Maintenance Diary: How To Change Fork Oil on a Dirt Bike

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Dirt bike on a center stand ready for front fork oil change

Do you know how to change fork oil on a dirt bike? What are the benefits of changing oils on front forks and how does the service affect riding performance? What tools do I need to service my front forks?

It’s time for our maintenance diary where we give easy step-by-step instructions for dirt bike maintenance! The maintenance diary is a series of posts where we provide tips and tricks that make regular maintenance easier and quicker. In these posts, we cover the typical things you need to work over time when riding dirt bikes.

So, keep reading if you are a beginner rider and need help understanding what the typical maintenance areas are and how you should be taking care of your dirt bike in general. Or if you are just looking for tips and ideas how other people do their maintenance, you have come to the right place.

In today’s post, we explain how to change fork oil and go through the main steps involved in the front fork oil service. You will also find out why you should service the forks regularly.

Front Fork Oil Service Interval

The front suspension typically requires service every 20 to 40 hours depending on the dirt bike and riding conditions. The forks have oil in the lower fork legs and the oil needs replacing periodically.

If you ride in rivers, creeks and in muddy conditions, you will likely need oil service more often compared to riding in dry conditions. Water, dust and debris travel inside the main seals and dirty the oil faster. You can prevent this by cleaning the dust seals often.

In WP KTMs and similar forks, you can usually start noticing performance issues around 30 to 40 hours. And above 40 hours, the forks typically will have a noticeable slowness as the oil gets dirty.

A suspension, rear shock and fork maintenance done by a third party service company can set you back anywhere from $300 and up per service.

Learning how to perform a regular oil service on the front forks and how to change fork oil is a great way to save money. You will also be able to get back to riding immediately and you don’t need to wait for the service to be completed.

Fork oil service will take about 2 to 3 hours to complete. You do need some special tools, but you save money even on the first service.

Let’s start with covering required tools and how to prep the dirt bike and get things ready for changing the fork oil on a dirt bike.

Read also: How To Clean Leaking Fork Seals On a Dirt Bike

Tools and Preparations for Front Fork Oil Service

While each dirt bike will require specific tools, all modern dirt bikes have very similar designs on the forks. So, the following tools apply to most fork oil services.

Start by washing the dirt bike well and setting it on a center stand.

Position the dirt bike so that the front tire is not touching the ground. You can set a wrench under the front of the skid plate to lift the front slightly.

Next, prepare a good stable and clean tabletop or similar where you can lay down all parts and tools. Prep the table with disposable cloths or cardboard and paper. Cleaning the forks can get messy.

Several tools required for changing fork oil set on a table
Typical tools required for a dirt bike front fork oil service.

The oil service is a fairly simple process but a few special tools will make it smoother and safe. In addition to new fork oil and possibly new seals, these are the typical tools you will need for changing the fork oil:

  • Seal bullet (the orange plastic cone)
  • Seal driver (the brass colored metal parts)
  • Fork cap tool (the blue wrench)
  • Hex tool for opening the lower fork cap
  • Picks or small screw drivers to help pull seals and clips
  • Fork oil, oil pan for old oil, oil measuring cup
  • Heat gun or blow torch
  • Service manual for checking the needed fork oil quantity
  • Brake or contact cleaner

Read also: How To Adjust The Front Fork Height And Why

Front Fork Removal for Oil Service

Start by removing the front tire and opening the triple tree bolts to remove the front forks from the dirt bike.

After you open the bolts, rotate each fork leg so that they slide down easily without damaging or scratching the upper fork leg.

Twisting the fork towards the fender on each side will loosen the triple tree clamps and allow the fork legs to slide down without resistance.

Once you get the fork legs out of the triple trees, clean them again well. You don’t want any debris or sand get into your clean service area and inside the forks to ruin the oil.

You can use compressed air and a brush to clean the forks well. Also clean the triple trees properly. Clean any dirt in the clamps and inside the triple tree housings.

Important: For both fork legs, count and write down the current clicker setup. It’s important to close both clickers completely (turn clockwise until stopped). This will ensure that you won’t have any issues while reassembling.

