Trailer Hub Oil Bath Cap / 9K-10K Axle Service
Billet Aluminum 3.5"-12 Trailer Cap Kit with O-Ring & Plug for Dexter 21-88 / 8-430 10K GD Hub
Heavy-duty billet aluminum oil bath cap kit for compatible 9,000 lb and 10,000 lb general-duty axle hub applications where the 3.5" thread outside diameter and 12-thread configuration match.
Part Number: 21-88 Billet-KIT 3.5" Thread O.D. Billet Aluminum Cap Kit
Overview
This R&P Carriages billet aluminum oil bath cap kit is a durable aftermarket replacement solution for compatible Dexter 21-88-style oil cap applications. It is designed for trailer hubs using a 3.5" thread outside diameter and 12-thread cap configuration, commonly found on listed 9,000 lb and 10,000 lb general-duty axle applications. The machined billet aluminum construction offers a stronger service replacement option when the original clear plastic cap is cracked, damaged, leaking, or no longer holding up in daily use.
R&P Service Insight
Oil bath hub caps are small parts, but they matter. A cracked or poorly sealing cap can allow oil loss, contamination, and hub service problems.
-
Material Upgrade: Billet aluminum construction provides a more durable cap body than the commonly broken clear plastic style.
-
Seal Check: This kit includes a new rubber O-ring and rubber fill plug for a cleaner service installation.
-
Fit Verification: Confirm thread outside diameter, thread count, hub/drum casting, and axle rating before ordering.
R&P Knowledge Hub
Compatibility: Intended for compatible Dexter 21-88-style oil bath cap applications where the 3.5" thread outside diameter and 12-thread configuration match.
Vehicle / System Fit: Commonly used on 9,000 lb and 10,000 lb general-duty trailer axle hub applications with listed compatible hub and drum configurations.
Installation Notes: Verify the existing cap dimensions, hub/drum casting number, thread style, O-ring seat, and oil bath system condition before installation.
Performance Insight: A billet aluminum cap body helps reduce repeat failures associated with cracked or damaged plastic oil caps.
Brand & Fitment Note
Selling Brand: R&P Carriages
Application Reference: Dexter and Dexter 21-88 references describe compatibility target and application style only.
What This Means: This listing is for an R&P Carriages billet aluminum replacement oil bath cap kit, not a guaranteed OEM Dexter-branded cap unless specifically stated elsewhere and verified.
Before Ordering: Verify axle rating, hub/drum casting number, thread outside diameter, thread count, and included components before installation.
R&P Repair Insight
Before replacing an oil bath cap, inspect the full hub area. Oil loss may come from the cap, plug, O-ring, seal, hub face, or damage around the threaded area.
-
Measure First: Confirm the cap thread outside diameter is 3.5" before ordering.
-
Count Threads: Match the 12-thread cap design to the hub currently on the trailer.
-
Inspect the Hub: Check for damaged threads, a rough sealing surface, or oil contamination before installing the new cap.
Features
Machined Billet Aluminum
Solid billet aluminum cap body offers a stronger service replacement option compared to commonly broken clear plastic oil caps.
3.5" Thread O.D.
Designed for compatible hub applications using a 3.5" thread outside diameter oil bath cap.
12-Thread Cap Design
Built for matching 12-thread cap applications where the hub thread pattern and cap configuration align.
Service Kit Components
Includes the billet aluminum oil bath cap, new rubber O-ring, and rubber fill plug for the cap assembly.
Specifications
| Specification |
Detail |
| Part Number |
21-88 Billet-KIT |
| Brand |
R&P Carriages |
| Product Type |
Billet aluminum oil bath cap kit |
| Application |
Trailer hub oil bath cap replacement for compatible 9K-10K general-duty axle applications |
| Material / Finish |
Machined billet aluminum |
| Thread O.D. |
3.5" |
| Thread Design |
12-thread design |
| Replacement Reference |
Replaces Dexter 21-88-style oil bath cap applications where dimensions and configuration match |
| Included Seal Components |
New rubber O-ring and rubber fill plug included |
Fitment / Application
| Category |
Details |
| Vehicle / System |
Oil bath trailer hub systems |
| Primary Fit |
9,000 lb - 10,000 lb general-duty axle applications using the matching 3.5" oil cap configuration |
| Drum Casting Reference |
8-415 drums after July 2009 |
| Additional Fitment Reference |
8-430 hubs with 9-123 drums after April 2009 |
| Notes |
Confirm hub casting, drum casting, thread outside diameter, thread count, and oil cap style before ordering. OEM names are used as application references only. |
Included Components
| Quantity |
Item |
| 1 |
Billet aluminum 3.5" oil bath cap |
| 1 |
Rubber O-ring |
| 1 |
Rubber fill plug |
Verify all measurements, hub/drum casting numbers, thread outside diameter, thread count, oil bath configuration, and included components before installation. Dexter and other OEM references in this listing describe application fitment only unless the product is specifically identified as OEM and verified.
21-88 Billet-KIT billet aluminum oil bath cap, 3.5 inch oil bath cap, Dexter 21-88 replacement style cap, 9K axle oil cap, 10K axle oil cap, trailer hub oil bath cap, billet trailer oil cap, 8-415 drum oil cap, 8-430 hub 9-123 drum oil cap, rubber O-ring fill plug oil cap kit
Thank You For Choosing R&P Carriages As Your Online Shopping Source!
