Mercury specs...

Straight from the Mercury service manual:

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1. Place impeller in wear ring bore and push to one side.
2. Measure clearance between impeller blades and wear ring with a feeler gauge. If clearance is over 0.100 in. (2.54 mm), replace impeller and wear ring.

NOTE: Impeller wear usually accounts for 75% of the wear. Reducing the clearance can improve both top speed and acceleration performance.

COMPONENT SPECIFICATIONS
Wear Ring Bore Diameter 7.273 - 7.283 in. (184.73 - 184.98 mm)
Impeller Outside Diameter 7.225 - 7.235 in. (183.52 - 183.77 mm)
Clearance between Impeller and Wear Ring 0.038 - 0.058 in. (0.96 - 1.47 mm)
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My wear ring looks like your description. With the stator installed and measuring with feeler gauges, I have ~0.015 inches of clearance all around. That's 0.030 cumulative inches based on their "shove it to one side" procedure above, well within specs and actually below the low range of their clearance spec (last item above).

Get a set of feeler gauges and see what you really have.