Tech Q&A: Fork Oil Drain Plugs

A fork oil drain plug on a 1998 Kawasaki.
A fork oil drain plug on a 1998 Kawasaki.

Q: Why don’t late-model motorcycle forks have oil drain screws? Didn’t they used to? I need to change my fork oil but really don’t want to take the fork legs off the bike. Can I drill and tap my own drain holes?

A fork oil drain plug on a 1989 Honda.
A fork oil drain plug on a 1989 Honda.

A: I feel your pain! Through the ’90s, motorcycles with damper rod-type forks typically had drain plugs to allow changing the fork oil without removing the fork legs from the bike. Cartridge-style forks must be removed and disassembled to properly change the oil, but many newer motorcycles still have damper rods, so why no drains? Based on what I have been told by the OEMs along with personal experience, I believe it’s a combination of factors, cost cutting among them. It is cheaper to manufacture forks without drains since the casting boss, oil passage, screws, etc., all add cost to the motorcycle. Lack of use may also be a factor—service departments failed to sell fork oil service to the customer. This is unfortunate since clean, fresh fork oil makes a noticeable improvement in fork performance. Most riders can feel the improvement when fork oil is changed yet most riders (and mechanics) ignore fork service until the seals are leaking. By that time the oil is badly degraded, dirty and the fork wear-and-tear components all need replacing.

It may be possible to drill and tap your Suzuki’s fork sliders for drain screws, but keep in mind that you will have to remove and disassemble them to do so. If that sounds too daunting, you may as well have your dealer perform the fork oil service. For a period of time, some sliders still had a drain casting boss that was easy to drill and tap. Current forks do not have such a casting, so in addition to drilling the hole you will need to create a flat spot around it for the screw head to seal against. All of this risks weakening the fork leg, another possible reason the manufacturers have moved away from including drain holes.

If you choose to drill and tap drain holes, great care must be taken. The fork slider must be inspected internally to confirm there is sufficient material available to support steel screws in the softer aluminum, and to confirm the appropriate location. I typically place them on the backside of the slider to facilitate draining, and drill a 4mm hole with a flat-bottomed counterbore for an O-ring or, if the surface allows, a sealing washer under the screw head.

Advertisement

LEAVE A REPLY

Please enter your comment!
Please enter your name here