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The following was posted by Terry Hay of Shock Treatment Australia on DRN back in 2003. Its a great piece and I'm sure Terry would not mind it being posted again here.
Fork seals - Why do they leak?
The fork seal has become the scourge of the modern motocrosser. Leaking at the most in-opertune time and creating the highest level of incovenience. But what causes them to leak. I have been reading many of the posts on this forum and it would appear that fork seals create the greatest amount of havoc for the greatest number of riders. Fork seals, contrary to popular opinion, rarely blow. Mostly they leak because they have a foriegn object lodged between the seal lip and the fork leg, holding the seal open and allowing oil to leak out. Usually this is dirt or mud, sometimes grass and so on. As modern forks grow in diameter to increase rigidity we also see an increase in seal drag due to the increased suface area. This additional drag creates unwanted harshness due to additional stiction or - static friction. In order to reduce the unwanted stiction the modern seals have less tension against the actual fork leg. Now when your wheel hits a bump we see the impact work in two directions. One tries to compress the fork and absorb the hit, the other tries to break the fork in half and push the front wheel back into the motor. This secondary force causes the forks to flex, usually at the lower triple clamp area. As the forks flex the seals are stressed and any dirt on the fork tube may get past the primary seal lip, lodging uderneath the seal and creating a path for oil to escape. Another from of stress to the fork tube is caused by the brake rotor. As you apply the brake there is a rotational toque applied to the wheel, opposing it's natural momentum. This force also creates stress to the fork leg which can affect the seals and create leaks. Hence the reason why most leaks occur on the brake side first. Obviously the dirt or foriegn matter needs to be removed from the seal to ensure correct performance. This is best done by removing the seal. Whilst the seal itself may not actually be damaged, I would still recommend replacing it with a new one. Only use genuine seals as aftermarket ones seem to leak shortly after installation. To the idiot that first come up with the suggestion that you dislodge the dirt by shoving something between the seal and the fork leg, I would like to see your balls in a vice and I will happily do the cranking. Ninety nine percent of the time you will simply shove that dirt further into the fork where it can do some real damage. Someone is making a tidy profit out of seal-savers. You can actually produce a more effective seal saver for almost nothing. Cut a small strip of foam appr. 10mm x 10mm x 160mm long. Apply a small amount of fork oil and massage in, similar to an air filter. Remove the dust seal and place the foam in the cavity between the dust seal and oil seal. Replace the dust seal and ride. Remove and clean the foam every time you do your air filter. Re-oil and replace. If any dirt gets past the dust seal it will be trapped in the foam before it gets to the oil seal. The oil on the foam lubricates the leg and further reduces stiction. Sometimes fork seals can leak because the seal lip burns due to excessive friction. If bikes are left for a long time or are washed with aggressive detergents the chrome tubes can dry out. When you next ride the seals are subjected to increased friction and a hardening of the seal lip occurs. This hardening reduces the seals flexibility and further promotes leaks. If you haven't ridden for a while or often use harsh detergents, apply a little lube to the fork leg to lubricate the seals before you ride. Good Luck!
Terry Hay

Original Article