Advice: How to replace fork seals

Date 18.12.2013

Presented by
  • Motorcycle Trader


From Motorcycle Trader issue 260, Aug/Sep 2012


As well as making a bike look neglected, a weeping fork seal left unattended poses a threat to a motorcycle’s handling and also its braking efficiency. Here’s how the experts go about the important task of replacing those seals…

Instead of the first weep from a fork seal being treated as a warning that urgent attention is required, it’s often seen as another item for the some-day-soon list or the too-hard basket. The problem here is that loss of fork oil poses risks to a motorcycle’s safety on two fronts.

The falling oil level alters the front suspension characteristics and therefore the bike’s handling and if left for long enough, the oil can soak into the brake pads, causing a pretty dramatic drop in braking efficiency.

In reality the task of replacing fork seals can be handled efficiently and without too much difficulty by riders with a reasonable amount of DIY experience.

Mick Hone Motorcycles of Mont Albert, Victoria, was happy for us to bring a camera into the workshop and pick up a few pointers from technician Brian Anderson as he fitted new fork seals to a customer’s Yamaha FJR1300.

Brian explained that seal life is dependent on usage. While seals can survive for 40,000km or more in a calmly ridden bike, high-stress and high-impact loads on forks can shorten seal life to 10,000km or so.

The following information is based on Brian’s work on the Yamaha on the day in the well-equipped Hone workshop combined with our DIY-experience at home in the shed.

Brian checks the position of the top of the fork stanchion (tube) relative to the top surface of the upper triple-clamp. This will enable him to reinstall it at the original height to avoid changing the bike’s steering geometry.

liding the leg down a bit and then reclamping it in the lower triple-clamp allows Brian to get a spanner on to the cap without having to remove the handlebar. The cap can then be loosened (only a couple of turns at this stage).

With the leg still clamped in place, a mirror gives Brian the opportunity to carry on with the job despite limited access. Here he checks the dust seal clip shape so he can choose the right tool to remove it. Now to remove the first leg…

A sticker on the stanchion (inset pic) can prevent it from sliding down through the triple clamp. This calls for the triple-clamp eye to be wedged open temporarily, using a flat screwdriver blade in the expansion slot.

Twisting the leg as you pull it down makes the removal process easier. A good spray of WD40-type lubricant on the stanchion above the bottom triple-clamp can also help to get the thing sliding. Old high-mileage forks may require  surface rust in the same area to be removed using 400 grit wet ’n’ dry paper.

It’s vital to keep some pressure on the fork cap for the last few turns of its fine thread to prevent spring preload firing the cap at you when it’s finally released from the stanchion’s internal thread. This also enables you to note the correct installation sequence of the washers, shims, spacers etc.

Whether you drain the old fork oil with the leg still on the bike or after removing it, good housekeeping is important. Oil spills are a safety issue in the shed and then there’s the domestic-bliss risk of tramping oil into the house. Proper disposal of the old oil is another  important matter for the DIY-er.

The jolt of Brian’s rattle gun usually removes the damper rod anchor-bolt easily. With hand-tools you may need to slide a suitable tool inside the stanchion to stop the damper-rod rotating. Now all items can be separated.

Once the fork components are separated and laid out in re-assembly order, the next stage of the job is all about thoroughly cleaning each item. Brian paid particular attention to removing all the ‘treacley’ oil residue from deep inside the fork slider with an appropriate solvent.

Linishing the stanchion removes wear marks and imperfections to protect the new seal. Use 400-grit wet ’n’ dry paper (and plenty of WD-40) with a combined rotating/sliding motion for a finish like a honed cylinder bore.

Make sure you get the new seal right way up – the top of the seal usually displays the seal ID info. When sliding the seal onto the stanchion it’s important to lubricate both the seal and the stanchion to make the task easy. A clean, thin plastic bag will protect the seal’s important inner lip from damage as it’s pulled over the sharp end of the stanchion.

