Motorcycle Front Forks, Replacing the Seals

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Motorcycle Front Forks, Replacing the Seals

John H Glimmerveen Licensed to

Before the front forks on a motorcycle can be disassembled to replace the seals, it will of course be necessary to drain the fork oil and (depending on the type) depressurize them.

For front forks with gas or air pressure assistance, the mechanic must release the pressure before attempting any disassembly or draining of fluids. He or she should refer to a shop manual for specific information on his or her motorcycle.

Safety Note: It is very important to release the pressure from front forks with safety uppermost in mind. Generally the pressure in most fork designs of this type is relatively low, however removing the Schrader valve, for instance, can be hazardous and eye protection must be worn.

With the air pressure released (where applicable) and the oil drained, the mechanic can begin the disassembly process. Needless to say, the forks must be removed from the motorcycle in most cases.

Clamping Fork Legs

In the vast majority of fork designs, the oil seal is located in the leg. It is typically held in place with a circlip or snap-ring and protected from dust and road dirt by a rubber cover. To remove the seal it is first necessary to separate the fork leg from the stanchion. To do this, the fork leg must be held firmly but it is imperative that it does not get damaged in a vice, for example. Therefore, the leg should be wrapped with a shop rag and then held between some aluminum soft jaws that are circular in shape.

The retaining bolt, which holds the leg and the stanchion together, is located at the very bottom of the fork leg; however, the mechanic must hold a tube which is located inside the stanchion before attempting to loosen the clamping bolt. For most motorcycles made after the 60s, a special tool will be needed to hold the inner tube, and to negate this problem, an impact driver (air or electrically powered) can be used to loosen the lower bolt. However, it is imperative that the socket used be a solid fit on the bolt.

Note: the retaining bolt can have either a hexagon or socket (internal wrenching) head.

Seal Removal

With the stanchion separated from the fork leg, the seal can be removed. As mentioned, the seal will typically be held in place by a circlip. Removing the seal should be done with care so as not to damage the fork leg; this is particularly important on aluminum legs, and the mechanic should use a piece of wood between any lever (screwdriver for example) and the fork leg.

Some of the older designs, such as the Triumph forks with external springs, have the seal located in a removable collar (see photograph).

With the forks fully disassembled, the mechanic can examine every part. If the forks have been disassembled as part of a restoration, it is good practice to replace all worn parts (bushes and seals etc.). In addition, the fork legs should be examined for pitting or corrosion. As fork legs are available for most of the popular bikes back to the 60s, it is more economical to replace damaged or worn legs than to repair them (by machining, and replating for example).

As with all mechanical work on a motorcycle, it is important to thoroughly clean all of the component parts before rebuilding the forks. After reassembling the forks, they can be placed back into the triple clamps ensuring that the legs on are in the same place on both sides (some fork legs protrude through the top triple, needless to say both sides must be through the same amount). The triple clamp bolts should be tightened to the manufacturer’s recommended torque settings.

Fork Oil

Replacing the fork oil is simply a case of pouring the correct amount, and grade, of oil into each leg. Some manufacturers specify a certain volume (for example 125-cc) and some specify an air gap. In the latter case, the forks will be fully extended and oil added until the level is a set distance below the top of the fork legs (a special tool is available for this process but a simple ruler can be used with care).

Once the fork oil has been added, the mechanic must slide each leg up and down to draw the oil through the various valves inside the forks. This process should be done slowly so as not to aerate the oil.

Reassembly of the remaining components is a reversal of the disassembly process; however, the mechanic must ensure full and free movement of the forks from side to side with the throttle opening and closing easily in all positions, and that no wiring can become trapped or chaffed.