outboard motor parts

DIY inboard outboard boat motor parts & accessories

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Inboard and inboard/outboard boats basically use two different applications of drive systems. Both systems act as transmissions and provide forward, neutral, and reverse only. Engine RPM and boat speed are altered by changing propeller type and pitch. We will be looking at propellers later in this section. An inboard application, figure 1, normally contains a transmission mounted on the back end of engine, then transfers power to the propeller via a long slim drive shaft. These applications are normally seen in some ski boats, and some larger boats not requiring a heavy duty drive system. Inboard/outboard applications, figure 2 use a different type of transmission called a "stern drive". Stern drives are used for small to large ski boats, large cruisers, high performance, and heavy boats needing a heavy duty drive. Stern drives are available in a few different options to be used for small light weight, lower horsepower boats, all the way through heavy high horsepower boats. The main difference in the two being that a stern drive can be raised and lowered for reasons of providing a way to adjust the way the boats rides when on plane, and also the for operation in much shallower water than the inboard.

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Inboard Drive System

There are only a couple of items of this system that can be serviced by the average person. First off, the transmission, figure 1, requires regular maintenance of oil changes. The frequency of oil changes depends on the severity of operation. If the transmission receives a lot of high strain use, the oil needs to be changed about twice a year. If average strain, then about once a season is adequate. The other item that might need attention is the prop shaft seal located in the fitting where the prop shaft goes through the hull. See figure 1 This seal can harden and loose seal after aging and a water leak can result. Access to this fitting is normally fairly easy, but can sometimes be hampered due to engine compartment design. A couple of things to keep in mind when changing this seal are, after loosening and retraction of the large nut that compresses the seal, and seal extraction, there is open access through the fitting to the outside of the hull. This is a very tricky procedure when attempting replacement of seal while boat is in water, as water will be gushing in through fitting, so make all effort to remove boat from the water if at all possible before repairs are made. The second note is that the seal is a specific size in thickness, and the same thickness of new seal must be used for proper compression when nut is retightened. Most common sizes are 1/4", 5/16", and 3/8".

Inboard/Outboard Drive System

These drives contain an "upper unit", and a "lower unit" and are much more sophisticated than the inboard type. There are many more moving parts that can become problems, but if regular required maintenance is performed as suggested, the drive will be mostly trouble free for the life of your boat. This maintenance can mostly be done by the average person in most cases, but if you do not feel confident in some procedures, let a qualified technician perform the service to limit that headaches that could occur. Stern drives also include a tilt/trim system for raising and lowering the unit for purposes of trailering, and also adjusting performance while underway. Tilt/trim will be discussed in further detail later in this section.

Inboard/Outboard Drive System Servicing

Lets start with Figure 3 that shows the location of oil drain and vent plugs, and also the nuts that are required to be taken off for removal of stern drive. The stern drive oil should be checked about 3 times a season, and changed once every season. Loosen and slightly remove bottom drain plug in lower unit just enough to allow a bit of oil to exit onto your fingers. If the oil is blackish blue, it needs to be changed right away. If it has a light whiteish milky look, or if pure clear water runs out, you have a seal leaking somewhere, and immediate attention is called for before further use. Due to the special tools needed in resealing the stern drive units, it is best recommended to let a qualified technician do this work for you. If the oil looks somewhat close to fresh, then all should be fine until time for the annual change. The annual change should be done in the fall, most commonly during winterization to make sure all moisture is removed from the internal parts of the stern drive. This obviously prevents corrosion from occurring. To change the outdrive oil, remove the upper vent plug located in the upper unit, and the lower drain plug located on the bottom side of the "bullet" shaped section on the lower unit. Allow oil to fully drain. This will take a few minutes due to gear oil viscosity. Once oil has completely drained, refill with type and weight oil that is recommended by the manufacturer. A very important note when refilling, you MUST fill stern drive from the bottom drain hole until oil runs out of top vent hole, then install top vent plug first, followed by drain plug. If you attempt to fill the stern drive from the top vent hole, an air pocket will develop, and the oil will not be able to fill the stern drive cavities. This is most often easily done with a hand pump (available as an accessory from most marine supply stores) made to work with quart bottles of new stern drive oil.

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To service the rest of stern drive, first place shifter handle on control box in forward gear. The drive is removed by taking off the six nuts that attaches it to the bell housing,Figure 3, and also removing the tilt cylinder anchor pin in Figure 4. Once the nuts and anchor pin are removed, it may take a little bit of convincing to get the sterndrive loose where it will slide back off the bell housing studs. You may gently use a pry bar or screw driver inserted between the stern drive and bell housing to get things started. Once the stern drive is loosened up, it should completely slide out of the bell housing with ease. After removal, remove and replace the o-rings on the drive shaft and grease the u-joints, figure 5. Also take a minute to check the gimbal bearing located by viewing inside and through the large upper accordian looking rubber boot, figure 6. Reach your hand in and with a couple of fingers, rotate the center race of the gimbal bearing back and forth. If it feels nice and smooth as it rotates, then all is well. If it feels rough and gritty as it rotates, it needs to be replaced before reinstalling stern drive. It takes a special puller to remove the gimbal bearing, so it would be advised to let a qualified technician remove and install new gimbal bearing for you. Before reinstalling the stern drive, check the accordion looking rubber boots and the water intake hose to make sure the are no cracks or holes caused from dry rot and aging. If any of them are found to be in bad shape, this is the time to replace them. Any cracks or holes in the boots, can allow water to leak into the boat, and the water intake hose being in bad shape could cause the cooling system to lack enough water intake for cooling purposes. To reinstall the stern drive, simply reverse the procedure replacing the stern drive gasket and o-ring that was exposed during removal. More detailed help with the aforementioned procedure can be found in the manual.

