omc 9.9 Shaft Length Differences

Lower Units Not Changed That Much in 30 Years : The same basic lower unit is still used on the latest motors for both the 2 stroke and 4 stroke motors. The gears and water pump are the same. After 1993 the exterior was changed slightly to incorporate a built in place for zinc anodes and the water intake screen was changed from an internal screen to 2 external screw on screens. Other than that the older ones and the newer ones appear the same. Although most of the internal part numbers of the lower units on the 4 stroke are the same as the two stroke, the housing is slightly different along with a different driveshaft. There are significant differences, so much so that it is not possible to interchange from the 2 to 4 stroke. The housing is physically smaller on the 4 stroke and has only 4 bolts holding it together.

Converting Motors With One Shaft Length to Another Length : This question comes up enough that it needs it's own article. First some background as to the whats and whys involved.

Shaft Length Differences : The standard motor comes to mount on a regular boat's 15" transom. This is designed so that the bottom of the boat is even with the cavitation plate of the motor. The cavitation plate is the flat fin in the lower unit housing above the prop. The difference in the longshaft unit is that it has a 5” aluminum extension the same physical size as the exhaust housing, to fit a 20" transom. This unit is supplied as a kit, comprising the extension, a new 5” longer driveshaft, a 5” longer shifting linkage rod, a copper water tube long enough to couple between the water pump and the old tube from the power head and 6 more bolts to assemble it to the gear case.

The SailMaster is basically the same 9.9 motor with electric start and a 10" shaft extension for use on sailboats, also called the extra long shaft for 25" transoms.

I have one reader who has a Extra Long 35" shaft motor that is mounted on his sailboat. However this one was surely not factory production as there is evidence of the driveshaft being professionally welded and trued. I am sure that the same was true for the shifting rod. Essentially this one used two, 10' extensions.

When replacing the water pump impeller if your motor has the longshaft, you first remove the upper bolts attaching the spacer from the exhaust housing. Once you have the gear case unit and the spacer off the exhaust housing, then you will have to remove the extension unit to get to the water pump that is still in it’s original location on the top of the lower gear case unit.

As you can see in the above illustrations, The different length motors either have no or 2 different height spacers installed between the lower unit and the upper exhaust housing. Then the oddball below. A conversion also requires the proper driveshafts long with other related parts.

Actual Removal of Gearcase or Lower Unit to Access the Driveshaft &Water Pump : If the motor has been used in saltwater to any degree, I will guarantee that at least a few of the bolts will be seized in and will be twisted off. I will cover this in a separate section. Have the motor mounted either on the transom of the boat, or on a motor stand or saw horses. It also will make a slight difference in the procedure whether the motor is a long shaft or not. There are 6 stainless, 1/4” bolts with 3/8” heads that come in from the bottom and are bolted up into the exhaust housing assembly. If it is a short shaft, they are the only ones there. If however it happens to be a longshaft, there is a 5” extension, or if a Sailmaster a 10" extension between the lower unit and this housing. If so, then remove the upper 6 bolts. The lower 6 bolts attach this extension to the lower unit.

When you get these 6 bolts removed, the gear case unit should drop down about 1/2”. If it does not you may have to tap it with a plastic mallet. Shift the gearshift into forward which will allow the shifting linkage shaft to drop down and align the retainer bolt with the gap between the housings. When the lower unit does slide down, there will be, inside the housing in the front right hand side, a connector that is a strap with 2, ¼” holes top and bottom and 2 bolts that are screwed in sideways. These bolts go in a circular groove of the shifting rod, which holds it in place. Remove only the top screw. It has a hex head and also a screwdriver slot. Once this screw is removed the gear case unit can be moved downward and off.

There is a stainless steel exhaust baffle screen that is slid into the rear partition's lower exhaust housing at the juncture where the gear case unit fits together on all motors except the first year or two. This baffle has many small holes in it and apparently helps reduce some of the exhaust noise.

The driveshaft may have a O-Ring around it on the splines that engage the motor crankshaft. The earlier motors had a groove around the shaft for this O-Ring, while the newer ones had the O-Ring in a internal groove in the lower end of the crankshaft. If it on the shaft, remove it until you are ready to re-install the parts.

Now on the gear case unit, you will find 4 bolts holding the water pump with the driveshaft going thru this pump housing. The water pump housing can be of pot metal or plastic, depending on the vintage. The plastic/nylon type is the newer style. If it is the older pot metal type and has been using saltwater, you may consider replacing the whole pump with the plastic style instead of just the impeller, as the internal portion where the impeller rubs will be corroded and will soon chew up a new impeller.

