YZ250 -2017, Fork bore/Bushing play

Hi guys, have a YZ250 2017 two stroke with 40hrs on it.

As I was putting in new seals and bushings in the outer tubes I feel a slight play at the upper bushing. I dit a comparison on a set of YZ450 forks and they are 100% tight.

So my conclusion is that my forks are bored out of spec. since theres no special wear on either the bushings or the tube.

Tried them with both the new bushings and the old and the play at least feels the same. So I guess I need to put a strip of metal shim behind the bushing to increase the O.D of the bushing.

I have the tools to do a proper measurement but is there any guideline to what the correct play should be?

/MA

Ok, so I did some measuring yesterday.

YZ250 17 fork:

Top bushing: 48.82
Tube top end: 49.04
Tube lower end: 48.99

Max clearance: 0.22mm

YZ450 14 fork:

Top bushing: 48.92
Tube top end: 49.01

Max clearance: 0.11

So I will add a strip of 0.05mm shims metal below the bushing and it should be fine. As I said I can´t see any obvious signs of wear so this must be normal for the YZ.

Now to the interesting part, the YZ450 fork have been to service and they knurled the surface to increase the O.D of the bushing. A pretty clever method that we used to do at work before for temporary fixing worn shafts or bearing surfaces(I was a maintenance engineer for mechanical automotive presses before).

 

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Edited by snoogans

Not much of a scholar when it comes to suspension, but how did you conclude that the existing clearance wasn't a design feature?

33 minutes ago, LSHD said:

Not much of a scholar when it comes to suspension, but how did you conclude that the existing clearance wasn't a design feature?

Haven´t really, but I´ve never felt that ammount of play in a healthy fork before!

I would run them as is.  A .05mm shim is pretty much nothing at all.  Oil will take that gap right up.  That may be the reason for the existing tolerances.

When you install the metal bushings in the upper tube do they drop right in or do they fit tightly?

You are right that 0.05 is a very thin shim, but is takes the play down to half.

The upper bushing snaps in just like normal new ones, but I can compress them a bit by hand.

They do vary a lot from one fork to another ,I'm not sure there is is spec as such , too tight and they will bind when flexing

I would ride it without shimming. If the fork seal starts weeping then add the shim.

That's normal.  Your asking for big trouble shimming or knurling like that.  The bushing must float and rock freely.  You cannot measure the required clearance when the fork is flexing under load.  Marzocchi 48s have problems with rapid wear of the uppers.  Of the few sets I have worked on, the longest lasting set had the most clearance.  My personal set actually had the bushing grab and scrape the upper very badly. 

Ok, so I get what you are saying, but stubborn as I am I have a real hard time accepting that there is no guidelines to what the correct play is.

I accept that there has to be play to allow flexing and heat build up, but I have never had another fork with as much play as this one. 

Look at this video, I´m not alone =):

 

It's a long video ?

Have a look about 2min in!

As a starting guideline you can check what's the hydraulic company that manufacture these bushing suggest. They usually provide the dimension of the piston to fit their parts.

I would worry about shim stock floating around behind the bush ?

The big question is:
What is the "best" clearance and where do you take the referene measurements?

I guess the wiggle test is a good one ,a few forks the same if one moves much more than the others then it's safe to assume it's incorrect

Sounds like an empirical methode haha

But I know what you mean. However, if you make it too tight, you ruin the coating of the outta tube...

As it seems there is a problem with kyb outta tubes on the latest models.
I had 4 sets in with 0 hours on it and all of them had rust inside the steel tube.

I've seen tons of kyb parts rusted over the years ,lots nearly new

I assembled the fork and still have a bit of play but at a healthier level, used MOGs empirical wiggle method =).

I see the risk in the shim rotating/floating out from under the bushing. But I don´t see any other method of getting a lager O.D of the bushing.

But I haven´t ridden it yet, it´s snow, cold and kind of wet here right now and I am a bit sensitive. Also I hate riding with studded tires. 

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