Boat Trailer Cool Hub Maintenance Oil Change

I recently changed the oil in the cool hubs on my Ranger Trail trailer. While doing so I put together instructions and a photo array of the process of a boat trailer cool hub maintenance oil change. Changing the oil in cool hubs can be an intimidating task for someone not comfortable with performing vehicle type maintenance. Hopefully these instructions and photos provide the confidence for anglers to perform their own cool hub maintenance and fluid change and save a few dollars to buy lures.

This instruction set will deal specifically with draining and refilling cool hubs. I will cover hub removal with bearing inspection, and brake pad inspection and replacement in future instruction sets.

I have to add a quick disclaimer; there are a few deviations from factory recommended procedures in my repair processes. These deviations will be indicated with a note. I have been a professional mechanic in one form or another since I was seventeen, culminating in 28 years of turning wrenches. Along the way, I have developed certain ways of doing things that have served me and my customers well. If there are any doubts about my substitutions, please follow the factory recommended portion.

Boat Trailer Cool Hub Maintenance Oil Change

My trailer is a 1998 Ranger Trail Model D4600 – Tandem Axle with Disc Brakes.

Tools and materials needed to drain and refill cool hubs.

3/16 Allen wrench
Floor jack
Jack stands
One 32oz bottle of gear oil
One tube RTV sealant (I prefer a gray like GM or Subaru brand)
Drain pan for waste oil
2-3 cans BraKleen solvent (optional)
Tire removal tools

To start, jack the axle until the tire is clear of the ground. Before removing lug nuts, grasp the top and bottom of the tire at the 12 and 6 o’clock positions. Simultaneously pull and push, trying to rock the tire, checking for lateral movement in relation to the spindle assembly. Movement indicates looseness in bearing tension. None was evident in mine. However, according to Ranger and UFP (Unique Functional Products) if there is more than 1/8″ of movement measured at the outside edge of the tire, the bearings need to be adjusted.

Note: I will go one step further and say if there is 1/8″ or more of movement it’s a good idea to remove and inspect the bearings and races for wear versus simply adjusting them.

Spin the wheel by hand and listen for any grinding or growling noise. This would indicate a bearing or race issue. Mine had no noticeable noises. Remove the tire center cap and lugnuts and set the tire aside.

Place a jack stand (1) under the axle for safety.

Boat Trailer Cool Hub Maintenance Oil Change






Rotate the hub assembly until the small Allen-head plug (2) is visible.

Boat Trailer Cool Hub Maintenance Oil Change






Place the plug at the 12 o’clock (straight up) position, and use a 3/16 Allen-head socket to remove the plug (3)
Boat Trailer Cool Hub Maintenance Oil Change







The Oil level (4) should be visible at the bottom of the threaded hole.

Boat Trailer Cool Hub Maintenance Oil Change







Place a small, clean drain pan directly under the hub and rotate the hub until the hole is at the 6 o’clock position. It is important to start with a clean pan so the oil can be inspected for particles. Let the oil drain into the drain pan (5).

Boat Trailer Cool Hub Maintenance Oil Change







While the oil is draining, look at the oil collecting in the pan with a bright flashlight. Inspect for excessive small metal particles which could be evidence of premature wear of the bearings.

Note: It’s hard for me to describe “excessive” particles, however if there are any doubts remove the hubs and inspect everything.

The oil will take a while to completely drain. If there is time, let it drain overnight.


Boat Trailer Cool Hub Maintenance Oil Change

Note: This is where I deviate and add my own process into this procedure. I rotate the drain hole back to the top and use CRC BraKleen (6) to flush the hub and bearings.





Boat Trailer Cool Hub Maintenance Oil Change

With the red spray tube attached begin filling the hub (7).






Boat Trailer Cool Hub Maintenance Oil Change

I filled my hubs 3/4 full with the solvent (8) and temporarily installed the plug.

Spin the hub around by hand for 30 seconds to flush the bearings. Pull the drain plug and rotate the hole back to the bottom letting the solvent and oil residue drain out. Perform this process three times on each hub or until everything running out is clear. I used three cans of BraKleen total between my four hubs.

The BraKleen solvent will completely evaporate in about one to two minutes. After evaporation is complete, it’s time to fill the hubs with the gear oil. In my case the factory recommends 50 weight oil or 90 weight gear oil.

Note: I decided to go with synthetic gear oil in my hubs. This is personal preference, using Ranger recommendations is completely acceptable. Before trailer maintenance is performed, be sure to look up the recommendations for the specific trailer year model.

Rotate the drain hole back to the top. Cut just the tip off of the provided nipple on the gear oil bottle. (9)

Boat Trailer Cool Hub Maintenance Oil Change







Stick the tip in the hole, leaving room for air to escape (10).

Boat Trailer Cool Hub Maintenance Oil Change







Very slowly fill the hub with new oil. Watch the window to see when the hub becomes full (11).

Boat Trailer Cool Hub Maintenance Oil Change







Refilling requires patience. Trying to force the oil into the hub quickly won’t happen because it is a sealed system. Clean the drain plug threads and coat them with RTV silicone (12).

Boat Trailer Cool Hub Maintenance Oil Change







When oil level is correct, install the prepared drain plug. Correct oil level on my 1998 year model is 3/8″ below the fill hole. Tightened the plug to 7ft-lb (13).

Boat Trailer Cool Hub Maintenance Oil Change

Rotate the hub by hand four to five complete turns to fully bathe the bearings in oil and to work any air bubbles to the top of the hub. Recheck the level in the hub window; if all you see is oil, hub maintenance is complete.

Each hub took 5-6oz of oil; just keep an eye on the oil level window while filling.

Don’t forget about the hubs; periodically look behind the tire at the back of the hub seal surface for evidence of leaking. Some “seepage” over time is normal. But if it is wet, dripping or slinging oil to the inside of the tire, the seal needs replaced. I will cover seal replacement in another article (watch for it very soon).

Hopefully these instruction and photos will help you tackle this regular maintenance procedure on your own. Remember my trailer is a 1998 model, different years require different lubricants and/or levels. Be sure to identify the correct specifications in the owner’s manual. If you have further questions or clarification on a specific year model please call Rick Huddleston, Ranger Boats Trailer Department, at 870-453-2222.

Please visit my blog called The Ultimate Bass Garage for many other money saving maintenance procedures

Banjo Pickin Bass Hunter!
Rick Olson


  1. Rangerman July 18, 2016
  2. alan barnes October 14, 2017

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