iFJF Trailer Axle Oil Cap Kit 4-Pack for Dexter 21-36 Hubs — 4" Threaded Caps with O‑Rings & Dust Plugs

iFJF 4Pcs 21-36 Oil Cap Kit is a set of four threaded 4" replacement axle oil caps with O-rings and rubber plugs engineered to seal Dexter 21-36 hubs on 9K, 10K, 12K and compatible axles.

This replacement kit supplies everything needed to restore a proper seal on compatible Dexter trailer axles. Each cap measures 4 inches in outside diameter with 4"-12 threads and a 1-1/2" depth, and the package includes four matching O-rings plus four rubber dust plugs. Constructed from corrosion-resistant materials, these caps are intended to resist weathering and provide a durable, lightweight fit for common trailer maintenance tasks.

Which axles and part numbers does this oil cap kit fit?

These oil caps are designed to work with Dexter 21-36 hub applications commonly found on 9,000 lb, 10,000 lb, 12,000 lb and certain 15,000 lb axles. They replace several OEM part numbers and are intended for older and compatible Dexter units, excluding specific 2009-and-newer 10,000# general-duty models.

• Compatible: Dexter 9K (after 10-1-1989), 10K, 12K, and 15K where 21-36 caps are used.
• Replaces part numbers: 021-036, 021-036-00, 021-256, 021-256-00, 21-036, K21-301-00, K71-148-00.
• Not compatible with Dexter #21-88 10,000# general duty axles produced 2009 and newer.

How are the caps sized and what are the key dimensions?

Understanding the cap dimensions helps ensure a proper fit into the hub cavity and thread engagement. This kit uses a common 4" diameter cap with coarse 4"-12 threads for secure installation and a 1-1/2" depth so hub seals sit correctly.

• Outside diameter: 4 inches for hub face coverage.
• Thread specification: 4"-12 to match standard axle hub threads.
• Depth: 1-1/2 inches to position the cap over seals and bearings.

What does the kit include and how does it protect hub components?

The kit contains four complete cap assemblies with sealing components so all wheels on a tandem axle can be serviced at once. O-rings provide a fluid-tight seal to keep lubricants in and contaminants out, while the rubber plugs guard internal cavities against dirt and moisture during storage or service.

• Package contents: 4 oil caps, 4 O-rings, 4 rubber plugs for one full axle set.
• Materials: lightweight, corrosion- and oxidation-resistant construction to extend service life.
• Protection: helps prevent grease loss and keeps water, dust, and debris away from bearings.

How do I install these oil caps safely and quickly?

Installation is straightforward with basic tools. Clean the hub bore, inspect threads and seals, slide the O-ring onto the cap, and thread the cap in until snug—avoid over-tightening. Finish by inserting the rubber dust plug if needed for extra protection.

• Prep: clean hub and inspect for thread damage before installing.
• Seal: place O-ring in groove, then thread cap until hand-tight plus a firm turn.
• Finish: insert rubber plug and check for leaks after running the trailer under a short load.

Who is this for: This iFJF 4Pcs 21-36 Oil Cap Kit is aimed at trailer owners, fleet technicians, and maintenance professionals who need a reliable, OEM-replacement style cap set for Dexter-style hubs. It’s especially useful when performing routine axle service, replacing lost or damaged caps, or prepping trailers for long-term storage. Choose this kit when you require correct 4" dimensions, included O-rings and plugs, and a corrosion-resistant replacement that matches common Dexter part numbers.