The short version
- The bearing itself gets packed full. The cavity around it does not.
- Aim for roughly one third to one half of the free space in the hub, not all of it.
- Overfilling causes churning, which generates heat rather than preventing it.
- On a grease fitting, stop when it moves. Pumping until it burps blows the rear seal.
How much grease in a wheel bearing is a question with a genuinely counterintuitive answer, because more is not better past a certain point.
The confusion comes from treating the bearing and the hub cavity as one space. They want different amounts.
How much grease in a wheel bearing: two separate spaces
The rolling elements and the space between them must be full. That grease is what actually separates the metal surfaces.
The empty volume around the bearing inside the hub is different. It is a reservoir and an expansion space, and filling it entirely causes problems.
So the answer to how much grease is two answers. All of it in one place, about a third to a half in the other.
Why overfilling makes heat
This is the mechanism people do not expect.
Grease is a thick, sticky substance. Anything rotating inside a solid mass of it has to continuously shear and push that mass out of the way.
That shearing takes energy, and the energy becomes heat. The term for it is churning.
Elevated temperature thins the oil out of the grease and drives it away from where it is needed.
So an overpacked hub can end up starved, having started overfull. It is a self defeating loop.
Grease also expands as it warms. A hub filled completely leaves nowhere for that expansion except past the seals.
What heat does to bearing life more broadly is on the failure causes page.
Packing the bearing properly
The bearing has to be full, and the traditional palm method is still the reliable way to achieve it.
- Put a generous blob in your palm, more than looks necessary.
- Press the widest edge of the cone into it, so grease is forced through between the rollers.
- Work around the cone, rotating a little each time, until grease appears at the small end all the way round.
- Coat the outside lightly once the inside is full.
- Smear a film in the cup in the hub, without filling the whole cavity.
The full repacking sequence, including the adjustment afterward, is on the repacking guide.
Grease guns and fittings
Some trailer hubs have a fitting that lets grease in without dismantling anything, and using it correctly matters.
On a spring loaded grease cap, the correct amount is reached when the piston starts advancing. That is the hub telling you it is full.
Continuing to pump raises pressure inside a sealed hub with nowhere to go, and the rear seal is the weakest point.
A blown rear seal is worse than the problem you were solving. It lets water in and grease out, on a component you cannot see.
A hand grease gun generates far more pressure than people assume, which is why a few pumps too many does real damage.
Greasing without taking it apart
A common wish, and the answer depends entirely on what you have.
A sealed hub unit. Not possible, and not intended. It is packed and closed for life, and drilling or prying to add grease ruins it.
A serviceable hub with a fitting. Yes, and that is what the fitting is for. Stop when the piston moves.
A serviceable hub without a fitting. No. Adding grease at the outer end does not reach the inner bearing, which is the one that usually fails.
New grease pushed in at one end does not remove old, contaminated or water-emulsified grease from the other.
An annual pump through a fitting is maintenance. It does not replace a proper strip and repack.
How to read the old grease when you do strip it is on the used grease page.
Signs you overdid it
Overpacking announces itself, usually within the first drive or two.
A hub noticeably hotter than its opposite number after a run. That comparison method is on the hot hub page.
Grease appearing on the inside of the wheel or on the brake backing plate, which means it went past a seal.
Grease on the brake friction surfaces, which is a safety problem rather than an inconvenience.
What a slight overfill looks like once it works its way out is on the escaping grease page.
Common questions
How much grease in a wheel bearing is right?
The bearing itself gets packed completely full. The cavity around it in the hub wants roughly one third to one half, not all of it.
Why is overfilling bad?
Because the rotating parts have to shear through the excess, which generates heat. That heat then thins the grease and drives it away.
How much grease in a wheel bearing hub cavity is too much?
Filling it completely. Grease expands as it warms, and a full cavity leaves nowhere for that expansion except past the seals.
How do I know the bearing is fully packed?
Grease should appear at the small end of the cone all the way round after working it in from the wide edge.
When do I stop pumping a grease fitting?
When the spring loaded piston starts to move. Pumping until it burps out builds pressure that blows the rear seal.
Can I grease a sealed hub without removing it?
No. Sealed units are packed and closed for life, and opening one to add grease ruins it.
Does pumping through a fitting replace a repack?
No. New grease at one end does not remove contaminated or water-emulsified grease from the other.