This is a 3/4 inch Axle Castle Nut used to secure your trailers Hub and Bearings onto your trailer spindle. The 3/4 inch spindle size is a less common spindle diameter, so you will generally see this castle nut used on older spindles and some agricultural axles and spindles. You will generally use a cotter key or tang washer to secure the nut into place and keep the castle nut from backing off while the trailer is in motion. This nut is the last thing that is installed onto your spindle, and once secured, allows you to install your Dust Cap and finish your project. If you have a smaller or more obscure trailer axle, this 3/4 inch spindle castle nut might be what you need to secure your hubs to your trailer axle.Specifications- 3/4 inch Trailer Castle Nut- Steel Construction- Fits 3/4" diameter trailer spindles