Hub cap without side fill for top-fill oil bath lubrication service on commercial truck wheel ends High-strength steel and aluminum construction with corrosion-resistant coating for long service life Six-bolt mounting pattern for secure installation on standard commercial truck and trailer axle hubs Rubber or O-ring sealed gasket type for a leak-resistant flange-to-hub interface in oil bath applications Clear polycarbonate or acrylic window material for at-a-glance oil level monitoring after installation Top fill configuration without side port for use where access from the front of the cap is preferred