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Steel Washer 3/8"

Depending on the manufacturer it can be better to use 5/16" washers for 3/8" bolts and threaded rod.  The 3/8" rod fits through them fine but with a better tighter fit.

The Hillman Group 830506 Stainless Steel 3/8-Inch Flat Washer, 100-Pack 

Steel Washer Zinc Plated 3/8in


5/16 in. Zinc Plated Cut Washer (100-Pieces) 


http://www.homedepot.com/p/Crown-Bolt-5-16-in-Zinc-Plated-Cut-Washer-100-Pieces-46050/203639702         $9.95 for 100    (10 cents each)


It can be useful to also add a split washer to avoid having to also put a spanner on the far side.

The Hillman Group 300024 Split lock Zinc Washer, 3/8-Inch, 100-Pack 

https://smile.amazon.com/gp/product/B00CAO5BQW/ref=ox_sc_act_title_3?smid=ATVPDKIKX0DER&psc=1    $5.63