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Old 11-08-2011, 06:03 PM
BleedMarshall's Avatar
Intermediate Class
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Last Online: 08-28-2012 11:40 AM
Location: Kentucky
Posts: 104
Default 2006 YZ125 KYB Front Suspension Rebuild


2006 YZ125 48mm Kayaba Fork Rebuild

*Parts of this write up are intended for those attempting this job for the first time, or any mechanical rebuild for the first time. Experienced builders should bear with certain aspects. Thank you.

**I also had to steal some screen shots off the web to replace corrupted pictures I took. For some reason, some pictures became corrupted on download and were erased. My apologies.

To start, I would like to say that the write up has taken longer to do than the actual teardown and rebuild of the forks themselves. That took about an hour and half to complete, while this write up has taken around double that. I have tried to be as thorough as possible while not boring you to death either. I hope I succeeded; however, I could be wrong.

When my seals let go in my 2006 YZ125 I called several shops to see what it would cost to replace what amounted to $50-$75 worth of parts. I got prices ranging from around $200 to $400, however, upon looking into what it would cost me to do it myself I found I could buy the parts and tools needed for around $100-$150. If I went this route as I could then replace the seals and such later for far cheaper. I opted to buy my tools and parts and break into this task myself. It is fairly simple, if you have any mechanical ability and a few tools. Below is a list of tools and parts you will need and those I bought. Please note, I had several of these tools already, which I will denote with an “*”.

KYB Fork Wrench w/ Cap
48mm Seal Driver
*5” Bench Vice (Soft Jaw Needed. You can use folded rags as well.)
*Small to Med. Flat Screwdriver (Snap Ring Keeper Removal/Installation)
*Small to Med. Phillips Screwdriver
*1/4” &/or 3/8” Drive Ratchet
*17mm Socket (Damper Assembly)
*15mm Open End Wrench (Damper Assembly)
*12mm Socket (Axle Pinch Bolts & Caliper Bolts)
*10mm Socket (Fork Pinch Bolts)
*8mm Socket (Fork Guard Bolts & Number Plate)
*3/4” Open End Wrench (Fork Cap)
*Dead Blow Hammer

Now, my job became a bit more intense as I had to replace a free piston on the base valve assembly for the non-caliper side of my forks. I could have paid about $65 for a new plastic free piston. However, and me being as I am, I found that the better alternative would be to replace them with “works” style aluminum free piston assemblies for about $75 each. I opted to spend the $150 for two free pistons. My price total was just over $315 now, but I will not have to worry about free piston breakage, nor having to buy the tools and I am still only going to be out around $50-$75 in parts from here on out. Here is my part list for the entire rebuild of my 2006 YZ125 Kayaba Front Suspension.

2: Aluminum Free Pistons
2: Wiper Seals
2: Oil Seals
2: Metal Slide Bushings
2: Metal Bushings
2: Quarts of Fork Oil

To begin, make sure your work area and tools are clean and free from debris. Under any circumstances do you want any foreign material to enter the forks. After you have cleaned your area, make sure you have a way to support the bike, as the front end will be removed from the bike. If you do not support the bike correctly, it will tip over and could cause serious injury or damage to the bike. If this is your first time rebuilding a dual chambered, inverted fork system, you may want to have a friend help you. Also, make sure to have your manual handy, as you will need it for certain aspects within this rebuild. Here is how I supported my bike by using two tie down straps and an overhanging support rafter within my work area.

I also placed the bike into 1st gear to help it from freely rolling while I worked on the bike. This also allowed me to raise and lower the bike, as I needed as the tie downs were ratchet straps.

I next removed the fork guards, brake caliper, number plate (as doing so allowed me easier access to the fork caps and adjustments), and front wheel.

I then released the pinch bolts from the triple clamps so I could remove the forks individually. However, when doing this be sure to only allow the forks to slide down enough so you can still get your fork wrench onto the tops of the forks, as it is easier to loosen then while they are on the bike. After you loosen them, then you can relieve tension on the pinch bolts and fully remove the fork(s) so you can lay out your parts on your workbench or work area.

Now we can start to fully disassemble the forks one at a time so we do not get any parts mixed up. Begin by setting your adjustment screw (The single slotted screw in the picture) all the way in clockwise and count how many times it clicks. The click will be only slight so go slow and pay attention. Once you have counted how many times it clicks in, write it down for reference on your rebuild, and then adjust them all the way out counter clockwise.

Using your fork wrench, loosen the outer assembly from the inner as shown below. (This is where it is much easier if you did this prior with the fork still on the bike.)

Once accomplished you can drain the oil from the outer fork tube and then begin the tear down and removal of the inner assembly from the outer. (You will notice two small holes on either side of the inner fork assembly. Oil may come out of these holes, so be careful and it is a good idea to rotate the inner assembly to accomplish this.) Once the oil has drained completely, hand tighten your outer and inner assembly again. You will now need your fork wrench and your vice at this stage, as well as your 15mm wrench and 17mm socket as well.

You will now want to secure your fork in the vice using the caliper mount or axle boss so you can secure the fork properly since we will need to apply a significant amount of force for a step in a moment or so. You will need to have a soft jaw vice for this, or you can fold up a few rags and place between the vice jaws and caliper mount or axle boss.

Once secured, you will now need to loosen your damper adjustment from the bottom of the fork with your 17mm socket. To do this, simply back off your adjustment as we did before and write down the clicks for reference later on the rebuild. Now, using your 17mm socket, loosen the damper assembly from the fork. Once it is loose, you can apply force to the top end of the fork, which will push the inner tube outward towards the end. You will need to take the slotted end of your fork wrench and place it behind the locking nut to hold the inner assembly in place so you can remove the bolt cap.

