Replacing lower unit

Replacing lower unit DEFAULT

Tips for Replacing your Outboard Lower Unit

The main part of the lower unit of an outboard motor is really nothing more than the gear case. The gear case is also known as the “business end” of the boat lower unit. When most boaters talk about changing the lower unit they are talking about disconnecting the speedometer cable, removing the gear case, and replacing the old, worn out gear case with a new or a rebuilt/remanufactured gearcase. It’s always important to purchase the correct lower unit parts needed for your repairs and replacements. Remember, the suggestions below are merely guidelines and your specific boat engine service manual should be consulted.

How to Replace a Lower Unit

The first thing to do is to take the key out of the ignition switch and pull the emergency lanyard, which is the line that clips to the operators life jacket out of the emergency stop switch. Next move the engine’s control handle into the neutral position. Now remove the prop and the gear case with a socket wrench.

Locate the splice connector which is comes out the top of the front side of the gear case and pull apart the two ends .On the top of the rear of the gear case there is a plastic plug, which you should pry out with a small screwdriver. Underneath that plug, there is a bolt; using a socket wrench, with an extender if necessary, loosen the bolt completely and remove the trim tab which is just above the prop.

The next step would be to remove the gear case by removing the bolt below the place where the trim tab was mounted and remove both bolts on either side. The gear case will not come out easily so it is easier to pry it open gently with a screwdriver ad removing the gear case by pulling it out straight. Now grease the drive shaft and shifter shaft splines by wiping them with a rag covered with lithium grease. Visually inspect the control handle to make sure that it is in neutral.

Gently pushing the drive shaft upwards into the middle part of the motor, when it reaches the crankshaft. When the crankshaft and drive shaft realigned, lower the drive shaft just enough to insert the speedometer through the hole line up the shift shaft, push the gear case all the way up and insert one bolt finger tight to maintain alignment. Replace the prop.

Finally, set the motor control handle to the forward position and then try to spin the prop. The prop should not turn. Return the control handle back into the neutral position; the prop should spin freely in both directions. Set the control handle to reverse. The prop should not move.

Replace and tighten all the bolts, reinstall the trim tab, push the ends of the splice connector on the speedometer cable together. Then replace the gear case cover, replace and tighten all bolts and test-run the motor.

Remanufactured lower unit is frequently used in the gear case replacement processes. Most shops use only quality parts with a good warranty, according to the manufacturer guidelines. They also include all internal parts and are fully integrated and interchangeable with OEM drives.  Be sure to find a reputable shop that has been in business for many years.  Check the warranty you receive on the unit which should be around one year.  Do your homework before investing in a lower unit replacement as purchasing a cheap unit could leave you stranded out on the water.


Confused about replacing the ENTIRE lower unit?



Moderator & Unofficial iBoats Historian
May 19, 2001
Re: Confused about replacing the ENTIRE lower unit?

Welcome to iboats! Sorry to hear about your collision.

SEI has a real good reputation.

Your mechanic is OVER THE TOP!

The OEM cost from is $3,517.38 USD for the gearcase assembly and the availability is limited. Kick in a water pump for about $60 more. ............. You do the math

If it is all that needs replacing you are talking about 1 hour of labor max.

Now if you have damage to the driveshaft, exhaust housing and more then yes the price goes way up.

Do you have access to the motor and the ability to take some pictures. You might be able to walk away from this cheaper then you think.

I will move this to the Evinrude/Johnson section and we will wait for your reply.
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Replacing An Outboard's Water Pump

Don't be put off by the fact that you have to drop the lower unit. Here are some tips to get you through it.