Read also: How To Align Dirt Bike Forks – aka Twisted Forks

Front Fork Leg Disassembly for Oil Service

After you have cleaned and prepped the dirt bike, start the fork leg disassembly so that you can change the fork oil.

We usually focus on one leg at a time to avoid mismatching any parts.

Removing the Upper Fork Leg Cap

Start by opening the fork cap. You may need to use a soft jaw vice or a bench to hold the fork leg in place. You can also loosen the cap when the legs are still on the dirt bike if you don’t have a soft jaw vice. Be careful not to damage the upper fork legs.

Next, slide the upper fork leg down to reveal the spring. Use an adjustable or fixed wrench to hold the nut inside the spring to break the cap loose from the suspension internal parts. To do this, push the spring down so that you can put the wrench in place and loosen the cap.

Once the fork cap is loose, set it aside. Then remove the spring and possible spacers under the spring as well as the adjuster rod (the long push rod inside the center piece). Set the parts on the table for cleaning. Pour out the old fork oil into an oil pan.

Removing the Lower Fork Leg Cap

Next, open the lower side of the fork leg cap. Again, you can use a vice to grip the lower fork leg. Put the oil pan under the vice to catch dripping fork oil. Rotate the fork leg so that you can access and twist the hex cap open.

Once you open the lower cap, more fork oil will pour out. Set the lower fork cap aside for further cleaning.

Breaking the Fork Legs Apart for Final Cleaning

Next, separate the lower and upper fork legs to access seals and perform a final cleaning of all parts.

Start by removing the dust seal. Wedge a small screwdriver between the fork leg and the seal. Carefully push the seal outward by wiggling and twisting the screwdriver to drive the seal out.

Next, remove the retainer clip. This clip holds the actual oil seal underneath it and it also acts as a stopper for the fork legs preventing them from separating. Use a pick to remove the clip and slide it down on the fork leg. Also remove the fork leg spacer.

Next, break the legs free to access internal parts for cleaning.

Start by heating the oil seal area with a heat gun or a torch. Make sure to only heat the one inch area on the fork leg and not the dust seal or the spacer. Heat evenly until slightly too hot to touch.

To take the fork legs apart, grab both lower and upper fork legs firmly. Hold the forks level in front of you, on the chest level. With one motion, pull both fork legs apart. You may need to do this few times before the oil seal comes loose.

Sometimes the oil seal is stuck and it might be difficult to break the legs apart. It can feel as if something is holding it in place. If you have removed the retainer clip and heated the fork leg properly, use the entire length of the fork to get some momentum and pull them apart. They will break loose eventually.

Once you have successfully taken the fork legs apart, place each parton the clean table top for cleaning in the order you took them out from the lower fork leg.

Front fork leg ready for final cleaning
Front fork leg ready for final cleaning.

Cleaning the Front Fork Internal Parts for Reassembly

Clean all parts with contact cleaner and paper towel.

Inspect the seals and other parts for wear and tear. It is not necessary to change the dust or oil seal every time you change the fork oil. If you have recurring oil leaks and if you see any damage on the seals, it may be time to replace them.

Seal sets are usually relatively cheap and last anywhere from 100 to 200 hours with proper care.

SKF seal sets (click to check the current price on MotoSport.com) are a great choice as they seem to last long and are a good fit for WP forks. They are available for most dirt bikes.

Read also: How To Set Up Dirt Bike Suspension For Trail Riding

Front Fork Reassembly with Fresh Fork Oil

Now that the internal parts are clean, it’s time to start putting things back together.

Reinstalling Seals and the Retainer Clip

Push the seal bullet in place. If you don’t have a seal bullet, wrap a piece of tape around the sharp edges on the lower fork leg to prevent seal damage.

Use suspension grease or fresh fork oil to lubricate all seals before assembly. Lube the outside of the oil seal slightly for easier installation and removal. This helps make sure the seal wont stick into the fork leg housing.

Slide the dust seal, the retainer clip, the oil seal, the retainer ring, the smaller lower spacer and finally the bushing in place. Pay attention to the assembly order.

Make sure you have greased the oil seal also on the outside and slide the upper fork leg in place.

Wrap the seal driver around the fork leg and push the oil seal in its place with the seal driver.

When installing the oil seal, make sure it sits fully in its place and the retainer clip groove is fully visible.