Unscrew these 4 bolts, slide the water pump housing up and off the driveshaft. For the older motors the front 2 screws are longer than the rear ones. Keep this in mind when you reassemble the unit. Later water pumps utilize all 4 bolts the same as the older shorter ones. Make note of which direction the impeller vanes are facing, as you need to install the new impeller the same way, otherwise it will not pump right. Now also there will be a key that holds the impeller to the shaft. Save this also for reassembly.

If you take it apart and the impeller appears still intact, you should consider inspecting it further, as it is not uncommon for the inner hub to become unbonded from the rubber impeller, allowing the hub to spin inside the impeller rubber or a vane may become hardened and break off. Also some aftermarket impellers can be thinner than the originals, not sealing on the sides, sucking air, creating erratic pumping.

Short Shaft Converted to Long Shaft : Motors can be converted from short to long, or long back to short with the proper parts. When converting a short shaft to a long shaft, you will have to purchase a conversion kit, which includes a 5" gearcase extension (#331152) with the 6 bolts(#324334), a new 5" longer driveshaft (#329511), longer shifting rod (#319298), a water pump tube extension (#319340), exhaust seal (#326922, water tube extension (#320025 and it's O-Ring seal (#202893. The reason for the water tube extension is that you DO NOT want to try to replace the upper water tube as that requires the powerhead removal because of the place the upper tube is secured in the block/exhaust tube.

The long shaft itself measures 27 9/32". These can be purchased off e-Bay many times, but possibly without all the needed parts.

Shafts are anchored in the lower unit by the water pump impeller key being retained inside the water pump housing. Once the water pump is removed, the driveshaft simply lifts up out of the lower unit, sliding out of the splined drive gear. However be careful not to tip the lower un it over as with the driveshaft out, there is now no upper gearbox oil seal.

Shown below are the gearcase extension with the black extension seal & white inner extension. Also shown are the differences in the short shaft & long shaft shifting rods & drive-shafts.
Also needed is the 5" water tube extension (shown in the 2nd photo below).

Shown below is the complete long-shaft conversion kit. The long black rubber guide (exhaust seal) rests in a pocket down inside the extension and is stopped by the ears you can see on it. In assembly you slide the 3/8" OD copper extension tube onto the existing tube that is attached to the powerhead, then push this extension down through the rubber guide (exhaust seal). This guides the now extended water tube so all you have to do is to then align the lower end into the larger flared water pump grommet.

Here is a complete 5" extension (long shaft ) kit

In the photo below is the 5" water tube extension needed for the long-shaft conversion. Also used is a O-Ring seal # 202893 which slid inside the tube extension and was secured inside the expanded ring. Also in the photo below is the grommet, just to identify the direction the tube is placed, as this tube will insert into the grommet in the water pump outlet.

In the lower photo below, I am not sure that this 5" tube extension is factory or not, but have seen many of them and have made my own as explained below. They are a tight enough slip fit to the upper tube that no appreciable loss of water is observed.

To do this conversion, you need to remove the lower unit just as if you were replacing the water pump impeller. You will need the proper extension assembly (usually the 5" white spacer adapter shown above) which has a rubber spacer inside for the alignment of the water tube, a plastic round hollow spacer that the driveshaft does thru. Also an appropriate longer driveshaft and shifting rod. You also need the another 6 mounting bolts and a water tube extension.

You should be able to buy these kits from any Johnson/Evinrude dealer(#392002), but the price may reach $300 as a guess. However if you go to some marine dealers they may have a used unit or enough parts to make up one. Or go to one of the used OB parts dealers online, they should have them for 1/2 of that. Or go to http://www.ebay.com/ , type in "johnson 9.9", they show up occasionally and can go for $50 - $100. However many times these guys may not sell the complete "KIT" and only the extension or the long shaft. The reason is many times the person selling the parts does not really know what it takes to do the job. Don't pay a high price for only part of the set. If you only get the normal parts of it (extension and driveshaft), you then need the longer shifting rod, the rubber water pump alignment spacer, the rubber exhaust seal, the white plastic tube spacer and a water tube extension, however these should be available from the dealers, at a cost of course. The water pump alignment spacer sells for $12, the longshaft shifter rod sells for about $13. So try to pick a unit that has as many parts as needed.

I have not been able to find in any of the parts manuals just a water tube extension. I make my own by using about 6" of 3/8 OD soft copper tubing. Be sure it is pretty straight and mount it in a metal lathe with about 1 1/2" protruding. Then I take a 3/8" high speed milling machine end mill, band-sand the non-cutting end to a smooth radius. Mount this end mill backwards in the lathe tailstock chuck, oil it, with the spindle turning slowly, the end mill oiled force the radiused mill into the copper tube 1/4", remove it, re-oil, do again to about 1 1/8". This gives you a little extra for overlap. You might check to be sure that the tube in the housing is round so that it will be a slide fit into your newly made extension. The fit is tight enough that you do not need any sealant. The new extensions have a expanded groove to accept a O Ring that acts as a seal however.

