I just asked my builder (Luis) and he explained it to me...
He gave me a link that I should share about clearancing..
Quote:
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Originally Posted by Racing Beat
Rotor Clearancing
A second concern for rotors used near or above 8,500 RPM is the clearance between the sides of the rotor and the side housings. The construction and assembly of the rotor and rotor gear is such that both sides of the rotor have a portion, referred to as the “land”, that protrudes out from the rotor “side” several thousandths of an inch. For measuring purposes, detailed below, the “land” on the side of the rotor containing the rotor gear is the face of the rotor gear itself. The “land” on the opposite side of the rotor is the circular portion of the rotor surrounded by the inner oil seal. These features need to be taken into consideration when clearancing the rotor.
The first step in preparing a rotor for clearancing is to measure the thickness of the rotor housing being paired with the rotor at eight evenly spaced points to determine the minimum or narrowest point of thickness of the rotor housing. This minimum thickness becomes the “base line” for the calculation process.
Now, measure the thickness of the rotor and rotor gear assembly at three different points from the “land” on one side to the “land” on the other side. Select the maximum, or widest, thickness of this assembly and subtract this number from the base line number previously obtained. We have found that the following clearances work well:
Clearance between side housings and rotor maximum “land” width:
12A Engines (.009” +/- .001”)
or 13B Engines (.010” +/- .001”)
“Land” protrusion from rotor side: .0055” +/- .0005” (each side). This “land” clearance is obtained by machining away material from the “side” of the rotor, thus leaving the “land” extending from the rotor “side” by the specified amount.
It is entirely possible, based on the production tolerances we have seen over the years that you may need to remove as little as .001” to. 002” from each surface to achieve the recommended clearance. In other words, some of the clearance required already exists.
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I was told my by builder that its not necessary on engines that does not run high RPM for extended period of time. And like most things, race and street prep pros and cons should be highly considered before doing such mod.