Mercury 40HP 4-Stroke Outboard Lower Unit Remount

After servicing your Mercury 40HP four-stroke engine’s lower unit or gearcase, you’re obviously going to have to mount it back onto the outboard. 

Getting the lower unit reattached is easier than it sounds. Follow the steps below and watch the video above to learn the quick reinstallation process of a Mercury outboard lower unit. 

Tools and Supplies Needed – Mercury 40 Gearcase Remount



  • Torque wrench


Mercury 40HP Tracker lower unit install

Reinstalling Mercury 40HP 4-Stroke Outboard Lower Unit

Step 1. Coat the upper splines on the driveshaft and the inside of the water tube seal above the impeller with marine grease.

Mercury 40HP lower unit remount

Step 2. Slide the driveshaft and shift shaft up into the driveshaft housing.

Mercury Tracker 40 gearcase remount

Step 3. Reinstall the four 16mm lower unit mounting bolts and torque them to 40 foot-pounds. 

PRO TIP: To torque the bolts that can’t be reached with a socket, attach a 16mm wrench to the torque wrench with an adjustable torque wrench adapter. Make sure the torque wrench is attached to the 22mm wrench at a 90-degree angle, otherwise the torque measurement will be incorrect.

Mercury 40HP gearcase remount

Step 4. Reinstall the 15mm bolt into the trim tab housing and torque it to 40 foot-pounds.

Mercury 40 lower unit trim tab housing

Step 5. Reinstall the screen into the trim tab housing.

Step 6. Reinstall the trim tab, then torque the 15mm mounting bolt to 16 foot-pounds.

Mercury 40 lower unit trim tab

Step 7. Reconnect the shift shaft linkage by pushing the pin back into the shift shaft and pressing the plastic retainer clip down.

Mercury 40 Tracker lower unit remount

Step 8. Reinstall the top cowling and you’re done.

Mercury 40HP lower unit install

Got an outboard engine that could use a little touch-up? If your Mercury outboard’s decals are worn out, watch the video below to learn how to remove and replace outboard decals




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