what is the best position to measure it? (vertical or horizontal )
Ok, to measure the drive-end clearance between rotors and end-plate:
Remove the outer casing, keeping only the drive-end endplate, with bearings and screws installed.
Keep the air-end vertically. So the screws press down. Now you can measure the clearance with a feeler gauge. (just push it between the screws and the endplate and see which one fits, to get the clearance.
(see photo below)
To measure the clearance at the non-drive end, put back the outer casing. Now you will see that the end of the screw is *almost* flush with the end of the outer casing.
Now measure this distance with a micrometer. When the end-plate is installed, this will be the clearance.
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