4-in-1 Axle Caps from Olson & Company. Sold as a pair.

Sometimes you can't tow a bus or semi-truck tractor backwards, you can't pull the drive shaft, and you just have to pull the axles. These caps  are equipped with an oil-access hole for adding oil to the hub prior to towing to prevent damage. Pull the axles, place caps, add oil and tow!

Axle caps fit most 8-bolt tandem and single-axle trucks on the road today. This is a very popular hub pattern.

Features:

• Prevent Damage: Caps feature an oil-access hole that allows you to add oil to the hub prior to towing
• Easy to Use: Axle covers do not need gaskets for attachment
• Outside Diameter: 8.5"
• Four models in one axle cover - 8 bolt
• Most popular model for tractor trailors (5/8" stud)
• Sold as a pair