Mercury 40HP Lower Unit Driveshaft Seals Replacement

Worn and failing driveshaft seals lead to oil escaping the gearcase, water getting in and quickly ruining the lower unit on your Mercury outboard. 

Follow the steps below and watch the video above to replace the input shaft seals on a Mercury 40HP 4-stroke outboard lower unit.

Tools and Supplies Needed – Mercury 40HP 4-Stroke Input Shaft Seal Replacement

Tools

• Ratchet and sockets
• Locking pliers
• Torque wrench

Supplies

• Upper oil seal 
• Lower oil seal
• O-ring
• Red threadlocker
• Marine grease

NOTE: If you intend to replace the prop shaft seals as well as the driveshaft seals, get a Mercury gear housing seal kit, which includes the upper and lower driveshaft seals plus all the output shaft seals.

Removing the Lower Unit From the Outboard

Step 1. Disconnect the shift shaft linkage.

Step 2. Remove the trim tab and the lower unit mounting bolt beneath the trim tab.

Step 3. Remove the four lower unit mounting bolts.

Step 4. Remove the lower unit from the outboard.

Watch the video below to see how to remove the lower unit from a Mercury 40HP 4-stroke.

Removing the Driveshaft Oil Seals

Step 5. Remove the lower unit drain screw and drain the gearcase oil.

Step 6. Remove the water pump assembly from the lower unit.

Watch the video below to see how to remove the water pump on a Mercury 40HP 4-stroke. 

Step 7. Remove the 24mm prop shaft nut and tab washer, then remove the propeller.

Step 8. Remove the thrust hub, and the two 13mm bolts and washers from the bearing carrier assembly.

Step 9. Remove the bearing carrier assembly from the lower unit using a slide hammer or puller tool.

Step 10. Remove the prop shaft from the lower unit.

Step 11. Hold the driveshaft with locking pliers and a rag (being careful not to scar the driveshaft), and remove the pinion nut from within the gearcase.

Step 12. Pull the driveshaft up and out from the lower unit, and remove the pinion gear and bearing from inside the gearcase.

Step 13. Remove the seal carrier from the driveshaft and the O-ring from the seal carrier.

Step 14. Place the seal carrier over a socket with a larger diameter than the oil seals, and use a hammer and a socket with a smaller diameter than the seals to tap out the old oil seals.

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

Installing the Driveshaft Oil Seals

Step 15. Remove any corrosion from the seal carrier and thoroughly clean it with contact/brake cleaner. 

Step 16. Apply red threadlocker around the new lower oil seal, and use a hammer and a socket with a slightly smaller diameter than the seal to press the seal into the seal carrier until it bottoms out. 

Step 17. Use a hammer and a socket with a slightly smaller diameter than the seal to press the new upper oil seal into the carrier until it bottoms out. 

NOTE: Don’t use threadlocker on the upper oil seal.

Step 18. Pack the space between the two new oil seals with marine grease.

Step 19. Install the new O-ring onto the seal carrier, then coat it with marine grease and reinstall the seal carrier onto the driveshaft.

Step 20. Apply red threadlocker to the pinion nut threads, then position the pinion gear and bearing within the gearcase, slide the input shaft back into the lower unit and reinstall the pinion nut.

Step 21. Tap down the seal carrier to be sure it’s fully seated in the lower unit, then hold the driveshaft with vice grips and a rag, and torque the pinion nut to 50 foot-pounds.

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

Reassembling the Lower Unit

Step 22. Slide the prop shaft back into the bearing carrier assembly and reinstall the assembly back into the lower unit.

Step 23. Apply red threadlocker to the two 13mm bearing carrier assembly bolts, then reinstall the bolts and their washer and torque them to 18 foot-pounds.

Step 24. 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 25. Reinstall the water pump assembly onto the lower unit.

Watch the video below to see how to reinstall the water pump on a Mercury 40HP 4-stroke.

Step 26. Remount the lower unit onto the outboard by reinstalling the mounting bolts and torqueing them to 40 foot-pounds, reinstalling the trim tab and reconnecting the shift shaft linkage.

Step 27. Refill the lower unit with gearcase lube and you’re done.

Watch the video below to see how to remount the lower unit onto a Mercury 40HP 4-stroke outboard.