No to both. The tapered stud where a ball joint seats into the steering knuckle or control arm must stay clean and dry — no grease, no anti-seize, and no WD-40. This is one of the few places on a ball joint where the usual "keep it lubricated" instinct is exactly backwards.
If you suspect a taper has already spun or slipped (persistent clunking that didn't go away after reassembly, or the joint feels loose even with the nut properly torqued): stop treating it as a lubrication problem. A taper that's already worn the mounting hole oval needs the knuckle or control arm inspected — and likely replaced — before the vehicle goes back on the road. Continuing to drive on a taper that isn't fully seated is a steering-system safety risk, not a maintenance inconvenience.
Why the Taper Has to Stay Dry
A ball joint's tapered stud is held in place by an interference friction fit, not a bolt clamping two flat surfaces together. The taper is machined slightly larger than the hole it seats into; tightening the castle nut pulls the stud into that hole and the two tapered surfaces lock together purely through dry metal-on-metal friction. There's no separate locking mechanism doing the real work — the friction is the mechanism.
Add grease, anti-seize, or oil to that surface and you cut the friction coefficient significantly. Two things can go wrong from there:
- The torque-to-clamping-force relationship gets thrown off. A lubricated taper reaches the same torque reading with less actual clamping force, so a nut torqued to spec on a lubricated taper may not be nearly as tight as the spec assumes.
- The stud can spin inside the hole under load. Once that happens, the taper starts grinding against the hole instead of staying locked, which enlarges (hogs out) the hole — permanent, irreversible damage. A knuckle or control arm with a worn-oval taper hole generally can't be salvaged by installing a new ball joint; the hole itself needs to be repaired or the part replaced.
The correct installation practice, per ball-joint manufacturer instructions (MOOG's tech tips are explicit about this), is to keep both the taper and the receiving hole clean and dry before torquing the nut to spec — check your specific vehicle's torque value rather than assuming a generic number.
Why WD-40 Isn't the Answer Either
WD-40 (the original spray) is a solvent-based water-displacer and light penetrant, not a heavy-duty chassis lubricant. Spraying it on or near a ball joint — often an attempt to quiet a squeak — causes two separate problems:
- It washes out the grease that's already there. A ball joint's internal grease (typically a heavy-duty NLGI #2 chassis grease) is what's actually reducing friction between the ball and socket. WD-40's solvents thin and displace that grease rather than adding lubrication, leaving more bare metal-on-metal contact than before.
- It can degrade the rubber boot. The petroleum solvents in standard WD-40 can soften or accelerate cracking in the rubber dust boot that's sealing the joint, which shortens the boot's life and lets contamination in faster.
One workaround that shows up repeatedly in forum threads — piercing a small hole in the boot to inject WD-40 or grease with a syringe — masks the noise temporarily but breaks the boot's seal in the process, which defeats the purpose of having a sealed joint at all. It's a stopgap, not a fix, and it usually accelerates the joint's failure rather than delaying it.
Where WD-40 is fine: as a penetrant to help break loose a rusted or seized joint before disassembly. That's a legitimate use. The rule is what happens after — any WD-40 residue needs to be wiped off before reassembly, and the taper needs to go back together clean and dry, same as if it had never been sprayed.
Two Different Parts, Two Opposite Rules
The confusion in this question usually comes from treating "the ball joint" as one surface that either does or doesn't get lubricated. It's actually two surfaces with opposite rules:
| Part of the joint | Lubrication rule |
|---|---|
| Tapered stud (where it seats into the knuckle/control arm) | Clean and dry, always — no grease, no anti-seize, no WD-40 |
| Ball-and-socket interior (if the joint has a grease fitting/zerk) | Grease regularly per the joint's service interval — this is the surface that's supposed to be lubricated |
If your ball joint has a zerk fitting, that's where routine grease maintenance belongs — not on the taper. A sealed (non-greasable) joint has no serviceable interior at all, and nothing about that changes the taper rule either way.
Related reading: What's the Best Brand or Material for Ball Joints, and Are They Covered Under Warranty?
