These cast iron pillow block bearings are for 3/4" shaft and have self-aligning ball bearing insert and grease zerk. Protective cap keeps dirt out of zerk. Two set screws on the inner race to lock shaft. Two slots are used for mounting. We use these with our 13 inch Traction Lug Wheel and Shaft Set on our 4WD WC DB and 6WD WC DB All Terrain Robot Platforms.