Mercury 40HP Lower Unit Prop Shaft Seals Change
A couple of drops of oil on the ground beneath the propeller of your Mercury outboard is a sure sign that you have a failing prop shaft seal in the lower unit.
Watch the video above and follow the steps below to learn how to change the outboard lower unit prop shaft seals on a Mercury 40HP 4-stroke outboard.
Tools and Parts Needed – Mercury 40 Lower Unit Output Seal Repair
- Ratchet and sockets
- Flat-blade screwdriver
- Torque wrench
NOTE: If you intend to replace the input shaft seals as well as the prop shaft seals, you can purchase a Mercury gear housing seal kit that includes the inner and outer prop shaft seals, plus all the input shaft seals.
Removing the Prop Shaft Oil Seals
Step 1. Remove the lower unit drain screw and drain the gearcase oil.
Step 2. Remove the 24mm prop shaft nut and tab washer, then remove the propeller.
Step 3. Slide the thrust hub off the prop shaft, then remove the two 13mm bolts and washers from the bearing carrier assembly.
Step 4. Remove the bearing carrier assembly from the lower unit using a slide hammer or puller tool.
Step 5. Remove the prop shaft from the lower unit.
Step 6. Note how deep into the bearing carrier assembly the existing oil seals are set. The inner oil seal is bottomed out within the carrier assembly, the outer seal is set just below the top of the carrier assembly.
Step 7. Pry out the oil seals from the bearing carrier assembly with a flat-blade screwdriver, being careful not to damage the needle bearing beneath the inner oil seal.
Step 8. Remove the O-ring from the bearing carrier assembly.
Watch the clip below to see the seal removal segment of our Mercury 40HP 4-stroke outboard prop shaft seals video.
Installing the Prop Shaft Oil Seals
Step 9. Apply red threadlocker to the new inner oil seal, and use a socket with a diameter that’s slightly smaller than the diameter of the bearing housing to drive the new inner oil seal into the housing until it bottoms out against the needle bearing.
Step 10. Apply red threadlocker to the new outer oil seal, and use a socket with a diameter that’s slightly smaller than the diameter of the bearing housing to drive the new inner oil seal into the housing until it’s just below the top of the housing. 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 bearing 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 install segment of our Mercury 40HP 4-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.
Watch the video below to see how to remount the gearcase on a Mercury 40HP 4-stroke outboard.