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  fork rebuilds Lnav
   4CS fork kit installation instructions  
  • Click on picture to enlarge.

We copied the rebuild instructions with mv cup here and created a new folder for all these images.  Then we used find and replace so it draws these images from the 'images_4CS_kit_ install_mvcup' folder.

We don't use the mv cup anymore, but we will leave the install instructions up as we have 54 dyno and press tests using the kyb cup.  We may want to refer back at some time.


    4CS kit pieces  
Click on pictures to enlarge.

    4CS kit - using stock mv piston  
'15 and '16 models use the stock mv piston.

- stk '13-'14 mv pist ht = 11mm
- stk '15-'16 mv pist ht = 13mm
- [also see section 14]

    4CS kit  - using KYB YZ 85 mv piston  
'13 and '14 models use the YZ 85 mv piston.

- stk '13-'14 mv pist ht = 11mm
- YZ 85 mv pist ht = 11mm
- [also see section 14]

    1)  Lock 2 - 8 x .75 nuts together to loosen jamb nut.  
Click on pictures to enlarge.

    2)  Remove base bolt.  Pour out oil.  Slide mv rod out the bottom.


    3)  Remove cartridge tube. [top]
Push ctg tube out from bottom.
Remove plastic spring holder, metal washer and preload rings.

    4)  Clamp tube in vise with 24.6mm clamp block and disassemble. [top]
Remove the seal assembly.
Remove seal assembly and slide off the icpiston (you do not have to disassemble the icpiston).  Check for oil inside icpiston. 

For some yet unknown reason, oil can accumulate in the icpiston.  This fix is to simply clean the parts and reassemble.  Not very scientific, but we haven't determined the cause.

We recommend replacing the 8mm ctg rod seal on a regular basis.  No need if the forks are near new.

CAUTION!:  Do not spray brake cleaners on the blue orings.  It can remove the blue coatings on the oring surface.
Check subtank (left end with screw driver), some earlier units have been known to be loose.  The subtank does not need to be removed for the kit installation.

If it's tight, it's on there with some good loctite.  Don't worry about breaking it loose with this quick check. 

If it's loctited and needs to be removed, heat should be applied.

    5)  Split check assembly and install plastic plug.
                ---> The plastic plug closes the top check and bleed hole.
Start with sharp wood chisel and progress to cold chisel.
Start with sharp wood chisel.
Finish with fatter cold chisel.
The plastic plug replaces the metal check shims and wave washer.
Press in the plastic plug (do not use check shim and wave washer).
Press together the pieces.
NOTICE!:  The 8mm bushing can pop out of the older style check assembles found in some '14's.

Our fix was to machine and press on a metal cap.  We also tried loctiting the bushing in place, but are unsure how strong the loctite holds.  
The factory corrected this in '15.

    6)  Install bottom sleeve with loctite.
        ---> This closes the bottom bleed hole.
A primer should be used when dealing with "inactive" metals such as anodized aluminum.  We recommend Loctite 7649 primer.  For maximum hold, we also use Loctite 680 retaining compound. But 680 is considered permanent, is expensive and hard to find, so red will do.

NOTE!  The primer is a necessity!  Also note that the primer can greatly reduce working time to as little as 3-5 seconds.

[click for additional info on primer]
Apply loctite and position the aluminum sleeve over bleed hole.

The aluminum sleeve does not press fit, but is somewhat loose fitting.  This makes for easier installation and gives a bit of space for the loctite.

    7)  Reassemble icpiston and seal assembly.  Use red loctite on seal assembly threads.


    8)  Remove mv piston and shims. [top]
The factory thread seal makes it difficult to remove shims.  We clean it with a 6 x .50 die.

Stock parts.

Notice the stock mv spring.  When installing the KYB mv cup, use the KYB mv spring included with the kit.  The stock spring wire diameter is too fat, and when compressed the spring will not fit in the cup.

The kit includes a mv cup, mv spring and three shims under the cup.

    9)  Mv spec sheet. 
        Print the 4CS spec sheet:  open any record > Forms > Fork Spec Sheet - 4CS mv cup
        ---> Notice this build uses the KYB YZ 85 mv piston, and this is recorded on the spec sheet. 
               We also use the stock '15 WP mv piston.  [click picture to enlarge]

    10)  Measure and install the mv cup and shims under the cup. 
          ---> This is used to determine mv float.

String the pieces on screw driver shank, spray with contact cleaner and blow with air hose.

It's important to build the mv with the intended float.  The most accurate and repeatable way is to measure the shim stacks and related pieces as shown in steps 10-11-12-13. 

Measure thickness and record.

The supplied mv cup and 3 - 15.3 shims will always measure 4.42mm.


This shows the cup protrudes .42mm above the 11mm collar on mv stem. 

This is used to determine the mv float.

    11)  Measure thickness of 7.5mm od shims* that fit above the slide collar.
                 [* 7.5 od shims are supplied with kit.  See section 14 for details]
          ---> This is used to determine mv float.
String the shims, spray with contact cleaner and blow with air hose.

For accurate measurements, it is important to spray with contact cleaner and blow with air hose.
Measure thickness and record.
Note location of measurement.

    12)  Measure and install the mv shims.
          --->  This is used to determine the mv float.
String the shims, spray with contact cleaner and blow with air hose.
Measure thickness and record.
Note location.

