Abstract: Ground opener units for agricultural seeding machinery and the like utilise a gauge wheel and associated depth adjustment mechanism for regulating the penetration depth of the disk opener in use. An improved gauge wheel adjustment assembly comprises a spindle adapted to be rotatably supported in a spindle sleeve, the spindle having a shaft with a first end with a rectangular boss adapted for engagement with a depth adjustment handle of the disk opener and a second end with a tapered spline engagement formation. A gauge wheel support arm has a complementary tapered spline formation for engagement with the spindle second end.