Valve stack

Can anyone tell me what the stock compression shim stack is for the forks of an '01 YZ426? I'm wanting to revalve mine more like a WR and I need to know what shims to omit/add. Also, what ID measurement do these shims have?

[ March 04, 2002: Message edited by: Victor Delgadillo ]


Don't have the factory stack in my records. Sorry.

I could help with a new stack if you want to provide what your current one is, and what you don't like about the ride.

I'm sure others here may have some favorites as well.

Inner ID is .312 or 7.92, which means you would order an 8mm inner ID.

Good luck!


Thanks DaveJ. Unfortunately, I haven't yet removed my base valve. I was hoping to remove it when I had all the shims I needed in hand. I've got some WR shim specifications that was posted by James Dean a while back that he reported worked very well. I primarily ride/race enduros but also ride a few scrambles. I'm looking to reduce the harshness of the forks. I don't have any issue with bottoming. I've read a number of your older posts and they are extremely informative. I remember reading in one of your posts that you simply removed one shim from the HS stack and it made a significant difference. Is that still good information? If so, which one? That might be a good starting point for me.

Hey Tex,

It sounds like you're headed in the right direction. Getting the right valve stack for what you want is mostly trial and error, so if James Dean has already done the work, it's best to repeat it.

Since you are trying to remove harshness, you will most likely be removing shims. So, you may not have to purchase any more than what you already have. In either case, it's always great to have a small collection, so don't worry too much about over purchasing.

And I too first broke into my forks to resolve a harsh ride issue. The factory configuration was a two-stage stack so removing shims from the second or high-speed stack really helps with this. There is, by the way, no magical one-shim that can be removed to resolve your issue, but you can soften the blow by pulling a few of the high-speed shims out.

The only thing to keep in mind is that you need to maintain plenty of low speed resistance. This is what keeps the suspension (and you) in control.

Lately, I've done away with the high-speed stack all together and just run a single stage, (nothing new, many people do this). It's much smoother, but I have yet to fully test this in how this works in combination with my re-designed mid-valve.

Most importantly, don't approach this whole shim-stack thing as voodoo. It's something worth getting comfortable with, and you'll soon find that it's much easier than most people assume.

I use to carry a small portable air compressor and air impact gun in my truck. With this, I could flip the bike over, pop out the base valve assembly, make my changes, and be back on the track all within about 5 minutes. It really can be that easy.

Let me know how things turn out.


Hey guys,

I too have been playing with the valving to try and get rid of some of the harshness in the chop. First I think the mid valve needs to be fixed since the stock one will wear and your damping will get progressively softer. I kept making my base valve stiffer then when I rebuilt my mid valve the forks started to deflect. Now I think I will try stiffing up the mid valve a little to try and make it more durable and softening the high speed portion of the base valve stack to smooth out the square edges. I have not decided what stacks to try yet.

Dave, I seem to rememeber you are on an 00, Did that come with a 2 stage base valve stack? My 01 came with a single stage stack. I converted it to a 2 stage to try to get it to be a bit more plush but may go back to a single stage, since getting sufficient low speed damping, on the 2 stage, seems to make the overall and high speed too stiff. Is this consistant with what you have found? I am using stock pistons.

What did you do to fix your mid valve? I am using C-cycle bottoming cones and plan to replace the first couple of the thin 0.1 shims (I haven't calculated how many yet)with a close to equivalent stiffness 0.2 shim, back that up with the remaining 0.1's and add some shims to fill in some of the gaps on the taper to try to add more support for the stack. Then back off on the base valve (mainly high speed) to smooth the ride out and compensate for the increase in active valving. I theorize the first shim on the mid valve is being over worked and a thicker shim in this location should hold up better, especially with the added support in the remainder of the stack.


Hey Jason,

Well it sure sounds like you have put some thought and time into your forks.

And yes, I'm still pounding on the 00. It did come with a two-stage by the way. I'm surprised to hear that the 01's were single. Did you purchase this bike used by any chance? The fiche shows different part numbers, so perhaps they switched. Good info to know.

So the mid-valve. At the moment I'm probably not in a position to brag about my fork design. Got back from ClubMoto tonight and the thing is suffering. It's a long story, but I think I have drifted too far from a proper balance in oil flow through the mid-valve to that of the base valve. In other words, too much oil is getting past the mid valve, not enough via the base, and the result is a soggy suspension, (and some tired arms).

I'm a little bummed at the moment, but this is the price one pays for doing a job by trial and error. Sure wish I had the smarts to generate numbers on this stuff.

I'm mean, wouldn't it be cool if you could run a suspension design on the computer before building it?

Oh well. And I too am running the C-Cycle cones. Those things are great. They more than cover medical bills that's for sure.

