Mercury 40HP 2-Stroke Upper Seal Replacement 

One of the easiest yet most important maintenance tasks to perform while your Mercury outboard’s lower unit (gearcase) is removed is to replace the upper (input shaft) seals.

Replacing seals prevents leaks, and it’s something that can be taken care of easily if you’re already working on the lower unit. Watch the video above and follow the steps below to replace the lower unit input seals on a Mercury 40HP 2-stroke outboard. 

Tools and Supplies Needed - Mercury 40 2-Stroke Upper Seal Replacement


  • Vise grip or channel lock pliers
  • Soft-blow hammer 
  • Bearing driver


NOTE: The seal kit comes with everything you need for whichever part of your Mercury 40HP 2-stroke outboard’s lower unit is leaking.

Replacing the input shaft seals requires removing the lower unit from your Mercury outboard first. Watch the video below to see how to remove the gearcase from a Mercury Tracker 2-Stroke outboard

Mercury 40 2-Stroke Input Shaft Seal Replacement

Step 1. Remove the housing, which is held in by O-rings, with a pair of vice grips or channel lock pliers.

Mercury 40 2-stroke input shaft seals replacement

Step 2. Use a soft-blow hammer and a bearing driver to drive out the seals.

Mercury 40HP 2-stroke outboard upper seals replacement

Mercury 40 2-stroke outboard input shaft seals

Step 3. Clean up the housing and inspect it to make sure it's not damaged or corroded to the point it's not going to seal back up.

Mercury 40HP 2-stroke outboard input shaft seals housing

Step 4. Drive the new input shaft seals (included in the gear housing seal kit) in with a bearing driver and a soft-blow hammer.

Mercury Tracker 40 input shaft seals replacement

Step 5. Press the neoprene seal (it faces out) in until it bottoms.

Mercury 40HP 2-Stroke lower unit upper seals replacement

Step 6. Replace the O-ring, and put a little bit of grease in between the seals and the O-ring.

Mercury 40 2-stroke gearcase input shaft seals change

Mercury Tracker 40 2-stroke gearcase seals change

Step 7. Press the entire housing assembly back into position.

Mercury 40HP outboard motor input shaft seals replacement

Step 8. Replace the shifter sleeve with a new shifter sleeve (included in the gear housing seal kit) if yours is old and cracked, and you’re done.

Mercury 40HP 2-stroke outboard engine lower unit seals change

All that’s left is to remount the gearcase to the powerhead if you’re done making lower unit repairs. Watch the video below to see how to reinstall the lower unit onto a Mercury 40HP 2-stroke engine.




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