Tilt/Trim System Servicing

The tilt/trim system needs little if any service other than checking all trim hoses for deterioration or damage, and trim rams for nicks or bad spots. If no seepage is present, then all should be ok. The trim pump located inside the boat, contains a reservoir for the trim fluid. Check fluid on occasion to make sure no milky white color is seen, and simply keep fluid at correct level.


The propeller should be checked periodically for damage to ears. Any bent or gouged areas should be repaired for maximum performance. Prop should also removed on a regular basis to check for fishing line that could have been caught and wrapped around the prop shaft. Fishing line is an ever present problem on floating in waterways, and many prop shaft seals are damage causing leaks when this happens. Another thing to look for while checking prop is, with shifter in neutral position, spin prop and pay special attention to tip of prop shaft. If it has any wobble as it spins, it most likely is bent, and should be replaced. If you find that shaft is bent, it will require the services of a qualified technician for replacement. Special tools are required to tear down and reassemble lower unit, so the average person would not be able to perform this task.

Propeller replacement & selection

This subject can and does get into a lengthy discription if you might be looking to change or replace a propeller for change of performance. Lets face it, selecting the right prop for your boat can be a tough job if you don`t understand what the characters of a prop are. Props come in many designs and materials, and the manufacturers have the knack of producing big fancy advertisements trying to show their product as being the best. In the following, I will try to disclose a little bit of the mystery of prop design and selection.

Diameter and pitch

Lets start with the diameter and pitch. The diameter of a prop is pretty much self explanatory, but the pitch is sometimes a dilemma to a person. The pitch of a prop is defined as the theoretical distance the boat will travel with one complete revolution of the prop with 0 slippage. Obviously there is slippage in water, but this is the standard in which a prop is built. For example, when someone tells you that a prop has 21 inches of pitch, that means that the blades are set at an angle that will theoretically move the boat 21 inches in one complete revolution of the prop.

Ok, now no one cares how far the boat moves, only how it performs right? Selecting a prop for your new boat is normally made easier due to the fact that most dealers already have pre-installed the prop for you. Lets say that your boat does not perform to expectations though. What do you do? First off, if its a new boat, talk to your dealer. If you have a boat thats been in exsistance for a while, the first thing to do is check your RPM on the tach at wide open throttle. Outboards can handle, and should run a maximum of 5,500 RPM wide open. Inboards most generally red line at 4,500 RPM. If your boat does not run at or very near these RPM`s wide open, one of three things can be the problem. First, a mechanical problem can exsist, second, ambient air temperature can greatly affect the performance of a motor. If its an extremely hot day, performance will drop, thus RPM will also. Third, the boat does not have the proper prop installed. If your motor is mechanically sound, and its an average temperature outdoors, but your wide open RPM is lower than previously mentioned, you may need to go down an inch or two in pitch. If it runs above the specified red lines at wide open throttle, you may need to go up in pitch. Beware though, it doesnt take much to make a difference. Most times, pitch can be reset by a good prop shop, without the need for purchasing a new prop. Two inches up or down is normally the max though.

Designs and materials

Now that I have explained the theory of a prop, lets go to designs and materials. Props come in three materials, and have many designs. Props are made of aluminum, stainless steel, and brass. They all look pretty much the same you say? At first glance maybe they do, but when it comes to the technical aspects they differ greatly. In the beginning, props were made of brass and had only two blades for most applications. This was fine back then, due to the small and lightweight designs of boats. Todays applications require more bite to push and carry the load. Therefore, props with 3 blades are most common, but 4 and 5 blade props have been introduced. 3 blade props provide the best all around performance and good top end speed. 4 and 5 blade props are used for alot of bite for low end take off and to get on plane quicker, although they limit and usually reduce top end speed. For all around normal performance, an aluminum prop is probably best suited, and run from $60 to $200 depending on your application. If you want higher performance with your boat, consider upgrading to a stainless prop. Manufacturers claim different advantages of their prop over others. This is sometimes true, and sometimes just a sales pitch. Some claim higher speeds simply by installing their prop. There are a few out there that will do just that. When it comes to speed, the greatest affect comes from a couple of different things. Weight, horsepower, hull design, and lastly the prop design. If you have the power and good hull design, the prop can make or break your complete combination. For all out speed, choose a 3 blade polished stainless prop that has good cup to it. These run anywhere from $350 to $600, and will give you a good low end holeshot, and decent top end speed. If you dont mind emptying your pocketbook a bit, you could also consider a "shift prop". A shift prop? These are a fairly new design, and do exactly what the name states. They are a mechanical type design that use springs to determine when the blades pivot to change pitch. At idle and low speeds, the prop is in the lower pitch range, and at a higher RPM, the blades pivot to a higher pitch range, feeling just like a shifting transmission. The RPM that the blades pivot is determined by the spring setup. This is about the only way to have the best of both worlds, an unbelievable holeshot and outstanding top end speed with one prop. These get pretty pricey though and can run as much as $900.

If you should have any questions about your drive system that may not be answered in this section, feel free to email us. We will gladly help you in any way that we can.