Due to patent restrictions, Outboard Motor Corporation engineers were forced to come up with a power transference design that did not use universal joints (u-joints) to transfer power from the engine coupler to the stern drive. The u-joint design patent which belonged to Volvo which was also used by Mercury on their Mercruiser models, allowed the outdrive to be used at all speeds – while turned left or right while the outdrive was tilted up. Engineers designed the first OMC Stringer (Electric Shift model) to use an engine to outdrive power transference design which called for the use of two ball gears (sometimes called knuckle gears) to mesh together between stern drive and engine coupler via an intermediate drive shaft.
The early OMC models (1963 – 1972), used ball gears which had to be pressed on to the intermediate drive shaft and the pinion shaft in the outdrive. Today, in OMC circles, these ball gears are sometimes referred to as ‘press on ball gears’. The intermediate ball gear contained seven notches, or contact points – and the mating ball gear on the drive side contained seven contact points as well. When mated together and with the outdrive tilted all the way down; the result was that all seven contact points on the ball gears to remain in constant contact at optimal points with maximum contact surface. This allowed for all of the torque from the engine, through the coupler to the outdrive, to be evenly distributed to all seven contact points. However, due to a fundamental design flaw, the outdrive could not be used with the outdrive tilted up without causing rapid damage and wear to the ball gears. When tilted up, the stresses on all seven contact points began to shift to the upper knuckles on the gear; and the further the outdrive was tilted up, the stresses were shifted off of the lower knuckles and shifted to only the top three upper knuckles. The composition of the steel used, along with the size of the knuckles, were designed based on the theory the outdrive would only be run with not tilted up and all seven contact points would evenly handle the torque. The design of the gear could not withstand the stresses caused when the boat operator used with outdrive when tilted up (more so at higher speeds), and rapid wear and damage would quickly begin to appear on the knuckles. Another weakness in the design was the clutch pack responsible for lifting / tilting the outdrive up – this clutch would lock the outdrive in place, however, when the stern drive was engaged into reverse gear and throttle was given (more prominent on high horsepower motors), the clutch could fail; unlocking which could cause the outdrive to begin to tilt up due to the reverse gear outdrive torque. This was especially prominent in heavier boats with large engines such as house boats. This slight lift in the outdrive would case excessive wear on the knuckles as well.
In 1970-1971, with the power transference design unchanged, OMC did redesign how they were installed onto the intermediate drive shaft and upper gear case pinion shaft. Instead of having to be pressed on and pressed off, the drive and pinion shafts were changed to a simple splined end design which allowed the ball gear (redesigned to have internal small splines) to be slide on via the spines; and simply held in place by installing a nut at the end tightened to the correct torque specs. A cover sat over the nut sealed with a compound to keep the nut protected from all elements.
How to Remove the Stern Drive Ball Gear (Press on Type and Newer Style)
Replacement ball gears can be purchased in a kit (kit includes both gears, o-rings, and all oil seals for shafts). Click here for kit details. This can be purchased through the order desk at T.C. Electronics/Marine (416-751-7326) – reference OMC GLM part # 22050.
Begin by undoing the four bolts and lock washers on the bearing retainer (OMC #313184, GLM #27960) that houses the ball gear. Use a large flat head screw driver to carefully pry out the assembly, and remove the colored shims (note color of shims if they need to be replaced). Apart from the ball gear being pressed onto the shaft, it is additionally secured with a nut on the end of the shaft. Punch out the cover that sits over the nut; and use a 1-1/8” socket to remove the nut (heat may be required to break the seal on the nut if a permanent compound such as 680 loctite was used). A press will be required to remove the ball gear and bearing off the shaft. After removal, slide off the bearing retainer and two oil seals. Note how the seals were installed back to back as these or new seals will need to be installed in the same manner. With the driven gear assembly removed, the bearing cup (seated towards the inside rear area of the upper gear case) needs to be removed along with the metal shims. Loctite may also have been used at time of assembly making this bearing cup difficult to remove – a slide hammer tool will be needed for removal. The removal process is identical for the newer style (non-press on type) ball gear, with the only difference being after the nut is removed the knuckle gear should simply slide off without the need for a press.
Removing the Intermediate – Engine Side Ball Gear (Press on Type and Newer Style)
Start by locating the oil fill and vent screw located just above the ball gear; this must be removed with a flat head screw driver, along with the oil drain screw located just underneath the steering gear. Approximately 1 cup of oil will drain out, which is responsible for lubricating the inner bearing. Once all of the oil has been drained, remove the four bolts (3 bolts on older models) on the retainer plate mounted to the intermediate housing. Remove the drive shaft assembly by pulling it straight out of the intermediating housing; this assembly should slide out fairly easily, however if water had entered the housing and the shaft is rusted a slide hammer may be needed to remove the shaft assembly. Once the intermediate drive shaft has been extracted, use a slide hammer to remove the seals and bearings. For newer style ball gears, punch through the cap covering the nut and remove it, then undo the nut and remove the ball gear (you may need to remove any silicone or sealant around the nut first). If you’re working on an older model with the ball gear pressed on, use a press to remove the knuckle gear off the shaft.
Installing New Ball Gears (New Style)
Use a seal installer tool to install new oil seals back to back into the drive shaft oil retainer. The lip of one seal must face the intermediate housing, with the lip of the other seal facing the ball gear (the flat sides of the seal with sit against each other). *Note, some early models only had one oil seal installed. Next, use a press to install both roller bearings onto the shaft; the first bearing pressed on should butt up against the start of the splines. Next, coat the splines with a sealant and install the new oil seal into the shaft, follow by the retainer plate. Inspect the splines to make sure all oil is cleaned off, and install the ball gear (OMC #908063, GLM #22010) onto the shaft. Torque the nut on the end of the shaft to 95-100 ft-lbs. using a 1-1/8” socket, and apply some loctite around the nut and cover area before placing the nut cover on; making sure to wipe off any excess loctite after cap installation. After reinstalling the drive shaft into the intermediate housing, make sure to fill the oil reservoir back up with oil as this oil is needed to lubricate the bearings on the shaft.
For the stern drive shaft, install it into the upper gear case without the shims or o-rings and install the bolts into the retainer to keep it in place while you install the ball gear (OMC #908069, GLM #22020) onto the end of the shaft. *Note – make sure to clean the splines of all oil or dirt before installing the knuckle gear. Torque the nut to 95 ft-lbs, and install the cover over the nut with a small amount of Loctite; removing any excess Loctite after cap installation. Next, remove the bolts from the retainer to housing and pull out the shaft assembly to begin the shimming process.
Tips For Use and Installation
- Always make sure to use a Loctite product on the cap that covers the nut
- Torque the nut to 95 – 100 ft-lbs for the intermediate and stern drive nuts
- Always make sure the splines are clean of oil or dirt before install the ball gears onto the shafts
- Never use the outdrive when it is tilted up unless absolutely necessary. If used while tilted up, keep the engine speed to less than 1000 rpm, preferably 500 – 700 (idle speed) and lower the outdrive as soon as possible to prevent excessive wear the knuckles
- When inspecting knuckles on gear, once 25% wear occurs or the start of a straight edge wear pattern emerges replace gears. Overly worn knuckles can cause misalignment issues inside the upper gear case which could lead to serious internal gear issues.