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The shafts that are taken out from inside the engine, such as the driveshaft and change shaft, have oil seals to prevent oil leakage. The key to these seals is the flexibility and smoothness of the lip, which can cause oil leaks if its performance deteriorates over time. If you find oil oozing on the shaft or near the seal, you need to replace it as soon as possible.
Table of ContentsOil seal to prevent oil leakage by lip contact
It varies from model to model, but on this motorcycle (Yamaha AT90) the clutch release arm is on the left side of the engine, and the pushrod that disengages the clutch goes through from the left side. Therefore, three oil seals are press-fitted around the drive sprocket.
After removing the dirt mixed with chain rub and mud that had accumulated to the extent that the oil seal could not be seen, transmission oil began to seep from the oil seal on the change shaft. It is no wonder that the seal is deteriorated because it is an old motorcycle that has been manufactured for more than half a century.
Oil seals are incorporated into the outer tube of the front fork, the hub of the wheel, or in the case of an engine, the change shaft, output shaft, or clutch push rod. In the case of front forks, fork oil is inside the outer tube and the inner tube slides through it. The oil seal, which is press-fitted into the end of the outer tube, has the role of keeping the oil inside the outer tube from leaking out and keeping moisture and dust from entering from the outside.
From a structural point of view, an oil seal generally consists of two lips: a sealing lip portion that comes in contact with rotating or sliding parts, and a dust lip portion that prevents external intrusion (depending on the application, there may be a single lip).
If it is just a lid, oil leakage can be prevented by plugging the hole with O-rings or liquid gaskets, but oil seals that do not leak oil or grease inside while rotating or sliding parts penetrate the center of the seal require a very high level of precision and require a certain amount of care to maintain their performance. It is necessary to take care of them.
In order for the seal lip to stop the oil, it is important that the surface of the parts it is in contact with is smooth. As a matter of course, if the surface of the shaft is as rough as sandpaper, the lip will wear and not be able to provide an airtight seal.
In the case of the front fork, if the hard chrome plating on the sliding surface of the inner tube develops point rust, that rust will scratch the sealship area as it moves back and forth. The inner tube doesn't rotate like the output shaft that the drive sprocket connects to, and it's always in the same position, so the seal lip also gets scraped by the point rust in only one place, which expands the damage and eventually causes the fork oil to leak out. However, the oil seal set in the outer tube of the front fork is an easy place to notice abnormality because it is usually easy to see when riding the motorcycle.
The oil seals used in the engine, on the other hand, are in a different situation to the front forks. The oil seal on the output shaft is located behind the drive sprocket and is prone to accumulation of dirt from scattered chain lubes, which puts the dust lip in a tough situation. The clutch push rod that runs through the crankcase is neither rotating nor sliding, but reciprocating, pushing the dirty rod into the oil seal.
The oil seal that seals the change shaft is dirty with gravel and other chain rubs, just like the output shaft, although the angle of rotation of the shaft due to the shift change operation is slight. This is a feature.
In order to prolong the life of the oil seal in such a harsh environment, it is important not to leave the dirt around the seal unattended at any rate. The area around the drive sprocket is prone to oil-based and gravel/dirt-based dirt, and it is obvious that if that dirt is near the oil seal, it will cause damage to the dust lip and seal lip.
Even with proper care, it is inevitable that the oil seal, which is a rubber part, will harden and the lip will become damaged with age, so it is important to replace the oil seal as soon as it begins to leak. It is important to replace the oil seal lip as soon as it starts to leak. However, the leaked oil will accelerate the dirt inside the sprocket cover and may cause damage to other oil seals, so it is important to take measures as soon as possible.
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There are two types of oil seals: press-fit type and sandwiched type.
A view of the area around the output shaft of the Kawasaki Z1. You can see that the crankcase does not hold the output shaft, clutch pushrod, or change shaft oil seal.
The three types of oil seals are press-fitted into the transmission cover which is screwed to the crankcase. The Z1, Z1000J, Ninja, ZZR1100 and other water-cooled models also follow this specification.
This is what the transmission cover looks like when it is mounted on the engine.
There are two types of oil seals built into the output shaft, change shaft, and push rod that exit from the inside of the engine depending on how they are assembled. One is the press-fit type and the other is the sandwiched type.
The press-fit type is literally pushed into the housing of the oil seal from the outside. On single cylinder engines with split crankcase, all oil seals are press-fit type.
In contrast, the sandwiched type is sometimes used on the output shaft of the upper and lower split type crankcase, and the flange is set on the outer circumference of the oil seal to match the groove of the crankcase. The best known example of this type of oil seal is the Honda CB750F series output shaft oil seal.
