What's inside an outboard engine gearcase

Since we have a lower gear case in pieces anyway, I may as well write about how the bottom end of your outboard engine is put together. This is one of those clever mechanisms that we boaters usually just take for granted, and trust the shop to deal with when it fails. It's kind of cool to see how it all works.

To figure out what's what in a hodgepodge of parts like this, you'll want an assembly diagram (like this one). Don't even think about trying to assemble the thing without it. If you just want to see how it works, though, you only need to recognize a handful of key parts.

We'll begin with the clutch dog. This component is splined to the propeller shaft, so it rotates with (and transmits torque to) the propeller. It is free to slide forward and backward on that shaft. The dog has two teeth sticking out from each side.

These teeth mate with a matching recess cut into the inside of the two main gears. When the dog is engaged with one of the gears, power is transmitted from the gear, through the teeth to the clutch dog, then through the splines to the propeller shaft.

Here's the dog in "neutral" position on the propeller shaft.

The two main gears spin freely on the smooth parts of the shaft when the dog is not engaging them.

The forward gear has a built-in bushing, with an oil hole and oil channels, to let it spin on the shaft when the motor is in neutral or reverse. The reverse gear has to spin freely for extended periods at high speed, so it gets a larger, bronze bushing to reduce wear.

The vertical driveshaft from the engine ends in this small pinion gear. It is always engaged to both the forward and reverse gears, which (as we've seen) are spinning freely in opposite directions on the propeller shaft.

To move the clutch dog, we add a lever and a yoke connected to the shift linkage through the engine's midsection and, eventually, to the helm controls. Pushing down on this linkage rotates the lever clockwise, sliding the clutch dog into the reverse gear. Power is now being transmitted from the pinion, to the reverse gear, to the dog, then through the splines to the propeller shaft. The forward gear is spinning freely on the shaft. To engage forward thrust, we pull up on the linkage and slide the dog the other way.

A tapered roller bearing on the front of the shaft transmits the propeller's thrust to the gearcase housing. The tail bearing housing shown here contains the second and third roller bearings and the shaft seal pack. Engine exhaust is routed down the midsection, around this housing (hence its blackened colour), and out through the propeller hub.

All of this fits inside the torpedo housing of the lower gear unit. The seat for the tapered bearing is at the front. In this photo, you can see the splines of the vertical driveshaft, which would slide into the back of the small pinion gear shown earlier.

Note that there are a few missing parts and a lot of damaged metal in these photos- this is, after all, a gearcase that just committed suicide.



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