Lip might be more accurate, I think the cause is the mismatch of surface area, cam bearing is about 2.685 in width/length and the adaptor is about .625 thick , the adaptor is nothing more than a disk that bolts to the front of the pre eagle cam.
The difference in diameter between the two cams is about.070 give or take a.010 like Steff said really not enough thickness for a sleeve to be reliable.
I wouldn't take the risk of it turning in the bore and closing off an oil passage

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