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
- 3/8th ratchet
- Torque wrench
- Torque wrench adapter
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.
Step 3. Thread in the four 16mm main mounting bolts, two on each side.
Step 4. Torque the four main mounting bolts to 40 foot-pounds.
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.
Step 5. Reinstall the 15mm mounting bolt that sits above where the anode mounts.
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.
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.