They are zinc plated for a long lasting service life.
eBay
EgoTrailer 2pcs Replacement 45mm Metal Dust Cap Wheel Hub Trailer Bearing Dust Grease Cover
Aliexpress UK
Multi-Purpose Bearing Grease, 14 oz. We cannot be held responsible if you do not provide engine model and serial number with order. Most outboard parts are specific to certain models & year series.
eBay
Bearing Protector Grease Wheel Hub 1.98 W/ Dust Cap Cover for Boat RV Snowmobile ATV Utility Trailers
Aliexpress UK
52mm External Size. Internal fit. (this hub cap slots into the hub centre rather than over it). Essential to protect the wheel bearings from dust and grit which can severely shorten the wheel bearing life.
eBay
Metal Wheel Drive/Bearing Hub Rim Double Wheels Set for 1/14 Tamiya RC Trailer Tractor Truck SCANIA VOLVO ACTROS MAN Model Car
Aliexpress UK
60mm Internal Size. External fit. (hub cap slots over the hub centre rather than into it). Essential to protect the wheel bearings from dust and grit which can severely shorten the wheel bearing life.
eBay
Metal Wheel Double Wheels Set Drive Bearing Hub Rim for 1/14 Tamiya RC Trailer Tractor Truck SCANIA VOLVO ACTROS MAN RC Car
Aliexpress UK
Zinc plated for a long lasting service life.
eBay
Zinc plated for a long lasting service life.
eBay
Zinc plated for strength and durability.
eBay
Popular on trailers with Avonride hubs / axles but can also be fitted to many other trailers. 47mm External Size. Internal fit. (hub cap slots into the hub centre rather than over it). Essential to protect the wheel bearings from dust and grit which can severely shorten the wheel bearing life.
eBay