Dropping gearcase to access water pump

1. Dropping the gearcase to access the water pump requires removal of the mounting bolts.

Outboard water pumps are simple designs. Located just atop the lower gearcase assembly, they're easy to access and service, too. The pump houses a rubber impeller that's keyed to the engine driveshaft. Water enters the lower gearcase through inlets on the sides of the gearcase, just above the propeller shaft area. The driveshaft turns the pump inside a plastic housing with a stainless steel liner. The pump draws water through the intakes and up into the housing. The impeller blades (vanes) touch the liner at lower speeds as the driveshaft turns, but as the engine revs up, the impeller blades bend back away from the housing liner. The pump sends water out of the top of the housing through a brass tube, typically about a half-inch in diameter, up to the engine's powerhead, where it circulates through the system's cooling passages.

Salt, sand, mud, sticks, and other debris take their toll on the pump impeller, housing, and related parts. That's why most outboard technicians recommend replacement every season, or at worst every other season. There are legions of stories that go "I've had my outboard for twenty years and never replaced the water pump"; however, good preventative maintenance practice includes replacement of the pump and housing at regular intervals.

Removing bolt under trim tab

2. Don't forget the bolt located under the trim tab. Mark the position of the trim tab before removing it so you can correctly position it for reinstallation. If you don't, you will have to re-adjust it after water testing the rig, so it will steer neutrally and won't pull to port or starboard.

Water pump ready to be removed

3. Water pump is ready to be removed, using a fitting socket on the four mounting bolts. Notice that the gearcase is also being drained of old lubricant while the author completes the water pump job.

Remove old pump housing and impeller

4. Lay out and identify all parts on a clean cloth and then remove the old pump housing and impeller.


5. Old impeller (on right) blades have taken a set; compare to the new one on the left.

Applying sealant

6. Use of proper sealer on gaskets is mandatory to reduce the chance of leaks.

Pushing impeller into housing

7. The impeller is pushed into the housing liner with a slight clockwise twist.

Placing impeller key onto driveshaft

8. Impeller key is placed in the flat on the driveshaft, then the housing and impeller assembly is carefully lowered in place.

Tighten housing mounting screw

9. Be careful not to overtighten the housing mounting screws.

Backyard Mechanic

Buy a factory service manual for your engine, so you can follow the correct procedure and use the correct parts. An hour or so is all it takes to remove the gearcase, replace the pump, and re-install the gearcase on the engine.

The tricky part of the gearcase removal on some outboards is disconnecting the shift shaft. For example, on most Mercury's the engine must be shifted into forward gear first. On many Johnson/Evinrude outboards, the service manual specifically denotes which gear the engine should be in to make removal and re-installation easier. Temporarily removing the shift cable at the engine end will make things much easier as well, especially on engines where the shift shaft disconnection is inside the cowling. The engine should be tilted all the way up for gearcase removal.

Typically there are two attachments per each side of the engine; on some there is also a nut in the front. There are usually at least one or two hidden nuts or bolts up under the anti-ventilation plate (that flat plate just above the propeller). You may have to also remove the trim tab to access one of these attachment bolts. Before you completely remove that last bolt or nut, prepare yourself to catch the gearcase in case it drops right out. Typically this won't happen; it will usually require some wiggling and pulling to remove it, even with all the bolts/nuts out.

Stand It Up

The lower unit should be placed in a work stand. Clamping the skeg firmly in a vise will work fine; protect the finish by padding the jaws of the vise with a rag or other soft material. With the unit secured, you can remove the old water pump.


Remember that the gearcase must be in the correct gear before installation. Hopefully you didn't move the shift rod at all while servicing the water pump. When reinstalling the unit:

  • Clean the top of the drive shaft, then dab a small amount of marine grease on the splines — NOT on the top. Greasing the top may prevent the shaft from seating properly inside the crankshaft. Greasing the splines will make removal of the gearcase next time easier.
  • Be sure the exhaust adapter is properly positioned in the midsection.
  • While sliding the unit up into the midsection, ensure that the drive shaft, shift shaft, and water tube are positioned properly. Be especially careful that the water tube enters the top of the water pump properly; if it misses, the engine will not receive cooling water and will overheat.
  • After the installation is complete and you've reconnected the shift shaft and shift cable, start the engine and ensure that it is pumping water, running properly at the proper temperature, and shifting correctly. Consider also servicing the engine's thermostats at this time, so that the entire cooling system is refreshed and ready for service.

