Yamaha Thrust Washer Nut & Pin Kit Y1 Ye-Kt

Thrust Washer
The thrust washer is mounted on the propshaft, between the lower unit and the propeller, and it may or may not have splines. It's an important component of a boat propeller's installation, because it fits the shaft in such a way that will not allow either itself or the propeller to move forward under thrust, thereby transferring thrust to the shaft and protecting the gear housing and its seal from contacting the rotating propeller.
A matching taper on the propshaft and the washer is one effective design that stops a thrust washer from moving too far forward. The thrust washer also helps to keep the propeller centered on the shaft when the soft inner prop hub fails to do so under adverse high-torque conditions.

Spacer
Like the thrust washer, the spacer assists alignment of the prop to the shaft when the inner hub is unable to hold its shape, only this time at the aft end of the prop.
The spacer bears reverse thrust loads and prevents the inner hub from moving aft, with its large surface area. The final washer and nut prevent the spacer from moving aft.
Many spacers are splined and some, for numerous Mercury, Mariner and Mercruiser applications, are also cogged or pinned to work in conjunction with an outer tab washer to prevent the nut from turning loose.

Washer
The washer helps fill space between the spacer and the nut, where the splines on the propshaft end and the threads begin. This helps prevent fore and aft play on the shaft by the prop.
A tab washer can be used if the spacer is designed for one. Otherwise, a large traditional washer would be expected on an assembly that uses a cotter pin to retain the nut.
When used, a tab washer's primary function is to prevent the nut from rotating in relation to the spacer.

Prop Nut
The prop nut secures the prop to the shaft and helps transfer reverse thrust to the shaft.
Normally made of brass, a nylon locknut is used in conjunction with a tab washer or else a castellated (castle) nut is used with a cotter pin.

Cotter Pin
Where used, the cotter pin passes through a hole in the propshaft after the castle nut has been tightened with its slots aligned on both sides of the hole.