The advantage of this system is that it is not easy to pull out because the flange is sandwiched by the crankcase. However, if the seal lip is damaged and engine oil starts to leak, the repair work will be very large because it cannot be replaced without splitting the crankcase.
There are many engines that use press-fit type oil seals instead of sandwiched oil seals even in the upper and lower split type crankcase, and some of them use plate-shaped retainers to hold the oil seal from the outside to prevent it from coming out.
There are advantages and disadvantages to both press-fit and pinch-fit, but if we focus only on the difficulty of oil seal replacement, the press-fit type is easier.
Kawasaki's four-cylinder series is unique in the way it assembles the oil seal around the output shaft: Z1/Z2 to ZZR1100/Up to the 1200, the three oil seal sets for the output shaft, clutch push rod and change shaft are not set directly into the crankcase, but are press-fitted into the transmission cover which is fixed to the crankcase with screws. Therefore, when oil leakage occurs from the seal parts, the three oil seals can be easily replaced by removing the transmission cover.
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Press-fit type oil seals can be replaced from outside the engine
Stick the radiator hose picker with the pointed end into the oil seal and pull it out of the housing. Be careful not to scratch the shaft, as scratching the contact surface between the change shaft and the seal lip can cause new oil leaks.
There are no obvious scratches visible on the lip surface of the removed old seal, but the rubber has lost its elasticity, probably due to reduced pressure to tighten the shaft. The number "12 22 5" on the oil seal body indicates the body dimensions of "12mm inner diameter 22mm outer diameter 5mm thick".
To prevent the seal lip from being damaged by the end face of the change shaft when passing the oil seal, it is recommended to cover it with plastic before passing it through. Also, apply a thin coat of liquid gasket to the outer circumference of the seal to both lubricate and prevent oil leakage.
The oil seal replacement job shown here is an example of a Yamaha AT90 built in the 1960s. The vehicle was used in a previous post to wash the crankcase full of chain lube deposits with kerosene, and after cleaning the oil stains and changing the transmission oil for a while, the change shaft oil seal began to bleed wrinkles. It seems that the dirt that was acting like a sealant was removed, exposing the deterioration of the seal function.
Since the motorcycle is already 50 years old, it is natural that the oil seal is hardened like plastic, so it should be replaced together with the output shaft and clutch push rod. The crankcase of this engine is aligned right and left, and all the oil seals are press-fit type, so replacement is easy if you remove the old oil seals.
There are several ways to remove the oil seal. The method of slipping a thin hook-shaped tool between the shaft and the seal and pulling it out with a sliding hammer. Screw a tapping screw into the front side of the oil seal and pull it out with pliers. The method used here is to remove a tightly fitted radiator hose, using a tool called a radiator hose picker, which is inserted between the hose and the mouth of the radiator to widen the gap.
The hose picker has a sharp tip and a strong shaft, so the oil seal can be easily removed by inserting it into the gap between the dust seals and prying, taking care not to damage the shaft.
For parts that rotate, such as the output shaft, the contact area may be worn by the lip of the oil seal. However, there is a collar set on the outside of the shaft, and in many cases it is the collar that actually contacts the oil seal, so if it is the collar that is worn by the lip, replacing the collar and oil seal will stop the oil leak. If it is the collar that is worn by the lip, you can stop the oil leak by replacing the collar and the oil seal. In the case of the change shaft, it is not rotating but moving within a fixed angle, so it is unlikely that the friction with the lip will wear the shaft.
The oil seal in the area where the oil leak has occurred is a genuine part that is still supplied. The oil seal itself is sized according to industrial standards, so you can purchase a standard product based on the dimensional description of the seal itself. In the case of this shaft, we used 12mm inner diameter, 22mm outer diameter and 5mm thickness. However, not all parts of the oil are standard products, and some of them are model-specific products, so please be careful.
When installing a new oil seal, it is important to press-fit it straight into the housing without scratching the lip and without tilting it against the housing. Since the change shaft has a long protrusion, we pressed it in with a PVC pipe that matches the seal diameter.
In most cases, press-fit type oil seals can be replaced without the need for special tools. If you find oil oozing from the engine oil or transmission, take care of it as soon as possible.
Since the change shaft is too long to reach with a socket, the oil seal is press-fitted with a PVC pipe available at Home Depot. The most important point is not to lean against the housing.
In the case of this motorcycle, the oil seal is covered by a plain washer of approximately the same diameter as the oil seal and is positioned by a C-clip. This prevents oil and dirt from sticking to the chain, but to prolong the life of the oil seal it is best to clean it before it gets too dirty.
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