How to Refill a Milwaukee M18 Grease Gun: Cartridge Size & Instructions

Key Takeaways

• The Milwaukee M18 grease gun uses standard 14 oz cartridges and requires removing the battery before refilling for safety.
• Priming the gun after every refill is necessary to remove trapped air and ensure a consistent flow of grease through the system.
• Forgetting to remove the cartridge’s foil seal or skipping the priming process are the most common reasons grease fails to flow.
• The M18 cordless grease gun delivers professional-grade performance with 10,000 PSI maximum operating pressure for heavy-duty maintenance tasks.
• GenuineTools carries authentic Milwaukee M18 grease guns and accessories at up to 60% off MSRP with fast shipping and expert support.

Refilling Your Milwaukee M18 Grease Gun: What You Need To Know

Refilling the Milwaukee M18 Cordless 2-Speed Grease Gun (Model 2646-20) requires a standard 14 oz grease cartridge and a two-phase process of loading and priming. This heavy-duty tool delivers up to 10,000 PSI and a maximum flow rate of  10 oz. per minute, making it ideal for high-pressure maintenance. To begin, always remove the M18 REDLITHIUM battery to ensure safety before unscrewing the barrel.

The loading process involves locking the plunger rod, inserting the cartridge open-end first, and, most importantly, removing the foil seal to prevent flow blockages. Once you reattach the barrel, use the air bleeder valve to prime the system. This removes trapped air pockets. Skipping the priming step or failing to remove the cartridge seal are the most common reasons grease fails to dispense properly.

How to Load a Grease Cartridge Into the Milwaukee® M18

The cartridge loading process has 9 steps that must be followed in sequence. Rushing or skipping steps—especially priming—causes most post-refill problems in the M18 grease gun.

1. Remove the battery pack: Before anything else, pull the M18 REDLITHIUM battery pack from the gun. Set the battery aside somewhere clean and dry.
   
   
2. Retract and lock the plunger rod: Grip the plunger rod at the rear of the gun and pull it fully back toward you. Once it's extended, push it sideways into the retaining slot to lock it in place. If the rod isn't seated securely, it can spring back unexpectedly. This is dangerous and creates a mess.
   
   
3. Unscrew and remove the barrel: With the rod locked, unscrew the barrel from the gun body by rotating it counterclockwise. Pull it away cleanly. If there's residual grease from the previous cartridge, this is a good moment to wipe the barrel threads with a clean rag to keep everything sealing properly on reassembly.
   
   
4. Remove the cartridge cap: Take your new 14 oz grease cartridge and remove the cap from the open end. Don't remove the foil seal yet. Keep the cartridge oriented correctly (open end up) so you don't lose any grease before it goes into the barrel.
   
   
5. Insert the cartridge: Slide the cartridge into the barrel open end first. The fit should be snug. Make sure the cartridge is fully seated inside the barrel before moving to the next step.
   
   
6. Remove the cartridge’s seal: Now reach into the open end of the barrel and remove the foil or plastic seal from the cartridge. This seal is what keeps the grease contained during shipping. Leaving it in place is one of the most common mistakes people make. The gun will prime and trigger, but no grease will come out because the seal is blocking the flow path.
   
   
7. Reattach the barrel: With the cartridge loaded and the seal removed, thread the barrel back onto the gun body by rotating it clockwise. Hand-tighten it firmly. A solid connection maintains the pressure integrity the M18 is known for. Do not use tools to over-torque it, and double-check that the barrel is fully seated—not cross-threaded—before moving on. 
   
   
8. Release and engage the plunger rod: Reach back to the plunger rod and push it out of the retaining slot, then press it fully forward into the barrel. You should feel it make contact with the back of the grease cartridge. This step is what creates the pressure needed to push grease through the system. If the rod isn't pressed all the way in, the gun won't prime or dispense correctly.
   
   
9. Prime the gun: Fresh cartridges trap air in the system, preventing grease flow. Priming pushes that air out. If you skip this, the trigger won't dispense anything.

Refilling the M18 grease gun’s cartridge is simple.

How to Prime the Milwaukee® M18 Grease Gun

Priming is the step most people skip, causing them to troubleshoot why the gun won't dispense. The M18 grease gun uses an air bleeder valve specifically designed to make this process fast and clean. 

