Toy haulers on 7,000 lb axles come both ways: grease hubs, which is what every RV axle ships with, and oil-bath hubs, which some owners swap to after a bearing failure on a hot desert run. The question on the dunes boards is whether the swap is worth it, and the answer depends on one thing owners do not expect: the seal.
How the two work
A grease hub is packed by hand. Grease is forced through the bearing cages, the hub cavity is left partly empty, a lip seal at the back keeps grease in and dirt out, and a cap at the front does the same. It is tolerant of a small seal leak, of a dented cap, and of neglect, which is why it is the standard.
An oil-bath hub runs the bearings in a bath of gear oil. The front cap is clear with a sight glass, the hub is filled with SAE 90, 80W-90 or 75W-90 gear oil with as little air as possible showing in the glass, and the rear seal is a different part entirely: a unitized oil seal that runs on the spindle and is built to hold liquid rather than grease. The bearings run cooler, they are lubricated constantly rather than by whatever grease stays in the cage, and you can see the level every time you walk past.
Why it is not a seal swap
Owners on the boards ask for "the oil-bath seal for my 7K axle" as if it were an upgrade you fit to the existing hub. It is not. Oil-bath running needs the oil-bath hub, with its clear cap and its seal bore, and the matching unitized seal. A grease hub with an oil seal pushed into it will leak. The bearings and races are the same in both; everything around them is not. Converting is a hub swap, per wheel, and on a 7,000 lb axle that means the hub-and-drum assembly.
What goes wrong with each
Grease hubs fail through heat and neglect: a bearing packed three years ago, a rear seal that let sand in, a hub run with too much preload. They fail slowly, and a hand on the hub at a fuel stop catches most of it. Repacked yearly with fresh seals, they are the reliable choice for a trailer that sees sand.
Oil-bath hubs fail through leaks. A weeping rear seal puts gear oil on the brake shoes and the trailer loses a wheel's braking without any warning at the hub. A cracked or scored clear cap, which a rock or a careless tire change can do, loses the oil on the highway. Owners who switch to oil-bath and then stop looking at the sight glass have traded a system that tolerates inattention for one that does not. The boat-trailer crowd has gone back and forth on this for years for the same reason.
The choice, honestly
Choose oil-bath if you run long, hot, loaded miles, you will check the sight glass every stop, and you are prepared to replace the hubs and carry a spare seal and cap. Choose grease if you would rather a system that survives a season of neglect and can be serviced at any tire shop in Brawley. For a toy hauler that does a dozen dune trips a year, grease, repacked every fall, is the sensible answer, and the bearing failures that prompt the question are almost always a repack interval problem rather than a grease problem.
The parts that are the same on both
A 7,000 lb spindle takes the 25580 inner bearing and the 14125A outer, on the 25520 and 14276 races, with a 12-inch drum. Our 12-inch Timken kit is that set with the grease seal, and the bearings in it fit an oil-bath hub as well; only the seal differs. A greased hub-and-drum assembly is the fast fix when a race has spun or a drum is scored, a fresh cotter pin every time, and a hubcap that seals on a grease hub is the part most often missing when a bearing fills with sand. Our guides to repacking bearings and to bearing kits by axle rating cover the job and the sizes.
What we stock, and what we do not
We stock the grease kits, the greased hubs and the caps. We do not stock oil-bath hubs, oil seals or clear caps, so an oil-bath conversion comes from the axle maker's dealer, and we say so rather than sell you a grease seal for an oil hub. If you are staying with grease, we have everything, and we are in Upland on the way to the dunes if you would rather collect it.