I think tomorrow I'm going to start building a bench flow device to test my forks. If I can't can't calculate this stuff out, (by design) the least I can do is perhaps get some results in place before I make another drive.




I purchased my 01 new (canadian version) and it has a single stage stack stock.

I have not tried the ideas for the mid valve yet and won't get a chance for a couple of weeks. I still need to order shims and I leave for vacation today and get back monday.

I also ride at moto fairly often, mainly on weekends. I live in Vacaville. We should get together and compare notes some time.

The mid valve. I am pretty happy with it when it is in good (new) condition. Based on this I am hoping to make it more durable with a minimal effect on how it works, slightly stiffer would be good for me. I go about 192 without gear and ride at fast B to slow A pace. I am running .48's in the front and a 5.6 in the rear to hold the bike up. I was dragging pegs before.


OK, I removed my base valves night before last. With the impact wrench, removing them was a snap. I disassmbled the base valves. It's now quite clear how this all works. I removed, measured, and recorded all the shims in the stack. My stack is single stage. The most notable thing was that the first 10 shims were 24mm X .1mm and then it tapered off 1 shim at a time with a reduction of 2mm OD per shim all the way down to 12mm OD. Then it went to 3 18mm X .25mm shims. There was then a 12mm OD shim and then finally a 24mm shim.

Being the absolute novice that I am concerning valves stacks, the logical thing to me was to remove some of the first 10 24mm shims. I removed 2 of these per valve and then reassembled.

I got to test the changes a bit yesterday on a loop I have at my house. It felt pretty good. I have one 3rd gear section through a rock field and for the first time on this bike I got that "floating" feeling going through it. Before, I could feel the rocks. It was always managable but irritating.

This is very cool. If I knew what I was doing I'm sure I could tune the valve stack to near perfection for my riding style and application.

The fun begins.


Ummm...that sounds like a two-stack stack to me.

Everything was low compression until you got down to the 18mm shims. From there it's high compression.

I'm not sure what that concluding 24mm thing was. Did you have a thickness on that?

Almost three stage. Humm....

Anyways, trial and error is the way to go. Keep precise records of what you had in there, and what changes you're making along the way.

Also, keep in mind that too much or too little damping can make for a harsh or uncontrollable ride. It's a tricky balance.

Let us know what you find.


[ March 08, 2002: Message edited by: TexXR2YZF ]


I have all the shim dimensions written down at home. The last 24mm shim was basically covering an open area in the bottom of the base valve, around the manual external compression adjuster. The stack of three 18mm X .25mm shims looked to me to be very inflexible. Do you think those three shims are the entire high speed stack?

I guess I should have taken a picture of it when I had it all apart.

When I get home I'll post or PM you the shim specs.

[ March 08, 2002: Message edited by: TexXR2YZF ]

OK, here are my stock stack specs.

The bike is an '01 purchased in November '01

(10) 24mm X .1mm

(1) 22mm X .15mm

(1) 20mm X .15mm

(1) 18mm X .15mm

(1) 16mm X .15mm

(1) 14mm X .15mm

(1) 13mm X .15mm

(1) 12mm X .15mm

(1) 11mm X .25mm

(3) 18mm X .5mm

(1) 11mm X .3mm

(1) 24mm X .12mm

I guess it is dual stage. I just didn't see the function of the 3 18mm X .5mm shims. In the hand they are very rigid. Is there any rhyme or reason as to why they didn't use a tapered stack for the HS as opposed to the three 18mm shims? I do know removing two of the ten 24mm shims in the LS did make for a more plush feel, but all I really affected was the low speed characteristics, correct?

Humm...I've got to find me a book about this.

Hey Tex,

Here’s my take on your information.

First, this is essentially a three-stage stack, but it is clearly divided into a low and high-speed progression.

Low compression (LC) begins with the (10) 24mm X .1mm shims and concludes with the 12mm X .15mm. High compression (HC) begins with the (3) 18mm X .5mm and concludes with the 24mm X .12mm.

The two 11mm shims between the second and third stage are what’s called a crossover shim, meaning that the proceeding stack has to deflect beyond this shim in order for the next stack to take effect. These crossover shims produce a flat line in the damping curve.

Obviously, this is why these shims are thicker than the others.

Since LC provides the majority of wheel and rider control, this is where most of the magic is created - hence the complication and smooth progression of shim layers. Eliminating any of the LC shims will result in greater oil flow through the base valve, less oil flow through the mid-valve, and hence less control over the bike. Rod speeds will become higher and bottoming more common. Collapsing effects will occur more often and you may actually cause the bike to become stiffer since the HC stack may take effect sooner. Such a modification will ultimately result in a ride that feels softer, but at the cost of being mushy, (more plush, less firm). Not a good thing for the track or high speeds.

