Mercury 40HP 2-Stroke Prop Shaft Seals Replacement

Oil dripping from beneath the propeller is an indication of a leaking prop shaft seal in the lower unit. Replacing the output or prop shaft seals in a Mercury outboard requires more than a few steps, but otherwise it’s not that difficult of a job. 

Watch the video above or follow the steps below to see how to replace the output (prop shaft) seals on a Mercury 40HP 2-stroke outboard's lower unit. 

Tools and Parts – Mercury 40HP 2-Stroke Output Seal Replacement

• Ratchet and extensions
• 13mm, 24mm sockets
• Drain pan
• Torque wrench
• Red threadlocker

• Marine grease
• Inner oil seal 
• Outer oil seal
• O-Ring

Removing Mercury 40 HP 2-Stroke Prop Shaft Oil Seals

Step 1. Position a drain pan under the gearcase, then remove the lower unit oil drain screw and let the oil drain out.

Step 2. Remove the 24mm prop shaft nut and tab washer.

Step 3. Slide the thrust hub off the prop shaft.

Step 4. Remove the two 13mm bolts and washers from the bearing carrier assembly.

Step 5. Remove the bearing carrier assembly from the lower unit using a puller tool or a slide hammer.

Step 6. Slide the prop shaft out from the lower unit.

Step 7. Pry out the oil seals from the bearing carrier assembly with a flat-blade screwdriver, being careful not to gouge or damage the carrier.

Step 8. Remove the O-ring from the bearing carrier assembly, and clean any dirt or corrosion out from the O-ring’s channel and the oil seals’ housing in the bearing carrier.

Watch the clip below to see the seal removal segment of our Mercury 40HP 2-stroke outboard prop shaft seals video.

Installing Mercury 40HP 2-Stroke Prop Shaft Oil Seals

Step 9. Apply red threadlocker to the inner oil seal (the smaller of the seals), and use a socket or driver with a diameter slightly smaller than the diameter of the bearing housing to press the new inner oil seal into the housing until it bottoms out against the needle bearing.

Step 10. Apply red threadlocker to the outer oil seal (the larger of the seals), and use a socket or driver with a diameter that’s slightly smaller than the diameter of the bearing housing to press the new inner oil seal into the housing until it’s just below the top of the housing. 

NOTE: Make sure you install the outer oil seal with its metallic side facing outward.

Step 11. Pack the space between the two new oil seals with marine grease.

Step 12. Grease and install the new O-ring onto the bearing carrier assembly.

Step 13. Slide the prop shaft back into the bearing carrier assembly, then reinstall the assembly back into the lower unit.

Step 14. Apply red threadlocker to the two 13mm carrier assembly bolts, then reinstall the bolts and their washers, and torque them to 18 foot-pounds.

Watch the clip below to see the seal installation segment of our Mercury 40HP 2-stroke outboard prop shaft seals video.

Reassembling the Lower Unit

Step 15. Reinstall the thrust hub, the propeller, the tab washer and the 24mm prop shaft nut. Torque the nut to 55 foot-pounds, and bend the tabs on the tab washer to lock the nut in place.

Step 16. Refill the lower unit with gearcase lube and you’re done.