At a Glance

> Thread Match: Verify the hub uses a 4"-12 threaded oil cap before ordering.
> Seal Surface: Inspect the hub face and O-ring contact area for burrs, corrosion, or old gasket material.
> Plug Style: Match the center plug style to the cap opening before filling or returning the trailer to service.
> Check the Thread: Confirm 4"-12 thread before replacing the cap.
> Check the O-Ring: Replace damaged, flattened, cut, or swollen O-rings during cap service.
Brand: R and P Carriages
SKU: 21-36-BILLET-WRENCH-KITX2

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 Caps · Billet Aluminum Service Kit

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

Two-cap billet aluminum oil bath cap kit for compatible 4"-12 threaded hub cap applications. Includes matching center plugs, O-rings, and a billet wrench for installation and removal.

Part Number: 21-36-BILLET-WRENCH-KITX2 4"-12 Thread 2 Caps + Wrench

Key Measurements & Specs

Specification	Detail
Part Number / SKU	21-36-BILLET-WRENCH-KITX2
Selling Brand	R&P Carriages curated service kit
Product Type	Billet aluminum oil bath cap kit
Thread Specification	4"-12 threaded oil cap application
Material	Billet aluminum cap construction
Quantity Included	2 oil bath caps, 2 center plugs, 2 O-rings, and 1 billet wrench

Fitment / Application

Category	Details
System	Trailer oil bath hub cap applications
Primary Fit	21-36 style 4"-12 threaded oil cap applications
Compatible Drum References	9-44, 9-27, and 9-28 where the original cap thread and sealing setup match
Fitment Method	Verify existing cap number, thread size, cap opening, O-ring style, and plug style before ordering
Important Note	Will not fit hubs using a different oil cap diameter, thread pitch, plug style, or sealing arrangement

Top Features

Feature	Buyer Benefit
Billet Aluminum Caps	Durable metal cap construction for compatible oil bath hub service
4"-12 Thread Fit	Matches 21-36 style threaded oil cap applications when measurements confirm fit
Sealing Components Included	Includes center plugs and O-rings needed for the listed cap setup
Billet Wrench Included	Provides a dedicated tool for cap installation and removal

Overview

This R&P Carriages billet oil bath cap kit is built for compatible 21-36 style 4"-12 threaded hub cap applications. The kit includes two billet aluminum oil bath caps, two center plugs, two O-rings, and one billet wrench, making it a practical service set when replacing a pair of oil bath caps on matching trailer hubs.

R&P Service Insight

Oil bath caps are small parts, but they are critical to keeping lubricant inside the hub and contamination out of the bearing cavity. Confirm the cap thread and sealing style before installation.

Thread Match: Verify the hub uses a 4"-12 threaded oil cap before ordering.
Seal Surface: Inspect the hub face and O-ring contact area for burrs, corrosion, or old gasket material.
Plug Style: Match the center plug style to the cap opening before filling or returning the trailer to service.

R&P Knowledge Hub

Compatibility: This kit is intended for 21-36 style 4"-12 threaded oil bath cap applications. Drum references 9-44, 9-27, and 9-28 are application references only and should be verified against the actual hub and cap dimensions.

Trailer / System Fit: Designed for compatible trailer oil bath hubs where a threaded cap, O-ring, and center plug are used to retain hub oil.

Installation Notes: Clean the cap threads, inspect the O-ring seat, lubricate the O-ring as appropriate, and thread the cap carefully to avoid cross-threading.

Performance Insight: A properly sealed oil bath cap helps maintain hub lubrication and protects the bearing cavity from leaks, water intrusion, and road contamination.

Brand & Fitment Note

Selling Brand: R&P Carriages curated billet service kit.

Application Reference: 21-36, 9-44, 9-27, and 9-28 are used as compatibility and application references only.

What This Means: This listing is for an R&P Carriages billet oil bath cap kit, not a guaranteed OEM-branded item unless specifically stated on the product packaging or invoice.

Before Ordering: Verify the existing cap number, thread size, hub opening, plug style, O-ring style, and drum or hub application before installation.

R&P Repair Insight

When servicing oil bath hubs, cap fitment should be confirmed before filling the hub with oil. A mismatched cap can leak immediately or fail to seat correctly even when the thread appears close.

Check the Thread: Confirm 4"-12 thread before replacing the cap.
Check the O-Ring: Replace damaged, flattened, cut, or swollen O-rings during cap service.
Check the Cap Body: Do not reuse a cap or sealing component if the threads, plug bore, or sealing surfaces are damaged.

Features

Pair of Billet Oil Bath Caps

Includes two billet aluminum oil bath caps for servicing both matching hub cap positions on compatible trailer applications.

4"-12 Thread Specification

Designed for 21-36 style oil cap applications using a 4"-12 threaded fitment.

Center Plugs and O-Rings Included

Comes with two 046-032-00 center plugs and two 010-050-00 O-rings for the listed cap setup.

Installation Wrench Included

Includes one Billet-Wrench-2.5 tool for installation and removal of compatible billet cap hex profiles.

Included Components

Part Number	Quantity	Description
21-36	2	Billet aluminum oil bath cap for 4"-12 threaded applications
046-032-00	2	Center plug
010-050-00	2	O-ring
Billet-Wrench-2.5	1	Billet aluminum wrench for compatible 2.5" hex cap service

Important Fitment Note: Verify your original cap number, 4"-12 thread specification, hub opening, O-ring style, plug style, and drum or hub application before ordering. Application references describe compatibility only and do not guarantee fitment on modified, mismatched, or non-standard hubs.

View More 21-36 Oil Cap Options

21-36-BILLET-WRENCH-KITX2 billet oil bath cap kit 4-12 oil cap 4 inch 12 thread trailer hub cap 21-36 oil cap pair 046-032-00 center plug 010-050-00 o-ring billet aluminum oil bath cap with wrench 9-44 9-27 9-28 drum application oil bath hub cap replacement trailer axle oil cap service kit

Two - Billet Aluminum Trailer Oil Bath Caps 4"-12 Thread with Wrench (21-36-BILLET-WRENCH-KITX2)