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.
Watch the video above or follow the steps below to replace the input shaft seals on a Mercury 40HP 4-stroke outboard lower unit.
Tools and Supplies – Mercury 40HP 4-Stroke Input Shaft Seal Replacement
- Ratchet and sockets
- Locking pliers
- Torque wrench
- Red threadlocker
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.
How to Replace Mercury 40 4-Stroke Outboard Drive Shaft Seals
Step 1. 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.
Remove the Driveshaft Oil Seals
Step 2. Remove the lower unit drain screw and drain out the gearcase oil.
Step 3. Remove the water pump assembly from the lower unit.
Watch the video below to see how to remove the water pump from a Mercury 40HP 4-stroke.
Step 4. Remove the 24mm prop shaft nut and tab washer, then remove the propeller.
Step 5. Remove the thrust hub, and the two 13mm bolts and washers from the bearing carrier assembly.
Step 6. Remove the bearing carrier assembly from the lower unit using a slide hammer or puller tool.
Step 7. Remove the prop shaft from the lower unit.
Step 8. 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 9. Pull the driveshaft up and out from the lower unit, and remove the pinion gear and bearing from inside the gearcase.
Step 10. Remove the seal carrier from the driveshaft and the O-ring from the seal carrier.
Step 11. 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.
Install the Driveshaft Oil Seals
Step 12. Remove any corrosion from the seal carrier and thoroughly clean it with contact/brake cleaner, then 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 13. 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 14. Install the new O-ring onto the seal carrier, then coat it and the space between the two new oil seals with marine grease and reinstall the seal carrier onto the driveshaft.
Step 15. 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 16. Tap down the seal carrier to ensure 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.
Step 17. Slide the prop shaft back into the bearing carrier assembly and reinstall the assembly back into the lower unit, then apply red threadlocker to the two 13mm bearing carrier assembly bolts, reinstall the bolts and their washer and torque them to 18 foot-pounds.
Step 18. 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 19. 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 20. 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.
Watch the video below to see how to remount the lower unit onto a Mercury 40HP 4-stroke outboard.
Step 21. Refill the lower unit with gearcase lube and you’re done.
