Having a clutch that works correctly is key to being able transfer all the power your engine produces to the rear wheel (or wheels if you're a quad guy/gal). In this post I want to share some key clutch inspection techniques I use and recommend to help ensure your clutch works as it should. These tips are presented in a step by step format and are taken right from my book, The Four Stroke Dirt Bike Engine Building Handbook.
Inspect the driven gear which is secured to the basket. Look for damaged gear teeth and other imperfections. Grasp the gear and basket firmly, then try to twist the gear. The gear is secured to the basket either with rivets or fasteners. With use, the rivets or fasteners can loosen causing the gear to become loose. Most baskets use round rubber dampers to locate the gear to the basket, which are sandwiched behind the backing plate. The dampers can wear out and break, which will create excessive play between the gear and basket. Any looseness may have been accompanied by excessive gear noise or rattling sounds when the engine was previously running.
On baskets with loose gears and riveted backing plates the corrective action which will need to be taken is to either replace the basket or drill the rivets out. The idle gear may need to be pressed off in order to remove the backing plate. Once this is done, holes can be tapped and bolts installed which will secure the gear in place. Any rubber dampers that have worn can be replaced with aftermarket options. Check out this article for more details on clutch basket damper replacement: How to repair your clutch basket dampers for less than $30.
Inspect the needle bearing bore surface on the basket next. Run your fingernail across the bore feeling for signs of wear. The bearing surface should be smooth and free of imperfections. If the surface is grooved or worn the basket will need to be replaced.
Inspect the area inside the basket where the large thrust washer resides. Wear should be minimal in this area. If any grooving is present, the needle bearing and spacer the basket rides on may have worn causing the basket to wobble or the pressed in steel insert has backed out, ultimately causing the face of the basket to rub on the edges of the washer.
Check for bent clutch basket fingers on the basket. Then look for grooving on the basket fingers where the clutch discs come in contact with the fingers. Grooving is caused by the clutch discs slamming into the clutch basket fingers. Normally grooving will be more pronounced on the drive side fingers. Grooving is not abnormal and occurs through usage of the clutch.
If any grooving is present, use the end of a pick to evaluate how deep the grooves are. Any grooving that can catch the end of the pick is also likely to be able to catch the edge of the clutch discs. When this happens, the clutch will have difficulty engaging and disengaging. If your bike had clutch disengagement/engagement problems prior to disassembly, basket grooving is the most probable cause.
A file can be used to smooth the grooves so the discs no longer catch, however deep grooving is an indication that the basket is near the end of its life. When filing clutch basket fingers, attempt to remove as little material as possible and remove material evenly from all the fingers.
Some manufacturers provide a specification for the clearance between the clutch disc tang and the basket fingers. This clearance can be measured by temporarily installing a clutch disc into the basket and using a set of lash gauges to check the clearance between the two parts. Both the clutch disc tangs and basket fingers will wear so if the clearance is outside the service limit it may be possible to prolong the life of the basket by installing new clutch discs. This is a short term fix however, and replacing both components at once is advisable.
Inspect the clutch hub needle bearing and spacer for signs of wear. The needle bearing will be replaced with a new bearing, but if the spacer is in good condition it will be reused. Check for grooving or concavity along the surface of the spacer where the bearing rotates. While the needle bearing won’t be reused, it can be inspected as well to help confirm any problems associated with the clutch basket or spacer.
There are two main areas on the clutch hub which will wear. First, grooving can occur on the splines which locate the clutch plates to the hub. The grooves are a result of normal clutch use and occur when the steel clutch plates rotate back and forth in the spline grooves. Any grooving which catches the end of a pick should be considered problematic. Careful filing to smooth the grooves or hub replacement are the two options available for remedying the issue. The clutch plates must be able to easily slide back and forth along the hub, otherwise clutch disengagement/engagement problems will occur.
The second area susceptible to wear on the clutch hub is at the back face of the hub. This is where the outer clutch disc contacts the hub. When the clutch is engaged, the clutch disc and hub will rotate in unison. However, when the clutch is partially engaged or disengaged, the clutch disc will rub against the face of the hub causing both the hub and disc to wear. Look for uneven wear patterns and indications of how deep the clutch disc has worn into the clutch hub.
If the face of the clutch hub has worn excessively or unevenly, the hub should be replaced.
Pressure Plate Inspection
The interaction between the pressure plate and clutch disc is identical to the situation previously described between the clutch disc and clutch hub. Wear will occur on the face of the pressure plate which contacts the outside clutch disc. Determine the condition of the pressure plate by looking for signs of excessive or uneven wear on the face of the pressure plate.
Disc and Plate Inspection
Both the clutch discs and clutch plates are designed to be wear items which will need replacement from time to time. Thickness and straightness are the primary inspection criteria used to determine if either component requires replacement. If there are any problems with any of the discs or plates replacing them as a set is best.
Clutch Disc and Clutch Plate Inspection
Clutch discs are made out of various compositions of fibrous materials which wear at different rates, while clutch plates are made from steel. Service manuals will specify a minimum thickness that the clutch discs and plates can be. This thickness can easily be measured by using a caliper. Take measurements at three to four locations around the clutch disc or plate to confirm either has not worn unevenly.
Once all the disc and plate thicknesses have been measured, both should be inspected for warpage. This can be done by laying the disc or plate on a surface plate or other flat surface. A set of lash gauges are used to determine any warpage. The service manual should specify a maximum warpage value which is usually around 0.006” (0.15mm). Attempt to insert the 0.006” lash gauge underneath the clutch disc or plate at multiple points. If the feeler gauge slides beneath either of the parts, those parts are warped and should be replaced.
Clutch discs which have been overheated due to excessive clutch fanning by the rider, not only may warp, but also emit an unpleasant stinky burnt smell. If a noticeable smell is present, the discs have overheated and should be replaced. Likewise, clutch plates that have overheated will likely be warped and exhibit discoloration. The discoloration is a sign of excessive heat build up. Once the clutch plates have overheated, the material properties of the plate change, the hardness is reduced, and the plate becomes less wear resistant. This means discolored plates should be replaced.
Lastly, inspect the clutch disc tangs for wear, chipping, or damage. If any tangs are damaged the disc should be replaced.
Clutch Spring Inspection
Over time and due to normal clutch use, the clutch springs will shorten. Clutch spring minimum free length specifications are provided by manufactures and can easily be measured using a caliper.
Clutch springs that are shorter than the minimum spec provided by the manufacturer will not have sufficient spring pressure to keep the clutch from slipping under heavy loads. Any springs at or past their service limits require the replacement of all springs as a set. This way when the new springs are installed, even pressure is applied to the pressure plate.
I hope you enjoyed this passage from my book detailing clutch inspection. If you have additional tips you'd like to share please leave a comment!
If you want more technical DIY dirt bike engine information, learn more about the book on our website or on Amazon. Simply follow the links below!
Thanks for reading!
-Paul Olesen, DIYMotoFix.com