KF7 and some KG7 had the ribbed prop shaft with alternating fibre and stainless steel disc clutch. If yours has the clutch the clutch may be slipping as designed.
Mercury Service Tool Catalog shows two tools for holding flywheel when
removing or installing ratchet nut. M-60-5167 is for KG7
M-60-591 used for those models having auxiliary starter plate mounted
on flywheel.
Flywheel holding wrench OR a work around to hold flywheel might
be what you need. Catalog was posted at one time, If it is still up
you may be able to fabricate something workable.
Stressing everything between prop and flywheel may not be a good
idea.
Louis
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