoid a few complications this way.
Note: It is important to keep both fork legs the same,
If you overhaul one leg you should change the oil in the
other leg to ensure that the same type of oil is in each leg,
also, ensure the oil level (air gap) is the same in each leg.

Tools & parts.

• You will need:
  • New seals
  • In my opinion you should also replace the upper bush.
• You might need:
  • One 6 mm copper washer for each leg
  • One 8 mm copper washer for each leg
  • Bushes
  • M8 x 21 mm socket bolts with low profile heads.
• Seal driver that will work on 41mm fork tubes.
  Note: To make your own seal driver see "Foot note 5".
• Socket and rachet for fork tube caps.
• 6mm, long reach, HEX key (allen key).
• Flat head type screwdriver.
• One liter of 10W Fork oil.
• Some other common tools.
• Clean rags.

Disassembly.

1. Loosen the fork tube caps while the tubes are still clamped
   onto the yokes(triple clamps) but don't remove them yet.
2. Loosen the front wheel axle bolt.
3. Lift the front wheel off the ground.
4. Swing the front brake caliper off the disk and then
   remove the front wheel.
   Note: Do not hang the caliper off the hose or twist the hose.
5. Fully loosen the handle bar pinch bolts.
6. Loosen the yoke clamps.
   Note: Do not allow the fork legs to fall out.
7. Remove the circlips on the very top of the fork
   tubes.
8. Remove the fork legs from the yokes.
   Note: Do not allow the switch gear wires or brake hose
   to become strained.
9. Place the bottom of the fork leg in a tray.
10. Remove the oil drain bolts and their 6mm copper
    washers and pump the fork leg to remove all the oil.
11. Wrap the slider in a towel and place it in a vice.
    Note: Tightening the vice on the slider will damage it.
12. Remove the tube cap.
    Note: the fork tube caps are under spring load.
    Note: the caps have aluminium threads which are easy
    to damage.
13. Remove the spacer, the spring seat and the spring.
14. At the bottom of the fork leg there is a M8 bolt
    witch takes a 6mm allen key, Remove this bolt.
    Note: The damper piston inside the fork leg may fall out
    when this bolt is removed.
15. Remove the piston by tilting the fork leg to
    allow it to fall out.
16. Use a flat screw driver to lift off the dust seal
    and then take it off the tube.
17. Remove the stop ring that holds the fork seal
    in place.
18. Pull the fork tube out of the slider until the
    lower bush contacts the upper bush, then lightly
    tap the lower bush against the upper bush until the
    fork seal is diven out
    Note: Make sure the stop ring is removed before you
    start tapping.
    Note: 3 or 4 light taps should drive out the seal.
    Note: If the upper bush and fork seal are difficult
    to remove read "Foot note 1".
    
19. Remove the fork tube from the slider.
20. Remove oil lock piece, or if it is stuck in place
    inside the bottom of the slider just leave it in place.

Inspection.

Bushing inspection.

The buhsings are split steel bushings.
On the sliding side they have a layer of copper on
top of the steel, and a layer of teflon fabric over the bronze.
In my opinion you should replace the upper bush because the impacting on it
when driving out the fork seal can damage it resulting in scoring to the
chrome on the fork tube later.

• The sliding surface of the bushs are covered in
  black Teflon, if most of the black Teflon is worn off
  then the bush should be replaced.
• Replace bush if it has deep scores on it.
• Replace bush if it has any other sign of damage
  or distortion.
• Replace bushes if you had to tap hard to drive
  out the fork seal from the slider.
• Replace upper bush if there are straight vertical
  score lines on the chrome of the fork tube where the
  bush slides, since this was probably caused by a
  damaged bush.
• Replace the lower bush if there are scores or
  scuffing on the inside sliding surface of the slider,
  since this is probably caused by a damaged bush.
• Replace the back up ring if there are any signs
  of damage or distortion.

Fork tube inspection.

• Measure the runout of the fork tube, it should not exceed 0.2mm.
• Inspect fork tube of pits, scratches or dents, see foot note 2.

