A damp patch under the front axle is not a cosmetic problem.
It is your 4WD knuckle seals quietly emptying oil and chewing through gears.
Ignore it and you pay for hubs, bearings, and planetary parts instead of a few seals.
In this guide you strip the knuckle, replace all three 4WD seals, and keep the front axle tight, dry, and working.
Why four wheel drive knuckle seals matter
The knuckle housing keeps oil around the planetary gears and hub bearings.
Three key seals hold that oil in place and keep dirt out.
When seals fail, oil leaks out and water and grit creep in.
Running a dry hub destroys bearings, gears, and shafts very quickly.
Which axle and seals “Fitting 4WD Seals” covers
Different tractors use different front axle types and knuckle layouts.
You may need to identify which axle is fitted before you order seals.
Typical 4WD knuckle overhaul uses a knuckle housing seal, wheel hub seal, and pinion carrier O-ring.
Doing all three seals while you are in there saves time and repeat labour.
Parts and tools you need for fitting 4WD seals
- You need a new knuckle housing seal.
- You need a new wheel hub seal.
- You need a new pinion carrier O-ring.
- You need a 19 mm socket, size may vary by axle.
- You need a breaker bar and ratchet.
- You need an 8 mm Allen key.
- You need a soft punch.
- You need a pry bar.
- You need internal and external snap ring pliers.
- You need a hammer.
- You need a topped up oil can.
- You need clean rags.
- You may need a chisel for stubborn parts.
- You also need a jack, axle stands, and decent coffee.
Safety and preparation before you strip the 4WD hub
Park the tractor on firm, level ground.
Apply the handbrake and chock the wheels.
Loosen wheel nuts slightly while the wheel is on the ground.
Jack up the front axle and support it securely on stands or blocks.
Remove the wheel from the side you are working on.
Step one: drain the 4WD hub oil
Locate the drain plug on the knuckle or hub.
Place a drain tray under the plug.
Remove the plug and let the oil drain completely.
This step keeps “four wheel drive soup” off your bench and clothes.
Step two: remove trunnion pins and knuckle housing
Remove the top trunnion pin from the knuckle.
Remove the bottom trunnion pin from the knuckle.
Support the knuckle as you free the last pin.
Take the complete knuckle housing off the axle.
Clean the knuckle housing with a pressure washer.
Remove all mud, oil, and dust before you open it further.
Any dirt left now will be inside your new seals next week.
Step three: remove the planetary assembly
On the bench, remove the two cap screws that retain the planetary assembly.
Lift the complete planetary assembly away from the hub.
Inspect gears, pins, and bearings for chips, pitting, or wear.
Replace damaged planetary parts now rather than after a failure.
Remove the ring of bolts that hold the planetary ring gear.
Lift the ring gear out of the knuckle or hub.
Step four: remove the wheel hub and wheel hub seal
With the ring gear out, remove the wheel hub from the knuckle.
Take care with weight and balance as you pull the hub free.
The wheel hub seal now sits exposed on the hub or housing.
Use the soft punch and hammer to work the old wheel hub seal out.
Work evenly around the seal to avoid gouging the metal.
This part is not glamorous, but very satisfying once the seal pops out.
Step five: remove the knuckle housing seal and inspect bearings
Inside the knuckle housing you now see the inner 4WD seal.
Some axle setups have a roller bearing directly behind this seal.
Use snap ring pliers if any retaining rings hold parts in place.
Remove the old knuckle housing seal with the soft punch or pick.
Check any inner roller bearings for play, pitting, or roughness.
Replace worn bearings now while everything is apart.
Check the wheel bearings in the hub for wear and rough spots.
If the wheel bearings look ropey, replace them before refit.
Step six: fit the new knuckle housing seal
Clean the knuckle housing seal bore with rags and solvent.
Remove rust and old sealant but do not scratch the surface.
Slide the new knuckle housing seal into the bore.
Press the seal in square and even all the way around.
Use a suitable driver or flat plate so you do not deform the seal.
The new seal must go in clean, with no bending or dirt on the lip.
Step seven: fit the new wheel hub seal
Clean the hub groove or housing where the wheel hub seal sits.
Position the new wheel hub seal in place.
Drive the seal in gently with a soft punch or seal driver.
Work around the seal until it sits fully home and flush.
Step eight: rebuild the knuckle and check shim settings
Refit the wheel hub to the knuckle housing.
Make sure bearings seat correctly and there is no trapped dirt.
Refit the planetary ring gear into the knuckle or hub.
Refit the planetary assembly and tighten the two cap screws.
Fit the new O-ring on the pinion carrier or as specified.
This gives you three fresh seals for one strip down.
Refit the knuckle housing onto the axle end.
Refit the top and bottom trunnion pins.
Check shim packs under the trunnion pins and adjust as needed.
You want smooth movement with minimal play but no tight spots.
Step nine: refit the drain plug and refill with oil
Refit the drain plug to the knuckle or hub.
Tighten it hand tight, not “Hulk tight”.
Over tightening can strip threads and make future servicing painful.
Fill the hub and knuckle to the correct level with the specified oil.
Use the topped up oil can if the fill point is awkward.
Wipe down the assembly until it looks like you were never there.
Refit the wheel and torque wheel nuts correctly.
Lower the tractor off the stands and remove the jack.
Step ten: test the four wheel drive hub
Drive the tractor forward and back in 4WD on a clear surface.
Listen for smooth operation from the rebuilt hub.
Check again for leaks around the drain plug, hub seal, and knuckle housing.
Dry castings and quiet running mean the job is done properly.
Common mistakes when “Fitting 4WD Seals”
People skip washing and drag mud into the knuckle internals.
People reuse old O-rings and then chase slow leaks for months.
People only change one seal and end up stripping it again soon after.
People overtighten the drain plug and damage threads in the housing.
When to ask Nick Young Tractor Parts for help
If planetary gears or bearings look suspect, ask about replacement kits.
If you are unsure which axle is fitted, share photos and serial info.
Nick Young Tractor Parts can match knuckle seals, wheel hub seals, and O-rings to your axle.
Quick recap: replacing tractor 4WD seals in the knuckle housing
Jack and support the tractor, then remove the wheel.
Drain the 4WD oil and remove trunnion pins and the knuckle housing.
Strip and inspect the planetary assembly, ring gear, hub, and bearings.
Replace the wheel hub seal, knuckle housing seal, and pinion carrier O-ring.
Rebuild the knuckle, check shims, refit the drain plug, and refill with clean oil.
With this “Fitting 4WD Seals” guide, your blog turns a big 4WD job into clear steps and long term savings.