General Tips

Follow the water pump replacement in the factory manual carefully, paying particular attention to the cautions and warnings. Here’s a few general tips that apply to most water pumps, regardless of brand or engine size:

  • Use compressed air to clear debris from the intake passages and the top of the gearcase surrounding the pump. When you remove the pump and the impeller plate, use the air gun with an extended tip to reach far down into the intake passages to blow out any accumulated sand and silt. Blow compressed air up into the brass water tube in the engine midsection to clear any debris.
  • Use correct sealants when called for in the manual.
  • When installing the impeller into the housing, be sure of rotation (clockwise); then, install the impeller into the housing while turning it so the blades compress in the proper direction.
  • Use a dab of non-petroleum-based lubricant on the impeller blades when installing the impeller into the housing, so that it's slightly pre-lubricated upon start-up of the engine when it's all reassembled. Also, this makes compressing the blades into the housing easier.
  • Carefully tighten the housing screws; do not over-torque, or you may crack the housing and cause a leak later, which could lead to overheating.
  • After completing the pump installation, it's a great time to drain, check, and refill the gear lubricant.

How to Install an SEI Lower Unit

Tips & recommendations for replacing your gearcase lower unit.

It happens – just plain bad luck – hitting an underwater tree stump, rock or other invisible debris & your lower unit is shot.  After you get towed safely back to shore and have a chance to regroup, what are your next action steps?  We have written this article to outline the step you should take if you damage your lower unit or need a gearcase repair. Use this as a reference tool as you navigate through the process of repairing or replacing your gearcase lower unit. Damaged Prop = Damaged Gears?First off, if you think you have damage to your gearcase lower unit but are hoping that it will be okay if you ignore the problem, be aware that internal damage to your unit may not be noticeable right away.  Slightly damaged gears may seem to work fine at first but over time the off-kilter gears will wear away and cause eventual failure.  We recommend you have your motor diagnosed thoroughly to determine if you have internal damage.  Check out this forum for further evidence: What smacking a boulder does to your lower unit gears. We recommend calling the insurance company that has your boat policy before you do anything.  Some policies will cover accidental damage to your drive.  If your insurance policy covers the damage, your claims adjuster will give you instructions as to how to get your lower unit fixed or replaced.  Some insurance companies tell you where to go for a lower unit replacement or repairs.  Others will approve your repair/replacement estimate at the shop of your choice.  Either way, be sure you have a reputable boat repair shop that specializes in lower units.  Your local boat shop may purchase a boat engine replacement lower unit from a 3rd party for you so do your homework and find out where the unit is coming from. Three options for replacing your unit are discussed below. The choices you have when replacing a lower unit include a brand-new OEM unit, a remanufactured OEM unit or a new aftermarket unit.  I will discuss the pros and cons of each below.

Brand New Manufacturer Unit

This unit is made by your motor manufacturer and will contain original OEM parts.  Quality manufacturing is a guarantee and the unit will come with a nice warranty.  Warranties will vary by manufacturer so please check with a knowledgeable dealer regarding the warranty. The OEM Factory Lower Unit is the recommended choice if the price fits your budget. 

  • More expensive
  • Warranty depends on manufacturer
  • All parts are OEM
  • Outstanding quality drive
  • Usually ships in 24 hours

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New Aftermarket Lower Unit

The new aftermarket gearcase lower unit is usually manufactured in China.  The quality control is generally lacking on these units.  The up side is that they are significantly cheaper and some aftermarket manufacturers give a great, 3 year, fault free warranty. While this warranty is nice, it is a pain in the neck to deal with replacing the unit under warranty.  There is the obvious down time associated with diagnosing the problem, calling the manufacturer for warranty replacement, placing a replacement order, waiting for the unit to be delivered then waiting on the boat shop to install the unit.  Do your homework before purchasing an aftermarket unit!

