Honda S90 Fork Seal Replacement

When fork seals start to leak, road handling is affected as the forks loose their oil. Sometimes oil can also find its way onto the brakes and the bike may fail its roadworthiness check.

This document shows how to dismantle the forks and replace the seals.


Honda S90 Forks.



First the forks were removed from the bike. For this the bike was propped up under the front to lift the front wheel clear of the ground.

Then the wheel was removed by first detaching the speedometer cable and brake cable, and then undoing the main axle nut and the 4 axle cap nuts on the underside of the forks.

After the wheel was removed the Mudguard was next held in place by four M6 bolts.









To gain access to the top of the forks the handlebars were removed and the headlight housing released.

Then the pinch bolt at the bottom of the fork tree was released and each fork leg pull/drifted down until it came free.









With the forks on the bench the spring gator, spring and top trim part were removed from the fork tube. (The S90 forks have an external spring only).

The working area (arrowed) was checked for pitting or damage and found to be OK.

Any damage in this area can accelerate seal wear, but these forks looked OK.









To access the working parts of the fork the top cap had to be removed.

This had to be done without the specific Honda tool which was unavailable.

The lower fork body was clamped firmly in the vice but not so tight as to distort the body (it's only aluminium).

On one fork, a layer of gaffer tape with a hose clip clamped tight protected the screw cap from the grip of the mole wrench. This worked well enough, but the second cap was much tighter.

On the second cap something with more grip was needed and it was found that a standard scaffolding clamp was the right size and gripped well without damage.

With the top cap unscrewed it could be slid, complete with seal, off the top of the inner fork tube.

Undoing the fork cap with mole grips.

Undoing fork cap with Scaffold clamp









With the top cap removed, the top fork bush (arrowed) was also slide off the top of the fork tube.

This was removed and inspected for wear on the inner face.









After the top bush had been removed the complete inside of the fork was pulled out and inspected for wear and damage.

The aluminium piston did show signs of 40 years of hard service, but all the O-rings were in place and the forks had showed no free play prior to the strip down.

All parts were wiped cleaned before the re-assembly.









The top cap seal was removed using a combination of tapping out from underneath and levering out from the top.

Care was taken not to damage the O-ring (arrowed) although this was also removed and cleaned as part of the service.









The new seal was pushed in on the arbor press with a 27mm socket being used to apply an evenly spread force.









The fork cap was screwed back into place with some Copper grease on the threads in case it needed to be removed again in the future.

Then the lower washer, spring and gator were replaced.

120ml of 10w-30 oil was added to each fork.









Replacing the forks can be tough because the springs work against you.

One technique is to pull the forks up through the fork tree plenty tall enough (1) and the tighten the pinch bolt (2) temporarily.

Once this has been done to both forks, the top plate was screwed to both fork tops and the pinch bolts released to drop the top plate down on to the steering pivot.









Once re-assembly was complete including reconnecting the brake cable and speedometer cable the bike was taken for a steady test ride.

Fork rebuilt and back on the bike.