WooLee Winder bobbins are required with a WooLee Winder flyer.  The bobbins have a gear at one end that drives the level-wind mechanism.

What's included:

• One (1) WooLee Winder Bobbin

 Product Details:

Weight: About 5.8 ounces (165 grams)
Dimensions:  Bobbin is 4.25" (108mm) diameter x 5.780" (147mm) overall length, 4.625" (117mm) inside length, 0.625"(15.875mm) core diameter
Capacity:  64.2 cubic inches (1052 cubic centimeters)