4.0" Billet Aluminum Oil Bath Cap Kit — 4"-12 Thread for 10K–15K Trailer Axles
Single billet aluminum oil bath cap kit for heavy-duty trailer hub applications using the 4"-12 threaded 21-36 style cap. Includes the cap, O-ring, and center plug for a complete one-cap service replacement.
Key Measurements & Specs
| Specification | Detail |
|---|---|
| Part Number / SKU | 21-36 Billet-KIT |
| Product Type | Oil bath cap kit |
| Cap Style | 21-36 style threaded oil bath cap |
| Thread | 4"-12 |
| Material | Billet aluminum cap |
| Quantity Included | 1 complete single-cap kit |
Fitment / Application
| Category | Details |
|---|---|
| System | Heavy-duty oil bath trailer hubs |
| Common Application | Dexter-style 10K HD, 12K, 15K, and select pre-2009 10K GD hub applications using 4"-12 oil caps |
| Fitment Method | Verify original cap style, thread size, axle rating, hub design, and sealing components before ordering |
| Important Note | Not for 3.5" 21-88 style oil bath caps. Match by measurement and original cap style. |
Top Features
| Feature | Buyer Benefit |
|---|---|
| Billet Aluminum Cap | Durable replacement option for heavy-duty oil bath hub service |
| 4"-12 Thread | Matches the common 21-36 style threaded oil bath cap application |
| O-Ring Included | Provides the sealing component needed when replacing the cap |
| Center Plug Included | Completes the cap assembly for oil bath hub use |
Overview
The 21-36 Billet-KIT is a single 4"-12 threaded billet aluminum oil bath cap kit for compatible heavy-duty trailer hubs. This kit is intended for 21-36 style applications where the trailer uses a threaded 4" oil bath cap with an O-ring and center oil plug.
R&P Service Insight
Oil bath cap fitment should be confirmed before ordering because heavy-duty Dexter-style hubs may use different cap diameters depending on axle family and production year.
- Match the thread: This kit is for 4"-12 threaded cap applications.
- Check the seal: Replace the O-ring when servicing the cap to help prevent oil leakage.
- Confirm the cap family: Do not substitute this for smaller 3.5" oil bath cap applications.
R&P Knowledge Hub
Compatibility: Designed for compatible 21-36 style 4"-12 oil bath hub cap applications. Verify before ordering.
Vehicle / System Fit: Heavy-duty trailer oil bath hubs commonly found on 10K HD, 12K, 15K, and select pre-2009 10K GD axle applications where the original cap uses the 4"-12 21-36 style thread.
Installation Notes: Clean the cap threads and hub sealing surface before installation. Confirm the O-ring seats correctly and inspect for existing thread damage before tightening.
Performance Insight: Oil bath caps help retain hub oil around the bearing assembly. Correct thread engagement and a sound O-ring seal are critical for leak prevention.
Brand & Fitment Note
Selling Brand: R&P Carriages billet aluminum service kit
Application Reference: Dexter and 21-36 references are used for compatibility identification only. Match by original cap style, thread, hub application, and measurements.
What This Means: This listing is for a billet aluminum oil bath cap kit, not a guaranteed OEM-branded Dexter part unless specifically stated elsewhere on the product page.
Before Ordering: Verify axle rating, hub style, original cap diameter, thread count, O-ring fitment, and plug style before installation.
R&P Repair Insight
When an oil bath cap leaks or becomes damaged, replacing only the cap without checking the sealing surface can leave the same problem in place. Confirm the hub threads, O-ring groove, plug opening, and oil level during service.
- Thread Check: Make sure the hub accepts a 4"-12 threaded cap before ordering.
- O-Ring Check: Do not reuse a flattened, cracked, swollen, or cut O-ring.
- Application Check: If your original cap is marked or measured as 21-88 / 3.5", this is not the correct kit.
Features
Billet Aluminum Construction
Machined billet aluminum cap design for a durable heavy-duty oil bath hub service replacement.
4"-12 Threaded Design
Built for compatible 21-36 style oil bath hub cap applications using a 4"-12 thread.
Complete Single-Cap Kit
Includes the billet cap, O-ring, and center oil plug needed for one cap replacement.
Heavy-Duty Trailer Hub Application
Useful for compatible 10K HD, 12K, 15K, and select pre-2009 10K GD oil bath hub applications where measurements match.
Included Components
| Part Number | Quantity | Item |
|---|---|---|
| 21-36 Billet | 1 | 4" billet aluminum oil bath cap with 4"-12 thread |
| 010-050-00 | 1 | Oil cap O-ring, 3.734" I.D. x 0.139" thickness |
| 046-032-00 | 1 | Center oil plug |