If this was my stack, and my only complaint was vibration into the hands from sharp edge rocks and bumps, I would try one or more of the following.

1. Increase the thickness in the cross-over shims between the first and second stacks. This keeps the control offered by the initial low speed stack, but increases the flat line in the damping curve. Meaning you extend out when the HC stack takes effect.

2. Build a more progressive HC stack so that the stack does not come on strong, (less big shims, more smaller shims).

3. Add more shims in the progression of the LC stack beyond the initial (10) 24mm shims. This stretches the LC damping curve, meaning that it works longer and harder before the high compression takes effect.

4. Eliminate one or more of the HC shims.

I think options 1 and 2 are best unless you’re into the supercross style of riding. Then option 3 may be best. If it’s all woods and slow trail riding, option 4.

Note that you will be somewhat limited in the sense that the damping curve of the mid-valve may not allow you to feel the effects of changes in the base valve, (imagine two damping curves layed on top of each other). For example, if the mid-valve enters a high-compression phase during the flat line in the base valve damping curve, you will still have a pre-mature HC problem. Perhaps not as bad, but you’ll still have it.

This is the problem that I am now having with my set-up, and I also think this is where the less experienced suspension shops fail to improve on the factory ride.

Since I have yet to figure out how to configure base and mid-valve stacks that are in balance with each other, I have a ride that offers a “choppy” damping curve throughout the range of suspension movement. What a can of worms.

One of these days I would like to figure the math on this stuff, but ultimately I know it’s a matter of getting out on the bike and riding.

Let me know how things turn out and please seek the opinions of others.


Jason -

Good to hear from someone that understands the purpose of springs.

I'm usually a Tuesday guy at ClubMoto (3 to 6pm), but I could do a Saturday. It would be great to hook up, chat, and do a few laps. I wouldn't mind getting up to Sac again either.

But I still have parts all over the garage, so no hurry.

There's also another guy here in Campbell that's on TT. I still owe him a call, and user EBay is in Livermore as well.

One of these days we'll have to have a TT day at the local tracks.


Excellent information, DaveJ. Thanks for your input and explaination. I have an Enduro this coming weekend but after that I'm going to begin experimenting. I'll post my findings as they occur.

[ March 11, 2002: Message edited by: TexXR2YZF ]

DaveJ and I usually agree, especially how great the C-Cycle bottoming cones are, but I disagree with many things in this thread.

First of all, I consider that stack to be single stage. Those three 18 x .50 shims are not going to bend. They are just the "clamping shims", and they determine how far the stack can deflect.

Also, HS compression does not come into effect until you fully deflect the LS shims. If LS is too stiff, it doesn't matter what the HS is.

Tex, you didn't mention your weight. I recommend that you pull more of those 24mm shims until you get the feel you want. You could also try pulling the 20 and 16 or 18 and 14. You will know if it gets too soft. Just put back shims as required. I also recommend that you set your oil height to near the minimum, like around 130mm. IMO, FWIW, viewer discretion advised, blah blah blah...

ScottF -

So here's my take.

If they combine 5 shims of different thicknesses and diameters, there's a purpose other than clamping the LC stack above it. If not, they would just run a single washer at or less than 1.62mm.

Secondly, even though the low speed stack is aggressive, deflecting 10 .1mm shims into and beyond the effects of the remaining shims (1.05mm) is going to happen very quickly, and is a relatively short distance for the amount of oil flow that would take place during a high speed hit.

Lastly, 3 18mm X .5mm shims are going to deflect with the amount of force occurring under a high speed hit, and contribute to the higher end of the damping curve. It’s going to be sharp, but I think that was their intent.

Since there's no way of really knowing how far into the stack he's going when he hits a bump that he considers harsh, he will have to experiment.

Removing the last 5 shims would be an interesting lesson, and provide some great feedback..


Tex - In my opinion, ScottF really knows his stuff. He's a great guy to get advice from and certainly a good one to bounce ideas and concerns off of. It's good that guys like him keep guys like me in check.

The bottom line is that good suspension designs take time and a lot of testing. In most cases, what works for one rider, will not work for the other. Be patient enough to experiment, listen to our little debates, take good notes and then go at it with your own conclusions.

You'll learn how valuable this is when you find the perfect setup.

Gentlemen, great thread!!

Scott is correct, the base valve has a single stage stack. The 18 x .5mm are actually washers that are used for spacing purposes only.

The other stack (that is underneath the 18.5x3 shims) is what is usually referred to as the passive bleed shim stack. Kayaba designed those to reduce the front end from diving when you chop the throttle on the YZ four strokes. They actually create damping (resistence) in the bleed circuit.

Dave, I will take a pic next time I have one in my shop. The compression adjuster is built differently on the 01 models. It is kind of hard to describe, but a picture of both adjusters side by side will explain much better.

Take Care, John

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