Rebound piston surfacing - shim float?

3 replies to this topic
  • DaveJ

Posted February 19, 2002 - 09:29 PM


MX Tuner, John C, ScottF and others,

Over the past few days I've been working on some mid-valve designs and stacks. During the process of assembling the compression cylinders, I noticed that one cylinder had more rebound and compression resistance than the other, (just by moving the rod against air).

I eventually discovered that the piston surface was .002 to .0025 less at the edge than at center. This was causing a shim float that was barely noticeable by eye, but could be measured with a thin feeler gauge.

With some further testing, I was able to conclude that a .0025 difference could be balanced with 9 clicks at the screw. That seemed excessive.

So I was able to polish the piston to within .001 from center to edge. That in itself is a task.

This still left a noticeable difference between the two units, however it seemed minor enough to reassemble.

My questions are:

1. With consideration to rod speed while riding, do such small differences matter? In other words, when the pistons moves as fast as they do during low and high speed, is shim deflection even between left and right forks even when the static float is not the same?

2. Have you guys ever been concerned with piston surfaces and balance testing before assembly, or am I just being a little overboard?

Hope that made sense and thanks for any input.


  • MX_Tuner

Posted February 20, 2002 - 02:18 AM


RT recommends doing that (resurfacing) the Gold Valves during routine servicing. I've never had one that needed it. I'm of the "disable the mid-valve" group. I'd rather not have my set up change a few months down the road.

But to answer your question, I'd *think* that at virtually any realistic (during riding) shaft speed, the amount if additional oil bled by the imperfection you have would be negligible. This obviously is only my opinion.

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  • Scott_F

Posted February 20, 2002 - 10:08 PM


Dave, are you talking about the new '02 design, or the previous piston? The '02 is a different situation.

I wouldn't be worried about any compression difference, I would be more worried about leakage past the mid-valve on the rebound stroke. With enough pressure, the shims might bend "down" and seal the piston, as long as it is smooth and flat. But, that gap should not be there. Resurfacing the piston on a surface plate is a common technique.

It's hard to tell if you have a problem, but the two forks should feel nearly identical, with or without oil.

  • DaveJ

Posted February 20, 2002 - 07:46 PM


Thanks guys.

ScottF - I'm still 00. Ya know, take the mid-valve out, put it in, take it out, put in...etc.

I was okay without the pesky thing until I started focusing on the track. Now it's back in.

I'll give up when the 03's roll in.


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