How to Remount the Gearcase on a Mercury 40HP 2-Stroke Outboard

To replace the water pump or upper seals on a Mercury 40HP outboard, you need to remove the lower unit, and then of course remount it when you’re done. 

Here’s how to reinstall the lower unit on a 40HP Mercury outboard motor once you’re finished with repairs or maintenance. Use our Mercury 40HP 2-stroke lower unit diagram to get a better look at the gearcase.

Tools Needed - Mercury 40HP 2-Stroke Gearcase Remount

  • 13mm, 15mm, 16mm sockets
  • 16mm wrench
  • Two 10mm wrenches

Mercury 40HP 2-Stroke Lower Unit Remount

Step 1. Make sure the motor is in neutral by giving the propeller (or prop shaft) a little spin in both directions. If it turns both ways, you know the lower unit is out of gear. However, if it’s not in neutral, turn the shift shaft. 

Step 2. Lift the lower unit into position, and have one of the mounting bolts in hand before you do this so that you can get one threaded to hold the lower unit in place. Once it’s into position, you’ll feel it bottom out against the midsection.

Mercury 40HP 2-Stroke Gearcase Remount

Mercury 40HP 2-Stroke lower unit remount

Step 3. Thread in the four 16mm main mounting bolts, two on each side.

Mercury 40HP 2-stroke outboard gearcase remount

Step 4. Torque the four main mounting bolts to 40 foot-pounds. 

Mercury 40HP 2-stroke outboard lower unit remount

NOTE: The front bolts are difficult to get to, so we used a torque wrench adapter and a 16mm box end wrench to reach them.

Mercury 40HP 2-stroke outboard motor gearcase remount

Step 5. Reinstall the 15mm mounting bolt that sits above where the anode mounts. 

Mercury 40HP 2-stroke outboard engine lower unit remount

Step 6. Reconnect the shift shaft (which is separated in the middle on this unit) using two 10mm wrenches to tighten the pieces together and you’re done.

Mercury 40HP 2-stroke lower unit gearcase remount

Need to change out the lower unit oil on your Mercury outboard? Watch the video below to see how to do a gearcase lube change and pressure test on a Mercury 40 2-stroke outboard.