  • Better prices compared to OEM Lower Units
  • Made overseas where quality control and trained mechanics may be lacking
  • 3 year, fault free warranty from certain manufacturers
  • Built new to manufacturer specifications
  • Includes all internal parts & is fully compatible and interchangeable with OEM drives
  • Usually ships in 24 hours

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Remanufactured Lower Unit

A rebuilt or remanufactured unit should be built by quality, trained mechanics with OEM parts.  These units and discount boat parts offer the best of both worlds, giving you OEM parts, a unit assembled by trained mechanics in the US and a nice price savings over new OEM units.  This option is the preferred choice of most boaters.

  • Priced at deep discounts to save you money
  • Rebuilt by trained mechanics in the USA
  • Units usually include a full 1 year warranty but this will vary
  • Built new to manufacturer specifications
  • Includes all internal parts & is fully compatible and interchangeable with OEM drives
  • Usually ships in 24 hours

If you are looking for a reputable remanufacture of lower units, check us out: US Boatworks in Kansas City.  We have been in the Replacement Drive business for over 30 years and know your motor inside and out.  We offer personalized service and will get your unit to you with the exact specifications needed.  Insurance claims are always welcome.  Our owner personally handles all insurance claims and is happy to talk with you and your adjuster to get the best deal for you. Call us at 913-342-0011 or email us for more info.

Call Us: 913-342-0011

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Remanufactured Lower Unit availability and pricing from US Boatworks


Lower unit replacing

Disassembly is the challenge! Well said!

If you have a great relationship with a great mechanic they may be inclined to offer up what they feel you are capable or incapable of DIY. Then of course you buy the parts from them! Agreed with an above poster, its good to know that basic seasonal stuff and the schedules. If you use the boat as much as some of us you save a lot of time and $$ if you need to do a 100 hour service in July. Basic to me is knowing how to change or swap, (four stroke) engine oil, engine oil filter, the primary fuel filter, block zincs, how to drain the vst, how to drain and replace lower gear oil, spin on water / fuel filters, engine zincs and spark plugs. Anyone with a basic tool set and nominal skills can handle these tasks. Knowing how to do the above you can "winterize" the engine yourself too. Its just one more step.

When it gets to something like a water pump replacement, we have done the work but they can be real time consuming and if you run into disassembly issues a real knucklebuster. Back to having a great relationship, I drop my lowers and BRING THEM, to my mechanic for the pump service. That alone is a savings and I know the job is done right. When you put 150 - 200 hours on the engines (rec fishing), this becomes and annual job. I don't want to even think about a one engine issue 75 miles offshore.

As for bigger jobs, this is where he has told me what I can and cant do, something like a low pressure fuel pump was easy for us to do. 1,000 hour service, not my job to tackle. Tearing down stuff to find the thermastats on my engines, no thanks, (they are buried), I think you get my point? Develop a great relationship with a mechanic and it will pay off long term. Treat them well and with respect and it will go a long way. You guys are dropping off a case or beer and some coffee gift cards to the shop over the holidays, right? Bringing them some tuna loins or tog fillets? Little things go a long way - just saying!



How to Install an SEI Lower Unit

Boating World

Author: Chris Rossell

Here are the items we needed:

– 14 mm socket (and wrench)

– 12 mm socket

– Pliers

– Grease

– Gear lube and pump

– Torque wrench

– Large flathead screwdriver

If you boat long enough, you will eventually tag bottom, especially if you live in the Florida Keys. Earlier this year, we decided to go wakeboarding on our 23-foot Dusky center console, powered by a 2003 Yamaha HPDI 200. The place we always use for skiing is fairly shallow, but the depth is very consistent … or so we thought. Bam! Bam! Bam! Damn! And just like that, our skiing trip was done — just like the stainless steel prop, which was missing most of a blade.

