I stripped the kingpin assembly because after approx 5500 hours the seal between the bevel gear case and front wheel case was weeping and I’d noticed iridescence in the oil when draining the RH front wheel case. In the overhaul process I found confusing or incorrect details in the Kubota WSM and "Service Training Book for Front Drive Axle Disassembly and Assembly Procedure" and needed extra info from others including Wolfman. So this aims to help other M4500DT front axle virgins and if the pros have any suggestions/corrections I welcome their contribution/constructive criticism.
The Kubota manuals and parts lists remain essential (available via PDF) PM me if you need them.
(I also serviced the diff input and oscillating axle bearings— that's for a separate write-up).
Prep
Standard tools and consumables including:
Kubota Manual Issues
WSM:
What the kingpin drive really looks like inside:
For this write-up
Disassembly
Neither of these methods comply with bearing dismounting best practice, so I decided to replace all 4 bearings each side.
Inspection, replacement & wear
This assumes checking of backlash. Pros having overhauled a number of axles may have a view on whether it is necessary to check backlash as on mine only the FWC backlash was out of spec and then only marginally.
BGC prep:
Feedback
The Kubota manuals and parts lists remain essential (available via PDF) PM me if you need them.
(I also serviced the diff input and oscillating axle bearings— that's for a separate write-up).
Standard tools and consumables including:
- Hydraulic press (5–7 ton) + spacers for 40mm/50mm ID bearings.
- Fabricated press frame if a shop press is unavailable.
- approx 3m shelf/bench layout space for heavy components (up to 45kg) plus working area.
- Circlip pliers (large, say 300mm o/all length, 3.2mm tips).
- Dial test indicator (gear backlash), vernier calipers (checking shim numbers because they can stick together and separate when cleaning).
- 1 kg deadblow hammer, 6” vice, 300 N.m torque wrench.
- 50mm A/F socket (kingpin nuts), 70mm A/F (stub axles, if needed. I didn’t dismantle the stub axles).
WSM:
- Several diagrams are mislabeled or miscaptioned (e.g., incorrect M7500DT drawing).
- Procedures do not fully align with the M4500DT axle’s actual configuration.
- Fills some gaps in WSM, but still omits key steps.
- Backlash measurement guidance is inconsistent.
For this write-up
- Bevel gear case = BGC
- Front axle support = FAS
- Front wheel case = FWC
- Start with WSM, up to page S5.7. Much of S5.8 to S5.18 is irrelevant to M4500DT kingpin assembly.
- Leave the BGC attached to the front axle for ease of handling.
- Separate the #8 case cover from the FWC using the three long M14 jacking screws, each bearing on an M12 bolt screwed in from the opposite side.
one of 3 jacking screw holes in the case cover.
The long jacking screws can be used as handles for the case cover... - Keep a M12 bolt in place to avoid injury from the ~40kg case cover assembly dropping unexpectedly.
- Remove #25 bearing retainer, kingpin top nut (mild steel bar or bolt prop between lower bevel gear and FWC body as per manual).
- Extract the kingpin with FWC from BGC:
- Method A: Hammer + soft drift.
- Method B: Use #25 retainer + spacers + threaded rod to press out the kingpin, which also separates the BGC from FWC.
Neither of these methods comply with bearing dismounting best practice, so I decided to replace all 4 bearings each side.
- Remove #53 bearing case using jacking screw holes, kingpin bottom nut same way as top nut.
- Remove the BGC from the FAS
- Most consumable parts were readily available from my local Kubota dealer.
- The shimmed joints and seal collars (#43) allow moisture and resulting rust, up to the O-ring seals, so needed a lot of cleaning of surfaces and threads.
- All O-rings are 3.1mm section.
- Replaced the 8 bearings with Chinese “Mochu” brand... if these fail prematurely you’ll be the first to be told J
- Word on the street is that #50 lowest kingpin bearing tends to wear first, probably due to this bearing taking all the axial load, and on mine this was responsible for the iridescence in the oil. Next time I would consider replacing #50 with a 7308 angular contact bearing for extended life. What doesn’t help is users who space out the front wheels...
(NOT my tractor)
Reassembly
This assumes checking of backlash. Pros having overhauled a number of axles may have a view on whether it is necessary to check backlash as on mine only the FWC backlash was out of spec and then only marginally.
BGC prep:
- Press #39 bearing onto kingpin (measure against old witness marks).
- Assemble internal stack: #41 bevel gear, #37 collar, #38 bearing. Fit to FAS. Add and torque bearing retainer. . Important: #26 gasket thickness (0.5mm OEM) affects backlash.
- Should be no need to fit/torque top nut since the stack is all press fit on kingpin.
- Seat assembly firmly against #xx bearing retainer (because it takes the weight of the tractor)
- Check/adjust backlash. Approximate backlash response:
- 0.10 mm shim change ≈ 0.085 mm backlash change.
- Remove kingpin assembly from BGC and remove BGC from FAS.
- Install #44 seal into #45 collar, with seal spring facing upward.
- Press #48 - #40 bearings bearing/spacer/gear stack onto kingpin and insert into FWC.
- Torque bottom nut, seat assembly to 250 N.m against #53 bearing. Don’t stake yet.
- Fit #8 case cover and shims, with say 4-6 temporary bolts (torqued). I didn’t fit all bolts just for the backlash checking due to the ones furthest from #8 would have minimal effect on the backlash.
- Check/adjust backlash. Approximate backlash response:
- 0.20 mm shim change ≈ 0.14 mm backlash change.
- Grease or use pipe dope to minimise corrosion due to water entering through bolt holes and shim end gaps.
- Assemble #46 circlip and #47 seal into BGC.
- Insert kingpin/FWC into BGC; press stack onto kingpin.
- Torque/stake top nut then bottom nut Install #25 retainer, #30 wheel support, and #53 bearing case.
- Torque all #8 case cover bolts.
- Attach FWC/BGC assembly to FAS, torque all bolts per spec.
- Seeking input: anyone with a better separation method, bearing-friendly technique or any steps I’ve written incorrectly. I kept notes as I went but can’t guarantee I’ve got it perfect.
- Happy to answer questions.