Slider inspection.

• Check for corrosion, dents, scratches in the seal bore.
• Check for scoring or scuffing or distortion of the sliding
  suface inside the slider.
• Read foot note 3 for further info.

Assembly.

1. Rinse and clean all metal parts in high flash
   point solvent and then dry them.
2. Insert the piston, with it's rebound spring, into
   the fork tube and gently let it slide down until it's
   tip comes out the other end.
3. Insert the oil lock piece onto the tip of the
   piston and push it into the fork tube, hold the tube
   horizontal to keep it in place.
4. While holding the tube horizontal, insert it all
   the way into the slider, turn it vertical and insert
   and tighten the socket bolt (8mm copper washer)
   Note: If using a non-original bolt the head of the bolt
   might interfere with the front wheel axle, so insert the
   axle into the hole to make sure it fits.
   Note: Check to see if there is an old copper washer stuck
   inside the hole.
5. Drop in the upper bush and back up ring and then
   use the seal driver to drive them home.
   Note: Before you drive in the fork seal check to see if
   there is a jog between the fork tube and the slider,
   there should be little jog.
6. drive in the new fork seal.
   Note: Internal lips of the seal should point down.
   Note: If the fork seal is difficult to drive in: STOP,
   Remove the fork seal and see "Foot note 3". Note: Applying red rubber grease to the sealing
   lips can ease break in.
7. Install the stop ring and dust seal.
8. Insert the spring, spring seat and spacer.
   
9. Install and tighten the oil drain bolt with
   it's 6mm copper washer.
   Note: Correct torque is probably around 8 Nm.
   Note: To re-use copper washers see "Foot note 4".
10. Add 10W fork oil to each fork tube.
    Oil capacity:
    Bros 400/650 J: 133mm, 500cc.
    Bros 400/650 K: 128mm, 502cc.
    Bros 400/650 L: 133mm, 514cc.
    Note: Too much oil may cause hard front suspension
    or the oil seals to burst.
    Note: You could use 15W oil for a stiffer suspension,
    20W is too heavy in my opinion.
    Note: Heavy weight oil will not reduce the occurrence
    of bottoming out. Note: The mm valve is the air gap. Air gap should be
    measured with the fork leg fully compressed
    and the spacer, spring seat and spring removed.
11. Install and tighten the tube cap.
    Note: The aluminium thread are very easy to cross
    thread and damage.
    Note: Check the O-Ring for damage.
12. Torque values:
    • Front axle bolt (M14)(steel to steel): 60 Nm
    • Axle pinch bolts (M8)(steel to alloy): 22 Nm
    • Caliper mounting bolts (M8)(steel to alloy): 27 Nm
    • Fork bottom socket bolt (M8)(steel to steel): 17 Nm
    • Fork oil drain bolt (M6)(steel to alloy): 6-8 Nm ???
    • Top yoke pinch bolt (M7)(steel to alloy): 11 Nm
    • Bottom yoke pinch bolt (M10)(steel to steel): 50 Nm
    • Handle bar pinch bolt (M6)(steel to alloy): 10 Nm
    • Fork tube cap (M37)(alloy to steel): 23 Nm

Foot notes.

Foot note 1.
If the fork seal is stuck in the slider and won't
come out with light tapping then you'll have to tap
harder and harder until it does come out, however this will
destroy the bushes, so they should be replaced afterwards.
If you tap too hard then the lower bush may wedge inside
the upper bush which would be a complete disaster with
no easy remedy. When installing a new fork seal make
sure it's not too tight to prevent this problem occurring
next time.
_________________

Foot note 2.
Carefully inspect the swept area of the tube for scoring,
pitting or scratches, anything that will catch your finger
nail is a problem.
Also, when installing a new seal you will have to slide it down over
the top of the tube, if there are pits they might damage the seal on
the way down.