    13)  Measure and install the rebound shims.
          --->  This helps determine the bullet check needle stroke.
String the shims, spray with contact cleaner and blow with air hose.
Measure thickness and record. 

The reb stack thickness is not critical.  This measurement is only used to determine bullet check needle stroke.
Note location.

    14)  Assemble the mv. [top]
Install the KYB mv cup and shims under the cup.
Install the mv spring and shims.  Use the supplied KYB mv spring.  The stock spring wire diameter is too fat and the compressed spring length is too tall.

Before we add any shims above the slide collar, notice the shims sit flush or slightly above the stock slide collar.  Use extra care when installing mv piston so shims don't slip up and over the slide collar.
Install the supplied 7.5 od shims above the slide collar.

7.5mm od shims are needed above the slide collar.  8mm od shims do not work.

NOTE!:  8od shims DO NOT fit in the stock 15 mv piston indent.  7.5mm od shims are required.

Install the piston and reb shims.  Install ck needle facing correct direction.   We torque to 30 lbs/in.

This example shows the KYB YZ 85 mv piston.

- - The stock WP 14 mv piston and the YZ 85 mv piston are 11mm tall. 
- - The stock WP 15 mv piston is 13mm tall, and the 15 stem is 2mm taller. 
- - The 15 mv piston cannot be used on the 14 stem.
- - All things being equal, if you use the YZ 85 mv piston on the 15 stem, you will need to add 2mm spacer on top of the rebound stack. 
- - BUT, because the shims above the slide collar basically moves the piston up, the washer thickness needs to be adjusted accordingly (precise details below).

    15)  Measure bullet check needle stroke.
          --->  With any major change to the mv assembly (such as
          different thickness mv piston or adding shims above the slide
          collar), the bullet check needle stroke needs to be measured.
Once assembled, measure the distance from the top of the side hole in stem to top of the bullet.   This distance determines the check needle stroke.

This example shows the '15 bullet.
Take these measurements to determine check needle stroke.

The 2014, 2015, 2016 bullets are all different.  We recommend measuring your stem and bullet for correct dimensions.

This example shows a 2015 stem and bullet.

We have measured the bullet height (with check needle bottomed) for the 2014, 2015 and 2016.  We have noticed some variation in these dimensions so it is advisable to take this measurement yourself (as shown above).

14 stem = 80.8 mm when check needle is bottomed.

15 stem = 79.8 mm

16 stem = 45.5 mm

The ck needle stroke should be between 1.2 mm - 2.2 mm.  But in some instances it can go over 2.2mm with no ill effects.

    16)  Fill out spec sheet to determine mv float (see section 9). [top]

Record slide collar height.  Note location on spec sheet.

The 4CS (8mm) slide collar height is always 2.60mm.

Record piston indent.

Stock '15 mv piston indent = 1.03mm.
[We have measured many mv piston indents, and found a good average to be 1.03mm]
Side Note:
stk 14 mv pist ht = 11mm

stk 15 mv pist ht = 13mm

Stock YZ 85 mv piston indent = 1.02mm.
[A good average is 1.02mm]
Side Note:
yz85 mv pist ht = 11mm
On the spec sheet, subtraction the cup protrusion and mv stack thickness.  This float = .28mm.  Measured float will always be aprox .05mm less.

    17)  Remove stock bv piston and install mod piston. [top]

Unscrew hydrostop with wide blade and remove connecting nut.

The stock bv piston and base bolt will be replaced.
Split the bv piston from the steel housing the same as the mv check assembly  (#5).
Use the stock steel housing, check shim and spring.  Replace stk bv piston with mod piston.

The mod bv piston may fit loose in some housings.  We have noticed this in a couple 14's.
If loose, use tool to tighten top of housing. 

Tap down on stock steel housing until it is flush with top of tool, then push out.  This will tighten housing.
Press the pieces together.
Don't over do it.  There may be a slight gap between the mod bv piston and housing.

    18)  Drill stock connecting nut.

Drill 2 holes on opposite sides of the stock connecting nut.  Use aprox 1/16 or 1.5mm bit.

This is required on the 14 and 15 models.  The 16 has a regular nut and does not require drilling.

Use dial caliper and scribe a mark 2mm down.  Use center punch to mark location for drill.

Secure the connecting nut in drill press vise.  Drill slowly, especially when the hole is nearly through.

    19)  Assemble base valve.
          --->  These instructions are specific to the gp adj base bolt.

Do not cover the relief hole.  Use max 11mm base shim.

The base bolt will accept a low speed shim stack.

Measure the thickness of the bv stack.  The stack thickness must fit within a specific range. We use:
--> bv stack thickness range = 2.3 to 3.8 mm
--> we generally set the stack height around 2.35 mm

The assembled bv piston height must fit within a specific range.  In this example:

- - bv stack thickness = 2.95 mm
- - piston ht = 39.6 mm
- - connecting nut ht = 51.25

Use loctite and torque the nut 30 in/lbs.

This diagram shows the minimum and maximum range for the bv stack thickness and piston heights.

Untitled Document Untitled Document
 fork rebuilds rnav
4CS fork kit installation
4CS kit install instructions
4CS kit tools
4CS fork - fill ctg tube
4CS fork - setting oil level
4CS fork - spring preload
4CS kit explaination