When we replaced the prop, we could see it had some wobble from a bent propshaft. We checked around and found that replacing only the shaft would be close to $1,000 with labor, but other components could also be damaged. We looked at remanufactured lower units, but they cost $2,240. While our Yamaha runs great, that’s a lot of money to spend on a 10-year-old engine, since a good used one costs about $5,000. Then a friend of ours told us about SEI, a company in Oldsmar, Fla., that sells outboard and sterndrive lower units. When we discovered that a new replacement from SEI costs $845 and comes with a three-year warranty — and we could install it ourselves without any special tools — the decision became a no-brainer. Another advantage of doing this yourself is that you will be learning the same procedure for changing your water-pump impeller (enclosed in the white plastic housing on the new unit).

Within a couple of days, our SE421 lower unit arrived, and we went to work. Fortunately, SEI has a step-by-step instruction guide on its website (, in the Technical Guides section), so this is a project that’s very doable by anyone with decent mechanical skills.

Before starting work on the outdrive, shift the throttle lever into forward and remove the prop. Next, look on the top rear of the outdrive; remove the plastic access panel and use a 12 mm socket wrench to remove the bolt (photo 1) that holds the trim tab, which is used to minimize prop torque. After the trim tab is off, use a 14 mm socket to remove the bolt underneath (photo 2). Just in front of where the tab was mounted is another bolt that you remove with the same 14 mm socket.Replacing a Damaged Lower UnitBW_11-2013_DIY_02

Next, remove the water-pump hose attachment from the upper unit, then remove the six bolts that clamshell the upper and lower units together, using a 14 mm closed-end wrench to make sure you don’t strip the nut head (photo 3). A word of caution: If the bolts don’t come off easily, don’t apply too much force, because you can break the bolt off. To make removing the bolts easier, we tapped them a few times with a hammer, sprayed them with PB Blaster and let them sit. The new lower unit we got didn’t include bolts, so we reused the old ones, which were in good shape after being cleaned. We had to wiggle the lower unit back and forth a little to get the two sections apart, but it came off easily.BW_11-2013_DIY_03

Use pliers to turn the shaft counterclockwise until it stops, so you can shift it into forward to match the shifter, taking care not to damage the shaft splines. We used Red “N” Tacky to grease the driveshaft, taking care not to lube the very end, per the instructions. Grease any protruding pins as well as the water-pump hose hole before threading the hose through it.

Slide the lower unit carefully into place. There are alignment pins on the new unit, to make sure it seats properly, so make sure they line up (photo 4). If the splines of the new unit don’t seem to be matching up with the hidden gear of the upper unit, slip on a prop and gently spin it counterclockwise until it matches. Attach the water pump hose to the upper unit.BW_11-2013_DIY_05

Hand tighten all six connecting bolts. Then alternately tighten them with a 14 mm wrench (photo 5). If you have a torque wrench, all the bolts on this unit should be tightened to 29 pound-feet.BW_11-2013_DIY_06

Install the trim tab (which we recycled from the old unit) by first inserting the lower bolt and tightening it. Then install the other bolt just in front of it. Position the trim tab, and screw the upper bolt into place, then replace the plastic access cover.

Fill the gearcase with lube, which we got from SEI for $10. To do this, take off the upper and lower screws, and fill it from the bottom using the pump until lube seeps out of the upper hole (photo 6). Wipe everything clean and put in the upper screw. Unscrew the pump fitting from the lower hole (the lube won’t come out as long as the upper is in place), and install the lower screw.BW_11-2013_DIY_07

According to SEI, the break-in routine is very important. We varied our speed every five minutes and didn’t go above 75 percent throttle until after the first five hours of use. During those hours, we shifted into reverse more than 10 times. Then, between hours five and 10 we occasionally went full throttle. Between hours 10 and 20, we changed the gear lube. The next day, we were out hunting for lobster and fishing in the Gulf Stream, catching mahi mahi.


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