Why do you want to center your skis?  The main reason is that if you do this on a Skidoo it will align the carbide on the bottom of your ski directly in line with center of the spindle.  I first got this idea from watching a “tips and tricks” setup video from Skidoo rider Carl Kuster while being interviewed at haydays.  The benefit of having the carbide lined up with the center of the spindle is that the sled will be every so slightly more predictable as there will not be any small left or right hand forces being applied to the spindle as there is when the ski is offset to either side.  So what you gain is a touch more of predictability.

This mod will also increase your ski stance by ~1″,  which will in turn slighty increase the effort it takes to get the sled on its side, but it will also allow you to put the sled on its edge a touch farther until you panel out in a sidehill, in theory allowing you to sidehill steeper slopes.

This mod is quick, cheap, easy and can be done 1 of 2 ways.

First, remove the stock ski spacer.

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This spacer can then be cut in half, if you have the means to cut it straight and clean. Once cut reinstall the now two (2) spacers with one on either side of the spindle.
Or

You can get your self four (4) Polaris ski spacers (Polaris Part# 5137847) and install them.  As these will fit perfectly on a Skidoo XM.  Cost is roughly ~$2.50 per spacer and you will need four (4).
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Stock Skidoo spacer compare to Polaris Spacers
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Polaris spacers installed on a 2016 Skidoo XM
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There you have it, a cheap (or free) and easy mod to do to your Skidoo XM.   The difference you feel maybe small or maybe you wont even notice a difference at all.  But making a couple small changes here and there often lead to noticeable changes out on the hill.