The inner exhaust tube is the same for either the short or long shaft motors. There are no gaskets or seals needed for these conversions.

Long Shaft Converted to Short Shaft : This would would be encountered less often, as there are way more standard length shafts motors out there than long shafts, with the long shafts typically used as secondary motors on larger boats that need the extended length, while the bulk of these motors sold during the time frame they were made were used on standard or smaller boats. But if you can purchase a good long shaft and need a short shaft, here is what you need to do.

In doing this you need to remove the outer gearcase extension, not use it nor the inner tube spacer or water tube seal.

   I have shortened long driveshafts and welded them back together. Short shaft overall length is 22 9/32".   If in doing this you would finally have to be sure that it is straightened so there would be no wobble. Again if doing this, go to a welding shop that can weld stainless with a wire feed welder (more penetration), grind the mating ends to a cone type taper with the center about 3/16" dia left.   Place them in an angle iron that is mounted in a vise so it forms a Vee to hold it in line for the initial weld. Once welded and initially straightened, the weld can be rough turned down to slightly oversize to the outside diameter and at this time while in the lathe, ran, straightened, ran again until it is close with the least amount of wobble.

   Or another method would be to make a sleeve, 2 to 3" long that can be slid over the shaft, have it welded to the shaft on each end of this sleeve. This would change your installation procedure slightly as the water pump unit then would have to be slid onto the shaft from below before you installed it into the lower unit. The splice would need to be maybe mid point between the water pump and the upper end, or at least enough above the water pump so the pump body could be installed above it's final location, then slid down to mount up.

   The upper shifting rod is the same on either the short or longshaft motors, just the lower one in the lower unit is different.

The shifter rod part number for the short shaft is #318992. The long shaft rod can just be cut off and the bend put back in the top end. You will not have to re-groove it like the original, as you just need to grind a notch in the location that the bolt goes thru that attaches to the upper rod. Be sure however that the notch is in the exact same relationship from the top, but the 5" lower, as this times the shifting. Or unscrew the shifter rod from the lower unit, cut 5" out, then re-weld it back. If you do it this way stand the lower unit on end then unscrew the rod being careful to not jar it around as you will have to push the rod back in, insert it in the female threads of the yoke unit, plus if it is tipped over the gear oil will run out. You can adjust this slightly by how many threads you screw it into the lower unit shift linkage.

In converting a factory long shaft to a short shaft motor, you can simply cut off the 3/8" copper water tube. Cut off 5", or so that the tube stops 1/2" above the exhaust housing bottom. The best way to do this would be to mark it, use tubing cutter so that the new bottom end will be slightly rounded at the cut, making for an easier reassemble of that end into the water tube outlet grommet, or use a Dremel Tool with a part off stone and file the end so it is not sharp. Be sure to remove any rough edges, as this has to fit into the rubber grommet in the top of the water pump housing, you can't really see if it is going or not, as it is mostly out of sight at the time. Or there is the possibility it has a removable 5" extension #0320025 mentioned above in the longshaft conversion, if so simply remove this off the lower end of the tube.

One of you internet readers on the east coast inquired about doing this above conversion. I suggested he contact a marine repair shop to see if they had a used driveshaft instead of cutting and welding his. The mechanic said it was impossible to do this conversion. I wonder just how long he had been in the business and or if he was drinking more than just water.

Driveshaft Interchange : Now to really confuse you. Looking at actual various parts catalogs for these series of motors, you will see a considerable number of actual part numbers. From my experience, all of the 2 cycle 9.9/15hp motors all WILL use the same driveshafts. The splines are all the same. The length will be different depending on the design for the boat's transom height. However about 1986 or so the retainer slot for the O-Ring was dropped because of possible weak point. The shafts will still interchange, just grease the upper splines well before reassembly.

The older / pre 79 shafts are listed as obsolete, but the newer ones will fit. Probably the effort of maintaining interchange numbers after a good many years, some numbers fell off the end of the list. In the list below the date is the year of the parts list that I found these numbers. Usually the number stays good until the next parts list change or it is then later superseded. The only reason I am listing these is that a reader questioned me on a part number I recommended above. These are 2 cycle motors unless specified.

1978 Regular, or 15" Short shaft	1986, 20" Long shaft	1992 Sailmaster, 25" Extra Long shaft