Now, here are the priming steps. 

1. Reinstall the battery: Reattach the M18 REDLITHIUM battery pack to the gun. Make sure it clicks firmly into place. A partially seated battery can cause intermittent power loss mid-operation, which complicates the priming process.
   
   
2. Bleed the air: Locate the air bleeder valve on the gun and push it in. Hold it until air stops escaping and grease appears at the opening. Do not release the valve until the airflow stops completely.
   
   
3. Secure the hose: Before pulling the trigger, grip the hose at the spring strain relief, the coiled spring section at the base of the 48-inch flex hose. This gives you control over the hose end without putting stress on the connection point. Be sure to point the coupler end of the hose away from your body and anyone nearby. 
   
   
4. Cycle the trigger: With the hose pointed safely away, engage the lock-on/lock-off trigger and let the gun run for 10 to 20 seconds. This pushes grease through the 48-inch hose. When a steady bead of grease flows from the coupler, the system is primed. Use the lower speed setting for better control. 
   
   
5. Repeat if necessary: If you've pulled the trigger for 20 seconds and still no grease is coming out, don't keep holding the trigger. Go back to Step 2 and work the air bleeder valve again, as stubborn air pockets sometimes require two or three bleeder cycles to fully clear.

Cartridge Size

The Milwaukee® M18 grease gun takes a standard 14 oz grease cartridge. Undersized or oversized cartridges can damage the barrel threads or prevent proper sealing. Any standard 14 oz cartridge from a reputable brand works, provided the grease grade suits your application. 

Get Authentic Milwaukee® M18 Grease Guns at GenuineTools

GenuineTools carries a wide range of authentic power tools. 

If you need the Milwaukee® M18 2646-20 grease gun or any accessories for it, GenuineTools carries the full range: guns, combo kits, and everything that goes with them. Every item ships in original manufacturer packaging. No refurbished stock, no open-box surprises, no gray-market tools that throw off compatibility with your existing M18 batteries.

We offer up to 60% off MSRP—significantly higher savings than the standard 10%–30% discounts found elsewhere. Most of our orders ship within one business day, with delivery to the contiguous U.S. in 2–6 business days. If something isn't right, our return process is straightforward: 30 days, with fast refunds on approved returns. 

GenuineTools offers fast shipping and authentic Milwaukee® products.

We have built a strong reputation on TrustPilot from buyers who needed the real thing and got it. That track record matters when you're buying professional-grade tools and need to know what's showing up at your door.

Shop Milwaukee® Grease Guns at GenuineTools →

Frequently Asked Questions (FAQs)

What size grease cartridge does the Milwaukee® M18 grease gun use?

The Milwaukee® M18 grease gun uses a 14 oz grease cartridge. Any quality 14 oz cartridge from a reputable grease manufacturer will fit and function correctly. Ensure the grease grade matches your equipment's requirements.

Do I need to remove the battery before refilling the Milwaukee® M18 grease gun?

Yes. Always remove the M18 REDLITHIUM battery pack before loading a new grease cartridge. This is a mandatory safety step. With the battery in place, the trigger is live, and any accidental activation while your hands are near the barrel or rod mechanism can cause injury or a serious grease mess. Pull the battery first, every time, without exception.

Why is no grease coming out after I refilled my Milwaukee® M18?

The two most common causes are a forgotten foil seal or trapped air. If you missed Step 6 (removing the seal), grease cannot exit the cartridge. If the seal is gone, the issue is likely air in the line. 

How do I know when my Milwaukee® M18 grease gun needs a refill?

There are a few reliable signs that your cartridge is running low or empty:

• Grease output noticeably slows or becomes inconsistent between fittings
• The gun triggers but delivers little to no grease even after priming
• The clear barrel accessory (49-16-2646) shows the grease level visually
• The pre-set grease counter indicates you reached the programmed limit
• Air starts coming through the coupler during operation, which is a classic sign the cartridge is spent

How much does a Milwaukee® M18 Grease Gun cost at GenuineTools?

The Milwaukee® M18 2646-20 (tool only) and the M18 2646-21CT kit are available at GenuineTools for up to 60% off MSRP.

*Note: Pricing and/or product availability mentioned in this post are subject to change. Please check the GenuineTools website for current pricing and stock information before making a purchase.