Scores:

• Usually straight vertical scratches on the chrome caused by a bad bush.
• Sand off with 800 paper, then 1200 and then polish.
  Note: Try to avoid sanding thru the chrome to the nickel underneath as
  this will destroy the fork tube. Chrome has a slight blue tint to it and
  nickel has a slight yellow tint to it.
  Note: Try to minimise the amount you sand off as sanding can destroy the
  fork tube, however if sanding is the only way to put the fork tube back
  into service then it's worth a try.
  Note: It may not be nessacary to completely remove the score, simply
  removing it's sharp edges may be sufficient.

Pitting:

• Rust spots on the chrome caused by age, wheather and stone chips.
• Remove any sharpness with sand paper, 800 then 1200.
  

Scratches:

• Same as pitting.

Foot note 3.
If the fork seal is difficult to drive in then there may be
corrosion, dirt, dents or distortion of the metal where the
seal fits.
Note: If it is difficult to drive in a seal then it may be
impossible to remove it next time.

A 55mm flap disc fitted to a drill is good for cleaning the bore.
Then sand with finer paper.

Check the "out of round" or "ovality" of the bore with some
kind of bore gauge to see if this is the problem.

If the seal bore is just too tight then sand with 80 paper then
240 then 400 and polish.

Apply lots of red rubber grease to help the seal go in a
little easier and come out easier next time.
_________________

Foot note 4.
To reuse a copper sealing washer heat it until it is red
hot and then dip it in water, this should soften the
copper and a make a better seal, this process is called
annealing.
_________________

Foot note 5.

I made my owm seal driver by using a 15cm long piece of steel pipe with
an outer diameter of 51mm (2 inches)and an inner diameter of 48mm.
As you might know the fork tube has a diameter of 41mm so the internal
diameter of the pipe is to large. I fixed this by inserting one of
those plastic joiners for a 40mm plastic drain tube (the type of pipe
that connects up the drain under your sink, any hardware store
should have one). You should buy two of these joiners, one for each end
of the pipe. On my pipe the plastic joiners were two large to
fit into the pipe so I cut a slit in them lenght wise and then put a
piece of sand paper into the slit and rubbed it to make it wider
until I was able to hammer it in (Make sure to to sand off too much
as it needs to be tight). There is a lip inside the joiner
which will prevent it from fitting over your41mm fork tubes so use
a round file to wear it down, but be careful not to wear off too
much as you want it to fit snugly on your 41mm fork tubes. Make
sure that the side of the pipe that will be driving in the seals
is flat and free from any sharp bits as oil seals are very
delicate and are easily damaged. And of course the most important
thing is that there is nothing that might damage or scratch the
chrome plating on the fork tube as a pair of new fork tubes will
cost 550 Euro.

Trouble shooting

Fork leg leaking oil.

• Worn or defective oil seals.
• Damaged or worn fork tube.
• Defective copper washer seal on oil drain bolt
  or damper piston retaining bolt.
• Too much oil in fork leg.

Fork leg leaking oil after installation of new oil seals.

• If you used pattern seals they were probably defective (use genuine seals).
• Too much oil in fork leg.
• Damaged or worn fork tube.
• Damaged or worn bushings.
• Damaged or worn fork slider.

Front suspension too hard.

• Too much oil in fork legs.
• Bent or damaged fork tube(s).
• Damaged fork slider

Front suspension too soft.

• Insufficient oil in fork legs.
• Weak fork springs.

Front suspension noise.

• Slider binding.
• Insufficient oil in fork legs.
• Loose fork leg fasteners.

Hard steering.

• Steering bearing adjustment nut too tight.
• Damaged or worn steering stem bearings.
• Insufficient air in front tyre.

Steers to one side or does not track straight.

• Bent fork tubes.
• Bent front axle or wheel installed incorrectly.
• Unequal oil quantity in each fork leg.

Front wheel wobbling.

• Bent rim.
• Worn wheel bearings.
• Axle or axle holder not tightened properly.

Vertical scores or scratches on the chrome fork tube.

• Damaged or worn upper bushing.
• Excessive play between fork tube and slider.
• No oil in fork leg.