Essentials of Front Fork Maintenance | What is the Role of the Oil Lock Piece and How to Assemble it?

The condition of the suspension, which holds the key to handling stability, is maintained by regular internal cleaning and fork oil changes. When disassembling and servicing floating valve type upright forks, the seat pipe should also be removed, but be careful with the oil lock piece to avoid wasting the work.

What do you know, but don't know much about the role of the oil lock piece?

The cylinder (bottom) inserted inside the inner tube (top) of the Ceriani front fork contains a rebound spring to reduce the impact when the inner tube is fully extended. Although not shown in this image, an oil lock piece is inserted into the end of the cylinder (left end).

The upright front fork of the so-called Ceriani type consists of an inner tube, an outer tube, a cylinder (seat pipe) inside the inner tube, and fork spring. When changing the fork oil, remove the top cap, pull out the spring and turn the fork itself upside down to drain the oil. For more serious disassembly and maintenance, you can loosen the cap bolt at the lower end of the outer tube, remove the cylinder (seat pipe) inside the inner tube, and pull the inner tube out of the outer tube to remove the internal parts.

The name of the short pipe-shaped part that fits inside the inner tube is called cylinder, seat pipe, or damper rod, depending on the motorcycle manufacturer, or suspension manufacturer if you go back further. However, all of them have the same mechanism to generate damping force by resistance generated when oil moves through the space between the outer tube, inner tube, and cylinder.

When you pull the cylinder out of the inner tube, you will see a thin-walled aluminum collar-like part attached to the end of the cylinder. This is the oil lock piece. The oil lock piece has a tapered tip. The outer diameter of the tapered oil lock piece is slightly thinner than the inner tube's inner diameter. When the inner tube is compressed, the oil between the outer tube and the outside of the cylinder flows into the inner tube through the cylinder and the valve at the end of the inner tube. As the inner tube compresses further, the tip of the inner tube meets the bottom of the outer tube, but before this happens, an oil lock piece is placed over the inside of the inner tube tip, forcibly blocking the flow of oil. Since the oil does not change its volume due to compression, it becomes locked and the inner tube will not contract any further, effectively ending the stroke.

If the inner tube and outer tube butt against each other at full stroke, there is no need to use oil lock piece. If you intentionally apply the front brake hard while riding, you will feel a bottoming out sensation, but even if you feel a thumping impact, it is because the oil reduces the direct shock. Although the oil lock piece avoids contact between the tip of the inner tube and the bottom of the outer tube, some of the stroke lengths becomes invalid.

Therefore, in limited situations such as racing, the oil lock piece may be removed in order to give priority to the effective stroke length of the front fork over the safety margin. However, this is a special case, and it goes without saying that the merit of the oil lock piece is superior in general.

The explanation here is based on floating valve type upright front forks, but both inverted forks and inner rod types incorporate an oil lock piece that avoids collisions between parts at the end of the stroke.

Chewing on the oil lock piece can change suspension characteristics.

When assembling the disassembled front fork, insert the cylinder through the inner tube and then insert it into the outer tube with the oil lock piece set at the tip. When the sunken inner tube reaches the tapered part of the oil lock piece, the oil passage closes and no further stroke is possible.

When the cylinder turns when the cylinder fixing bolt is tightened, the fixing socket of the special tool should be pressed against the top of the cylinder to stop it from turning. When the oil lock piece does not fit well in the bolt hole position of the outer tube, you can also use this tool to guide the cylinder. However, depending on the inner diameter of the cylinder, the square pyramid may not catch.

When you are doing a DIY front fork overhaul, you may have the impression that the oil lock piece attached to the end of the cylinder is just one aluminum part, but as we explained, its function is very important. However, there are some unforeseen problems that may occur when you restore the removed cylinder and oil lock piece. A typical example of this is the biting of the oil lock piece.

When assembling the front fork, insert the oil lock piece into the end of the cylinder through the inner tube and insert it into the outer tube with the oil lock piece in place. When the cylinder touches the bottom of the outer tube, tighten the cap bolt from the lower end of the outer tube to the specified torque while pulling the cylinder in to secure it. When the cylinder rotates when the bolt is turned, insert the holder tool into the inner tube to hold it in place.

The sequence of events is very simple, but it is important to ensure that the cylinder is correctly centered when tightening it to the outer tube. If the female thread at the end of the cylinder, the bolt hole at the bottom of the outer tube, and the inner and outer diameter machining of the oil lock piece are all centered with zero machining error, you should be able to tighten the bolt without thinking about it. In reality, however, all parts have tolerances that are acceptable. Not only the parts mentioned above, but also the inner diameter of the outer tube, the bolt hole at the bottom, and the inner tube may or may not be exactly centered to the cylinder, even if the basic premise is that they are within the tolerances.

And each part of the slightest misalignment can cause the inner tube tip and the oil lock piece to be off-center, causing the two to physically come into contact at full stroke. It is a possibility. It is not a certainty that this will happen, it is just a possibility. However, there have been cases where the cylinder has been assembled with a slight tilt due to unintentional misalignment of the outer tube with the bottom of the oil lock piece. Also, we have found the oil lock piece with traces of the inner tube hitting it hard during disassembly.

Even if the inner tube and oil lock piece bite each other and friction loss occurs, the actual front fork will not lock at a full stroke because the reaction force of the spring or air spring will be stronger. However, if the inner tube and oil lock piece are not aligned, the damping force characteristics at the time of oil lock will not be as designed, and the degradation of the fork oil may be accelerated by the physical contact.

Centering during assembly prevents uneven hitting of the oil lock piece.

Install the cylinder fixing bolt after renewing the gasket and cleaning the outer tube seating surface. Clean the bottom of the outer tube where the oil lock piece is seated so that no dirt or foreign matter remains.

When the extension bar is attached to the cylinder fixing socket with the T-type handle and the fork itself is inverted, it is easy to tighten the bolt while checking the interference between the inner tube and the oil lock piece. If the oil lock piece interferes when the bolt is tightened to a higher torque, the oil lock piece may not be seated properly against the bottom of the outer tube and the cylinder may be slightly tilted.

If the oil lock piece is not in the center of the inner tube, it may not be a big problem under normal riding conditions. However, it is not nice to have a part that should be in the center not there. So how do you determine the position of the oil lock piece that you can't see by looking through the inner tube? To find out you can stroke the inner tube when assembling the cylinder.

If the cap bolt that fixes the cylinder is temporarily tightened and the inner tube is stroked with the oil lock piece off-center, you may be able to feel the two pieces rubbing against each other at the most contracted position. If you stroke the inner tube while turning it, and there is no change in the feeling anywhere, you can conclude that the oil lock piece is not in contact. If the contact is felt or lost depending on how tight you tighten the bolt, the bottom of the oil lock piece and the bottom of the outer tube may not be in perfect contact, and the oil lock piece (and cylinder) may be leaning more as you tighten the bolt.

It is not easy to find out what the real cause is, as this is happening inside the outer tube where it cannot be seen. However, changing the position of the cylinder and outer tube, and the position of the oil lock piece and inner tube, may symptomatically improve the rubbing symptoms. If the front fork is disassembled and the oil lock piece has no galling or rubbing marks, there is a good chance that the front fork can be reassembled without contact between the two pieces.

It is true that the oil lock piece only works under limited conditions near the full bottom, and under normal riding conditions, it is easier to feel the difference with preload and fork oil level changes. However, if you are going to overhaul your motorcycle, you want to assemble it so that it can function at 100% in all areas. For this reason, it is important to understand the role and importance of the oil lock piece at the end of the cylinder.