If you can’t borrow a seal driver you can use the old seal to protect the new one as you tap it down into its seat using a small hammer on a blunt, flat-blade screwdriver, working evenly around the seal’s circumference. Then refit the clip.
Now it’s time to install the damper rod.

Refill the leg with the specified quantity and grade of fresh oil (unless you’re intending to alter the bike’s handling). This is done before the spring is refitted. Take care to pour oil toward the centre to ensure that the damper rod is filled.
See separate note under ‘Top Tips’ about fine-tuning the oil level.

If your fork has a damper tube, allow it to settle back into the damper rod under its own weight to allow any trapped air to escape. Now it’s time to reinstall the spring, spacers, washers, fork cap and dust seal. Load the spring the right way up and get all the other components in the right order.

Before refitting the leg, stand it on the floor and push down to compress and extend it a few times. This gets rid of air bubbles and allows you to check for smooth operation. Also check for any oil leaks. All good? Well done. It’s time to slip it back into the triple clamps. Now there’s the other one to do!

Get it right: ROB’S TOP TIPS

Safety warning #1
Given that this job involves a total strip-down of the front suspension and temporary removal of front brake calipers, a sub-standard result will pose serious risks to the safe operation of your motorcycle. If you’re not an experienced DIY-er, get an experienced mate to supervise you.

Before you begin to remove the front wheel the bike must be upright and stable with all of the weight of its front section supported on a strong and stable jack or stand under the engine or frame. Protect engine castings and frame paintwork with a timber packer placed on top of the jack.

Appropriate eye protection should be used whenever the task at hand involves hammering of metal components or prising out retainer clips and circlips. Also, if your skin is sensitive to solvents or oils use suitable gloves to protect your hands.

KNOW YOUR BIKE: This article offers general guidance only. Inform yourself about your specifics applying ot your bike, including technique tips, special tools, etc, from a workshop manual and enthusiast websites. And ask for advice from mates with a good DIY track record.

LAY OUT THE PARTS: As you dismantle the first leg lay the parts out in a safe area in the correct order and with the correct orientation. Springs are generally not reversible – note which way is up. Use sketches if necessary or take digi photos. This stuff is vital, particularly if you won’t be finishing the job the same day.

ONE AT A TIME: Work on one leg at a time for two reasons. Having one intact is great for reference in case your neat display of parts gets scattered when you trip over the cat. Also it’s never good to swap parts between appartently identical assemblies because mating parts develop matching wear patterns.

DAMPER TUBE BOLT: Without a rattle gun it’s best to try to loosen the bolt before you strip the leg – the spring repload often prevents the rod rotating internally. Leaning hard on a half-stripped leg inverted onto a wooden broom-handle can work too – it can grip the damper rod enough for you to loosen the bolt.

THE BIG PULL: once the fork-seal retaining clip and the damper-tube bolt are removed, a bit of brute force is needed to separate the stanchion from the slider. Two or three tugs using a slide-hammer-like action should do it. Grab the rebound spring and any other loose parts that come out with the stanchion and seal.

LINISHING THE STANCHIONS: This has a big effect on the life of the new seals. The ideal diagonal cross-hatching finish can be achieved by wrapping 1 /2 turns of rag around the sandpaper on the stanchion – using the two rag ends as ‘push-pull’ sandpaper ‘handles’ produces the combined rotating/sliding motion.

INSTALLING DAMPER ROD BOLT: Good practice is to use a new copper washer on the bolt as well as some non-permanent thread sealer. As at the removal stage, it may be necessary to sue a special tool or ito improvise something to slide into the stanchion to prevent the damper rod from rotating.

OIL LEVEL: For a really professional result you need to check the oil level even after adding the specified amount of oil. This will be specified as the distance down to the oil surface from the top edge of the stanchion ( with no spring and the standhion pushed fully into the slider). A syringe is the best tool for the job.