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CSAR FE

Who surfaces their shock piston?

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The KTM/Husqvarna shop manual calls for surfacing a new shock piston on a surfacing plate, with 1000 grit before installing. Anyone actually do this or think it’s necessary?

 

I just serviced my shock, which was previously revalved by Factory Connection. It looks like they surfaced the original piston. I’m under the impression that machining these days is so good that it’s unlikely the shock piston surface isn’t flat enough for the shims to seal properly.

Edited by CSAR FE

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That's weird as they tend to dish pistons STD 

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The STD pistons have varied with dish, the 19 had the most dish I believe

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I would not remove the dish ,It gives a bit of preload to the stack which is a good thing imo 

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I’m pretty sure that sanding is to create a crosshatch to avoid the shim sticking to the piston lowering the opening force required for the stack.

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10 hours ago, Luis Velos said:

They say to sand to make sure the shims touch the piston perfectly.

Correct and to get a good mating surface with the shims, to avoid blow by

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54 minutes ago, EnglertRacing said:

Lapping is dumb,

The piston is slightly concave, you will be removing this aspect by lapping

I have an extra KYB shock and Im going to add more dish as I want to increase low speed damping without adding high speed, Roots/rocks.

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If you surface the piston too much you can actually make it convex.  The thin outer edged of the port will get sanded away, but the fat middle will not.  I've seen enthusiastically surfaced pistons with -.002" preload (convex).  And with piston preload, that's a lot. 

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50 minutes ago, valvinglogic said:

If you surface the piston too much you can actually make it convex.  The thin outer edged of the port will get sanded away, but the fat middle will not.  I've seen enthusiastically surfaced pistons with -.002" preload (convex).  And with piston preload, that's a lot. 

Thats a wobbly lap job.

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3 hours ago, slothspeed said:

I have an extra KYB shock and Im going to add more dish as I want to increase low speed damping without adding high speed, Roots/rocks.

it will, but beware preload creates a threshold that must be overcome

The transition from clicker to shim cracking will be more "defined"

And also in my experience boosting low speed and using something with a high speed blow off sounds appealing but in practice the wheel always accelerates through the low speed damping to reach the high speed anyways, the low speed is actually the first thing you feel on a high speed hit, its always there.  A because the suspension has mass that must accelerate. B The wheel rolls up even sharp objects.

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Should I try a ring shim first as a test to see if I can get something I like better?. Dont want wallowy and want the back to not kick on a root? Thanks for the advice. Bike is 2012 yz250. 46mm piston, 18mm shaft.

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Always best to try things that can be undone.  Preload ring is a good way to go.  Once you surface the piston, you can't go back.

 

Edited by valvinglogic

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