At a Glance

> Confirm the cap number: Match the existing cap to the 21-36 application before ordering.
> Check the thread: Verify 4"-12 thread compatibility on the hub or drum assembly.
> Inspect the seal area: Replace damaged plugs and O-rings when servicing oil bath caps to help prevent seepage.
> Leak check: If oil is present around the cap face, inspect the center plug and O-ring before reusing old sealing parts.
> Thread check: Damaged or crossed threads can prevent proper seating and may cause leaks even with a new cap.
Brand: R and P Carriages
SKU: 21-36-BILLET-WRENCH-KIT

Before You Order

Match the part number, measurements and connection points that apply to your trailer.

Part Number	Match the number stamped on the original bearing or seal.
Dimensions	Measure the mating surfaces when a number is unavailable.
Hub / Axle	Use capacity as context, not as the only fitment check.

Product Details

Oil Bath Hub Cap Kit • 4"-12 Thread • Wrench Included

Billet Aluminum Oil Trailer Bath Cap 4"-12 Thread with Wrench

Complete oil bath cap service kit with one billet aluminum 21-36 style cap, center plug, O-ring, and billet wrench. Designed for compatible 4"-12 threaded oil bath hub applications where the 21-36 cap style and measurements match.

Part Number: 21-36-BILLET-WRENCH-KIT 4"-12 Thread Application Cap + Plug + O-Ring + Wrench

Key Measurements & Specs

Specification	Detail
Part Number / SKU	21-36-BILLET-WRENCH-KIT
Product Type	Billet aluminum oil bath cap kit
Cap Application	21-36 style oil bath cap applications
Thread Specification	4"-12 thread
Material	Billet aluminum cap and billet aluminum wrench
Quantity Included	One complete cap-and-wrench service kit

Fitment / Application

Category	Details
System Type	Trailer oil bath hub and drum applications
Primary Fit	Compatible with 21-36 style 4"-12 threaded oil bath cap setups
Referenced Drums	9-44, 9-27, and 9-28 applications where the 21-36 cap style is used
Fitment Method	Verify cap number, thread size, thread pitch, sealing surface, plug style, and drum/hub application before ordering
Important Note	Application references are fitment guides only. Match your existing cap and hub specifications before installation.

Top Features

Feature	Buyer Benefit
Billet Aluminum Cap	Durable upgrade-style construction for compatible oil bath hub applications
4"-12 Thread	Matches 21-36 style threaded oil bath cap applications where measurements confirm fit
Center Plug & O-Ring Included	Provides the key sealing components commonly serviced with the cap
Billet Wrench Included	Helps with cap installation and removal without needing to source the wrench separately

Overview

The 21-36-BILLET-WRENCH-KIT is a curated oil bath cap service kit for compatible trailer hub and drum applications using a 21-36 style 4"-12 threaded oil bath cap. The kit includes the billet aluminum cap, center plug, O-ring, and a billet wrench for 2.5" hex cap service, making it a practical replacement or service option when the existing cap style and thread specification match.

R&P Service Insight

Oil bath cap fitment is measurement-driven. A cap that looks close may still leak or fail to seat correctly if the thread, sealing surface, or plug style does not match the hub.

Confirm the cap number: Match the existing cap to the 21-36 application before ordering.
Check the thread: Verify 4"-12 thread compatibility on the hub or drum assembly.
Inspect the seal area: Replace damaged plugs and O-rings when servicing oil bath caps to help prevent seepage.

R&P Knowledge Hub

Compatibility: This kit is intended for compatible 21-36 style oil bath cap applications using a 4"-12 thread. Verify before ordering if your hub, drum, or axle setup has been changed from its original configuration.

Referenced Drum Applications: Common application references include 9-44, 9-27, and 9-28 where the 21-36 oil bath cap style is used.

Installation Notes: Clean the hub threads and sealing surface before installation. Do not force the cap if the threads do not start cleanly by hand.

Performance Insight: The cap retains oil bath lubrication inside the hub and helps protect the bearing system. Proper cap fit and sealing are essential for oil retention and long-term hub performance.

Brand & Fitment Note

Selling Brand: R&P Carriages curated billet oil bath cap kit

Application Reference: References to 21-36, 9-44, 9-27, and 9-28 are used for compatibility and application identification only.

What This Means: This listing is for a billet aluminum oil bath cap kit assembled for compatible 21-36 style applications. It should not be treated as a guaranteed OEM-branded item unless separately verified.

Before Ordering: Verify cap number, thread size, thread pitch, sealing surface, center plug style, O-ring fit, and drum/hub application before installation.

R&P Repair Insight

Oil bath caps are often replaced because of damaged threads, leaking plugs, worn O-rings, cracked caps, or cap damage from removal. This kit is built to cover the cap and common sealing pieces while also including the matching billet wrench for service.

Leak check: If oil is present around the cap face, inspect the center plug and O-ring before reusing old sealing parts.
Thread check: Damaged or crossed threads can prevent proper seating and may cause leaks even with a new cap.
Tooling check: Use the correct wrench profile for the cap to reduce damage during installation or removal.

Features

Billet Aluminum Construction

Durable billet aluminum cap designed for compatible oil bath hub applications using the 21-36 style cap format.

4"-12 Thread Application

Built for 4"-12 threaded hub cap applications where the existing cap and hub measurements confirm fitment.

Service Components Included

Includes the center plug and O-ring, giving buyers the common sealing pieces needed during cap replacement.

Billet Wrench Included

Comes with a billet aluminum wrench for 2.5" hex caps, helping simplify future installation and removal.

Included Components

Part Number	Quantity	Item
21-36	1	Billet aluminum oil bath cap, 4"-12 thread application
046-032-00	1	Center plug
010-050-00	1	O-ring
Billet-Wrench-2.5	1	Billet aluminum wrench for 2.5" hex caps

Important Fitment Note: Verify your existing oil bath cap number, 4"-12 thread specification, sealing surface, plug style, O-ring fit, and drum or hub application before ordering. Drum references such as 9-44, 9-27, and 9-28 are compatibility references only and should be confirmed against your actual hub assembly.

View More 21-36 Oil Cap Options

21-36-BILLET-WRENCH-KIT billet aluminum oil bath cap kit 21-36 oil cap 4-12 thread oil bath cap 4 inch 12 thread hub cap trailer oil bath cap with wrench 046-032-00 center plug 010-050-00 o-ring billet wrench 2.5 hex cap 9-44 9-27 9-28 drum application trailer hub oil cap replacement oil bath hub cap kit R and P Carriages

Billet Aluminum Oil Trailer Bath Cap 4"-12 Thread with Wrench (21-36-BILLET-WRENCH-KIT)