Phat, looks like you win the overheat derby. I used my micrometer on the driveshaft and then my veneer calipers on the ID of the old Impeller. there's almost .020 slop. Seems Mercury used 2 different driveshafts on these engines and as you correctly called it that results in 2 different impellers and housings.
The PO obviously installed the wrong one. It's just enough that the pumping action just isn't there.There shouldn't be any slop between the shaft & impeller. There are two impellers that can fit that outboard. The 47-89981 has a .456 ID the 47-89980 has a .438 ID. The .456 is what I have, the .438 is what should be there.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
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