The impeller key is the small part that locks the water pump impeller to the driveshaft. If it’s worn, damaged, missing, or the wrong size, the impeller can slip—causing weak water pressure, overheating, and expensive damage.
 
This Mercury / Quicksilver / Mariner / MerCruiser water pump impeller key is commonly referenced by OEM part numbers 28-56654, 566541, and 8M0032834.
 
What the impeller key does
Inside the pump, the key seats in the driveshaft and index into the impeller hub so the impeller must turn with the shaft. Many “mystery” cooling issues come from a key that fell out during assembly, got mashed during install, or was reused after being deformed. Replacing the key during an impeller service is cheap insurance.
 
Part numbers for ordering
This part is commonly listed as 28-56654, 566541, and 8M0032834 across Mercury, Quicksilver, and Mariner catalogs, and it also appears in MerCruiser sterndrive parts lookups.
 
This impeller key is widely listed across legacy Mercury 2-stroke platforms, including 135 HP, 150 HP,Fits Mercury 2-stroke outboards by horsepower and engine family 175 HP, 200 HP, 225 HP, 245 HP, 260 HP, 280 HP, and 300 HP families spanning common displacement groups such as 2.0L, and 2.4L, 2.5L.n.
 
Catalog naming you’ll often see alongside it includes 135 HP V6 (2.5L), 150 HP V6 (2.5L), 175 HP V6 (2.5L), 200 HP XS 6 CYL (2.5L), 200 HP XS ROS (2.5L), 225 EFI Pro Max / Super Mag, 225X EFI Pro Max, SST-120, S2000, and S3000, plus legacy groups like V-150, V-175 XRi (EFI), and V-200 XRi (EFI).
 
Common gearcases and sterndrive families
You’ll often see this key referenced with Mercury gearcases such as Sportmaster and CLE, plus performance/race listings like SSM #4 Super Speedmaster and SSM #6 Speedmaster. It’s also commonly listed in MerCruiser cooling catalogs alongside MCM, R/MR, Alpha One, and Bravo families—always confirm by matching the OEM number in your diagram.
 
Quick install tip
A light dab of grease helps hold the key in the driveshaft while you slide the impeller down. Make sure the key is fully seated and not pinched before tightening the housing.
 
Part Numbers
28-56654 • 566541 • 8M0032834
 
Contact mike@buckshotracing77.com or +1-714-697-1716 with any questions.