To remove the bolt cap, take your 15mm wrench and hold the locking nut, you can then remove the bolt cap with your 17mm socket as shown below.

Once you have removed the cap bolt, you will also want to remove the push rod as well. It will slide out easily as it is held in place by the cap bolt. You can now release pressure by forcing the inner out and removing your fork wrench. You will now need your flat head screwdriver to remove the dust seal and snap ring. Make sure to lay your parts out in order and the way they came off the bike. This will help you later on the installation of the new parts. Be careful not to damage the inner tube with the screwdriver. The dust seal will take a bit of working at times, so be patient. Once the dust seal is removed, you can remove the snap ring. This is a part that can be reused, so be careful not to damage it when removing it. Remove the fork from the vice and proceed to the rest of the parts removal.

To remove the oil seal, simply remove the outer fork cap with your fork wrench and lay the inner fork assembly out on your workbench or area, as well as the upper spring seat and spring. Now, extend the outer tube until it bottoms out against the seal. With a few slight, yet forceful tugs the seal will come loose. Now remove the metal washer, piston, and bushing and lay them out as well. The metal washer will be reused, so do not lose or damage it either.

Now we can start our disassembly of our inner fork tube. You will need both the fork wrench and fork cap tool as we as your Ύ” wrench or socket. Place the fork wrench on the outer part of the inner fork, and the cap wrench on the inner section. By placing your wrench or socket on the cap, you can apply force to remove the base valve from the inner fork section.

Once the base valve is unthreaded from the inner fork tube you can remove it, however, there will be a certain amount of suction as there is oil within the inner tube. Be careful not to spill any oil, but drain it as you did the inner section. While draining the fluid, work the dampening rod in and out so you can remove all the old fluid from the inner tube. Inspect your parts, and especially the base valve. It should look as follows.

However, it is cracked, busted, or in any way broken, it must be replaced. My non-caliper side looked like this below. The tape was there to hold it together apparently.

If your free piston, or pistons are broken, you need to disassemble the base valve and inspect the parts for damage. However, be very careful, as there is what looks to be slid pieces that look like small plates. These are not solid, but several shims stacked together to form what is known as a shim stack.

Be careful not to lose or rearrange the way they are tacked together. I found a good way to keep them together is to use a small piece of wire to hold the parts in place. To disassemble the base valve you will need a 10mm socket to remove the nut at the bottom of the base valve. Again, be careful as there is a small plate that hides and houses a spring. Under that spring is a small bushing, then the shim stack. Once you remove the shim stack and have it secured you will need to remove a bushing/plate that holds the free piston and base valve spring in place. It threads off with less than considerable force. When you have it disassembled you should have it laid out in a similar fashion as below. (Note the aluminum free piston beside the broken one.)

You can go online at Suspension Direct KYB / Kayaba Motocross Components and Parts and find the parts you need to replace any OEM part within the base valve comp., however, I decided to use the aluminum free pistons machined specific for my KYB suspension via Riding apparel and gear, tires, helmets and more. Privateer Connection Rock Hill, SC (803) 980-7200 owned by Chuck Shirley. You will now simply reassemble in the reverse order and should end up with what you see below. Though only one free piston was broken, I decided to replace both at the same time for less worry and hassle later on.

Once everything is disassembled and laid out, it should look similar to the picture below. Now is a good time to inspect each fork tube for any damage, including any dents, dings, or scrapes to the inner fork tube. After a good inspection, you can clean all the parts in a mild solution to remove any debris from the forks.

Last edited by Woody_393; 11-10-2011 at 11:54 AM.
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Old 11-08-2011, 06:03 PM
BleedMarshall's Avatar
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Last Online: 08-28-2012 11:40 AM
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Default 2006 YZ125 KYB Front Suspension Rebuild

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Last edited by Woody_393; 11-10-2011 at 11:53 AM.
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Old 11-10-2011, 11:48 AM
Woody_393's Avatar
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Last Online: Today 07:56 AM
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Nice job!!! I see you went with Aluminum free piston. I think that's a good idea. That free-piston made of plastic/nylon is a bad idea IMOP.

Again, nice work!!!
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Old 11-10-2011, 01:44 PM
MXtras's Avatar
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Join Date: Jun 2006
Last Online: 09-23-2014 02:14 PM
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Nice write up!

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Old 03-11-2012, 06:11 AM
Welcome To ATM
Join Date: Mar 2012
Last Online: 03-12-2012 02:35 AM
Location: N Ireland
Posts: 1
Thumbs up excellent re-build instructions....

hi there

im just about to do the same for a 2008 yz250f i bought 2nd hand at the end of last year - complete with shattered free pistons.

Although i've found a few instructions, for example:

the smartperformanceinc com YZ Free Piston Mod and Cartridge Install (this is my first post so im unable to post the full link for you )

your picture step by step guide is excellent. many thanks.

may i ask a question of you (or any other readers with direct experience also) regarding the performance of your forks post rebuild and with the inclusion of the aluminium free pistons....

the original 'plastic free pistons' have no holes in them. you will note that in the smart performance article they recomend drilling 2 to 4 small holes to stop the pistons cracking. i see the new pistons you used have what appear to be quite large 'holes' already made for you.

Question: have you noticed much change in the performance of your forks with the new modded free pistons?

If so a i'd be most grateful for a little description of the 'new' handling as compared to the old. it would also be most helpful to know what sort of riding you are doing (i.e. motocross etc) and at what sort of level (hobby, amateur racing, semi pro, pro etc).

many thanks in advance for any info you (or others) can supply.


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Old 03-20-2012, 01:50 AM
Welcome To ATM
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Last Online: 03-20-2012 01:50 AM
Location: australia
Posts: 1

can you send me a link for the free aluminium piston in the base valve because i cant find one anywhere and awesome write up man. appreciate it tones
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