Next, reinstall the dust seal and the fork mudguard spacer in place.

Reinstalling the Fork Damping / Rebound Rod

Next, reinstall the damping or rebounding rod in place. Grab the rod and the lower cap and align them inside the lower fork leg.

Adding the Fork Oil and Bleeding Air Out of the Forks

Check your service manual for the correct oil quantity. Forks typically take around 600 to 650ml of light 4W oil for each fork leg (in KTMs).

There are few tricks to learn how to set the oil correctly.

In our forks, we measured 635ml of Motorex 4W fork oil for each fork leg.

Start by adding about 75% of the total oil quantity. Carefully pour the oil in and make sure the inner push rod hole does not get any oil in it. It is the clicker adjuster rod and it should never have any oil inside it.

After adding the first set of oil, move the fork push rod up and down a few times to push the oil further inside the fork. Add rest of the oil (25% of the total oil quantity) slowly moving the push rod up and down.

In addition to the total oil quantity, the service manual also tells you the oil level measured from the top of the fork leg. This is usually around 110mm from the top.

It’s important to note, that if you use this oil level measurement method, you need to make sure to properly bleed all air out of the fork first for accurate oil level measurement.

Reinstalling the Spring and the Fork Cap

Finally, it is time to install the spring and the fork cap back in place.

Installing the spring is simple. Just remember to check if the fork has any plastic spacers and install any spacers first. Many KTM WP forks have a plastic spacer on the bottom side of the spring.

Once the spring is in place, insert the adjuster rod inside the center of the suspension rod. Make sure you have threaded the rotating push rod down on the threads, so that the fork cap will completely seat inside the clicker housing. The threads need to seat first into the cap clicker and then on the cap larger diameter. Incorrect installation will prevent clickers from working properly and will rotate endlessly.

After final assembly, rotate the clickers to all open and then all closed. Then set the clickers to previous settings you wrote down at the beginning of the fork service or to the service manual recommended setting.

Note: If the clicker keeps rotating and doesn’t seem to stop on either end, it means that the push rod threads are not in place with the clicker threads. This can happen if the push rod threads are not fully threaded in where it shows most of the threads on the top.

Check the visible thread lenght for correct cap installation
Check the visible thread length for correct cap installation. To fix the issue, open and remove the fork cap, then thread the push rod housing down on the push rod. This thread goes inside the fork cap and allows the clicker reach the end of the adjusting rod correctly.

Naturally repeat the fork oil service for the other fork leg. Steps are identical for both fork legs.

Read also: 7 Best Mods To Make Your Dirt Bike Turn Faster

Reinstalling the Forks on the Dirt Bike

Once you have changed the fork oil to both fork legs, the last thing to do is to install the forks back on the dirt bike.

  • To install front forks back to the dirt bike, start by sliding the forks back to the triple trees. Rotate the forks while you slide them to the desired height and make sure the bleed screws are facing forward.
  • Next tighten the top triple tree bolts slightly to hold forks in place.
  • Install the front tire and brake caliber in place. Torque the axle bolt. Do not tighten the crimp bolts yet.
  • Next torque in the top triple tree bolts.
  • Rotate the front tire and apply front brake quickly to center the wheel on the axle.
  • Then take the dirt bike off the center stand and while holding the front brake, rock the dirt bike forward to push the forks in. This will help center the front end and release any tension.
  • Place the dirt bike on the center stand and torque the axle crimp bolts.
  • Torque the lower triple tree bolts.
  • Finish the fork service by installing any remaining plastics or other parts.

You can find detailed instructions on how to correctly install and tighten front forks in our article How To Align Dirt Bike Forks – aka Twisted Forks.

Read also: How To Set Dirt Bike Sag

Maintenance Summary

Suspension service can sound intimidating or difficult. But like anything else, with great instructions and some practice it actually is quite easy.

Servicing front forks can really help keep costs down and make the dirt bike feel plush and performing like new. We highly recommend learning how to change fork oil and service your own dirt bike front forks.

Make sure to check out our other posts related to front forks, such as how to adjust the front fork height and why or how to clean leaking fork seals on a dirt bike.

See you out on the trails!

Last updated: December 27, 2021