TOYOTA 2011 Sienna - Manual (Page 383)

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2-5. Driving information
Selecting trailer ball
Use the correct trailer ball for your application.
Trailer ball load rating
Matches or exceeds the gross
trailer weight rating of the trailer.
Ball diameter
Matches the size of the trailer
coupler. Most couplers are
stamped with the required trailer
ball size.
Shank length
Protrudes beyond the bottom of
the lock washer and nut at least 2
threads.
Shank diameter
Matches the ball mount hole
diameter size.