Mercury 40HP Outboard 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.

Mercury 40HP outboard prop shaft seals replacement parts

Tools and Parts – Mercury 40 Lower Unit Output Seal Repair

  • Ratchet and sockets
  • Flat-blade screwdriver
  • Hammer
  • Torque wrench
  • Red threadlocker

Buy Mercury outboard inner prop shaft seal

Buy Mercury outboard outer prop shaft seal

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.

Mercury 40 Outboard Prop Shaft Oil Seals Removal

Step 1. Remove the lower unit drain screw and drain the gearcase oil.

Mercury 40HP outboard prop shaft seals replacement

Step 2. Remove the 24mm prop shaft nut and tab washer, then remove the propeller.

Mercury 40HP outboard prop shaft seals change

Mercury 40HP outboard output seals change

Mercury 40HP outboard output seals replacement

Mercury 40 outboard output seals replacement

Step 3. Slide the thrust hub off the prop shaft, then remove the two 13mm bolts and washers from the bearing carrier assembly.

Mercury 40 outboard prop shaft seals replacement

Mercury 40 outboard prop shaft seals change

Mercury 40 outboard motor prop shaft seals change

Mercury 40 outboard motor prop shaft seals replacement

Step 4. Remove the bearing carrier assembly from the lower unit using a slide hammer or puller tool.

Mercury 40 outboard motor prop shaft seals change

Mercury 40 outboard motor output seals change

Step 5. Remove the prop shaft from the lower unit.

Mercury 40 outboard motor output seals replacement

Step 6. 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.

Mercury 40 outboard engine output seals replacement

Mercury 40 outboard engine output seals change

Watch the clip below to see the seal removal segment of our Mercury 40HP 4-stroke outboard prop shaft seals video.

Mercury 40 Outboard Prop Shaft Oil Seals Change

Step 7. 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.

Mercury 40 outboard engine prop shaft seals change

Mercury 40 outboard engine prop shaft seals replacement

Step 8. 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 outer 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.

Mercury 40HP outboard engine prop shaft seals replacement

Mercury 40 outboard engine prop shaft seals change

Mercury outboard engine prop shaft seals replacement

Step 9. Remove the O-ring from the bearing carrier assembly.

Mercury outboard engine prop shaft seals change

Mercury outboard motor prop shaft seals change

Step 10. Install the new O-ring onto the bearing carrier assembly and apply grease around it.

Mercury outboard motor prop shaft seals replacement

Mercury outboard motor output seals change

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

Mercury 40HP 4-stroke outboard motor output seals change

Step 12. Slide the prop shaft back into the bearing carrier assembly, then reinstall the assembly back into the lower unit.

Mercury 40HP 4-stroke outboard engine output seals change

Mercury 40HP 4-stroke outboard motor output seals replacement

Step 13. 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.

Mercury 40HP 4-stroke outboard output seals change

Mercury 40HP 4-stroke outboard prop shaft seals change

Mercury 40HP 4-stroke outboard prop shaft seals replacement

Watch the clip below to see the seal installation segment of our Mercury 40HP 4-stroke outboard prop shaft seals video.

Step 14. 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, then 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.

 

 

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