Changing a fork oil seal
without disassembly of the fork leg.


An oil spill under the fork means it's time to change the oil seal. It's better not to wait too long before the oil find its way to the brakes !

One often think that the reason is wear, dust, a scratch on the tube or also too much free play in the anti-friction ring. In my case this was simply due to the oil seal clip which was corroded with rust. The dust seal actually offers no protection against water ingress, and the clip is no stainless steel made. Rust particles deteriorated the oil seal inside one fork leg. The other fork leg was also contaminated with rust but the seal was OK and a good cleaning was sufficient.

The standard procedure to change the oil seal is to disassemble the fork leg elements using special tools. I actually used the hydraulic pressure method which does not require any fork leg disassembling and no special tools. This method is not new, but it is relatively unknown, yet it works fine : it's faster and simpler than the standard way.

Initial part

• While the machine is on the side-stand : unlock both caliper bolts, the allen bolt on the right-and side fork leg, the wheel axle, the upper fork bracket bolt, the fork cap bolt.

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(image)

• Put the machine on a stable upright stand, front wheel off and remove the front calipers, the mudguard and the front wheel.
• Remove the fork leg : loosen the two bolts on the under bracket then slip the leg downward.
• Clean carrefully and check the fork tube, hardened dust traces or scratches have an immediate destructive effect on the new seal.

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(image)

• Gently remove the dust seal with a large flat screwdriver, it comes out easily without effort.

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(image)
(image)

• Remove the retaining clip with a flat screwdriver without scratching the tube.
• Clean or replace the clip if it is too corroded.

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(image)

Removing the faulty seal

Now the operation becomes interesting : removing the oil seal from the lower fork element using hydraulic pressure !
It requires to fill the fork tube to the top with oil and then strongly compress the fork leg. This method may not work in case the oil seal is too much deteriorated.

Tools

• A trolley jack.
• One or two strong straps.
• About 500cc oil, any sort, used or new.
• A plastic bottle and adhesive tape.
• Rags, plenty of...

Operations

• Secure the bottle with tape to prevent splattering and also to block the fork tube on the lower sleeve.

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(image)

• Remove the upper cap bolt and extract the washer and the spacer.
• Extend the inner tube to the maximum.
• Fill the tube completely with oil : there should not remain any air inside.
• Replace the upper cap bolt and tighten normally.

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(image)

• Place the fork vertically on the trolley jack, place the strap under the jack and above the fork tube cap on a strait line.
• Place a rag around the top to protect the strap and keep it from slipping.
• Tighten the strap to the maximum possible.

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(image)

• Operate the jack frankly, taking care that the strap remains straight.
• Internal oil pressure quickly pops out the oil seal, and possibly the anti-friction ring (replace it by hand).
• Remove the bottle, remove the upper cap bolt and drain the fork.
• Finally drive the oil seal out.

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Fitting the new seal

Oil seal part number (Yamaha)

• TDM900 : 4PU-23145-01 (around 16€). It is actually manufactured by Athena, part BR3655K (43 x 55.1 x 9.5/10.5). This oil seal is common to many Yamaha bikes (R6 1999-2004 / XJR / FZ1 2006-2011 / 2011-2012 FZ8 / BT1100 2002-2004, etc.) and some other brands.
• TDM850 (4TX) : 4TX-23145-00.
• TDM850 (3VD) : 3GM-23145-A0-00.

Warning : the oil seal is a fragile part, handle with care to avoid any damage on the inside part.

• Protect the upper edge of the fork tube with a thin plastic film, put fork oil around the tube.

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(image)

• Unpack the new seal, put some grease on the inside part, then slip it gently straight onto the fork tube - number side on top. Push it down slowly with the help of the old seal and the mounting tool (see below).
• In case force is required to settle the new seal inside the lower fork part, always protect the new seal.

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• The new seal does not seat easily into place, an appropriate method should be used in order to avoid damaging it. It is best to avoid using a hammer ! In my case, I once again used the trolley jack as a hydraulic press, using a metal tube to push the seal inside the fork leg element. The tube has an internal diameter of 45mm and is 1mm thick, it is long enough to protrude some cm from the top of the fork tube. A butyl ring comes to protect the new seal from the contact with the metal tube. The old seal may also be used.

(image)

(1) - new oil seal.
(2) - butyl ring.
(3) - metal tube dia = 45mm / th = 1mm.

• Before putting the metal tube, protect the fork tube with some plastic tape.
• Place the whole assembly on the jack, put the strap on top of the metal tube and under the trolley jack, then fully tighten it up.
• Operate the jack slowly while checking that the assembly stay vertical and the metal tube gently pushes the seal into place.
• Once the seal is seated, remove the strap, jack and the metal tube.

• Install the retaining clip - put some grease on it to prevent future corrosion.
• Place the dust seal by pushing it with the mounting tool.

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• Fill the fork leg with oil according to the usual procedure.
• Replace the fork leg on the bike.
• Replace the front wheel, the front fender and the brake calipers.
• Do not forget to activate the brake lever several times before test ride the bike...

Preventing corrosion of the clip

• On a regular basis - ie every 20.000km, varying along with the local climate - raise the dust seal, remove the clip, check, clean and lubricate with grease. Check the oil seal (well the visible part of it...).
• In case moisture or rust are present, remove the clip, clean the space inside with a soft brush, dry thoroughly with compressed air, clean the clip or replace it with a new one, grease it before replacing it.

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Mounting tool

A special mounting tool is normaly required according to the workshop manual : a weighted cylinder and a 43mm diameter adaptator ring. This tool is necessary to slide down the seal perfectly straight on the tube (this is a fragile part - did I already said that ?).
A DIY tool can easily be made with a PVC tube, I made one with a flexible plastic sheet rolled around the fork led to make the right diameter. For the adapter ring, you can use the old seal, it works very well.

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Standard method to change the oil seal

This method requires complete disassembly of the fork tube. It takes a special tool for locking the damping element : a simple metal rod of sufficient length, terminated by :
• TDM850 1996-2001: a 19mm squared end (to be confirmed).
• TDM900 2002-2003: a 19mm squared end.
• 2004 + TDM900: a 29mm hex end.
This tool prevents the bottom socket bolt to turn freely !

It is also possible to remove the bottom socket bolt with an impact wrench, it works very well !
When installing the bolt tightening presents no problem, apply pressure on the fork tube to block the damping element.

I will not describe the standard operations, please refer to the workshop manual and also to the excellent slideshow made by Vincens66.
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