Clutch Cover Gasket

Before getting into the clutch itself, a few words about the cover gasket are in order.  Firstly, know that clutch cover gaskets are no longer readily available - if at all.  Take care of the one(s) you have.  They are quite robust but repeated removal/installation of the clutch cover can damage the gasket.  One of the first things I did to my bike was throw away the frustrating split spring-steel cover locating dowels.  I replaced them with “solid” dowels from Yamaha (P/N 99530-10114-00).  This makes removing the clutch cover much easier.  There is no need to remove the clutch hose!

When reinstalling the cover, I like to first put the locating dowels and gasket on the motor.  Then, to help hold the gasket in the proper place and assure it does not get bent, I temporarily install three M5 Phillips screws with their heads machined off.  You could just saw off the screw-heads but retaining the Phillips cross makes removing them easier.

One of three special temporary clutch cover gasket holding screws (this one's adjacent to the filler plug)

OSSA clutch cover gasket 9520020211 on flat-bed scanner with 6-inch steel ruler shown for scale.  The GasGas Pro gasket is quite different.

Clutch Master Cylinder Rebuild Kit

Outside the USA, this part is called a clutch pump.  Both old and new styles use mineral oil.

Older bikes use an AJP clutch master cylinder (clutch pump).  The rebuild kit is AJP part number 150.00.602C.

Newer bikes use a Braktec clutch master cylinder (clutch pump).  The rebuild kit is Braktec part number 853068MO0.  (Note that this part number contains the letter O as well as the number zero.)

Improving the OSSA Clutch

Something that immediately impressed me about my Electric Motion Race is how well the clutch works – especially considering how similar it is to the recalcitrant OSSA clutch.  Specifically, I was shocked by how little finger effort is required to operate the EM's clutch.

When my EM clutch was apart for service, I noticed the spring force was so light that I could easily push on the throw-out bearing with my thumbs to disengage the clutch.   That's not at all easy to do with the OSSA.

EM's use of 4 friction plates versus 3 in other designs does give more surface area, so the spring force could be reduced for the same torque-handling capability.  Conversely, the EM's 45 Nm maximum torque is probably greater than any other trials motor.  But an electric motor's peak-to-average torque (per revolution) is nearly constant, so that makes life easier on the clutch.

Bottom line, the EM clutch works beautifully, and it seems the OSSA's clutch could be improved.

Clutch Pack Thickness

With all other variables being equal, the thicker the clutch pack, the greater the effort required to pull in the clutch lever.  Conversely, the thinner the pack the greater the propensity for the clutch to slip.

The first two XiU-rdi clutch packs I received measured 9.91 to 9.92mm, which is too thick.  I ended up using one of my original worn friction plates for a while.  In the XiU-rdi system, the three fiber plates typically measure 2.12 to 2.13mm thick, and the steels are 1.49mm and 2.02mm. 

1.49 + 2.02 + (3 * 2.13) = 9.90

See the section My Upgrades, Clutch Control Ring for more information.

As a side note, steel plates for the GasGas are available in four different thicknesses: 1.3, 1.4, 1.5, and 1.6mm.  This is a great tuning aid, but no such option exists for the OSSA.  Unfortunately, the GG plates won't fit the OSSA.

GasGas Pro Clutch Dimensions

The Gas Gas Pro is similar to the OSSA clutch, both having 3 fiber and 2 steel plates.  I found the adjacent drawing in the GG's owner's manual.  Although somewhat cryptic, it shows two important dimensions.

I had no idea if these dimensions were even applicable to the OSSA, but it was a starting point.

I'm told that the 17.5 - 18.5mm specification just provides a method to determine pack wear without disassembly.  However, measuring that on the OSSA is not possible because the reference surface is blocked by the fingers themselves.

Still, I wanted to see what I could learn.

GasGas Pro clutch dimensions

Top row shows side with marking dimple.  Bottom row does not.

Fingers Manufactured by Stamping

When the fingers were manufactured, the punch press produced a smooth-edge side and a sharp-edge side (which also exhibits a small marking dimple).

All of my OSSAs were purchased secondhand, so a lot of people have had their fingers on those fingers.  Wear marks were evident on both sides.

Honestly, I had not previously given much thought as to which way they should be installed.  I typically use the old mechanic's maxim motors can't read and put any writing or marking towards the outboard side.

Initially, I felt that the fit was better if all the fingers were installed so that the sharp-edge (dimpled) side faces out.  That's why you'll see it that way in some of my photographs.

But in the end, I believe having the smooth-edge side of the finger outboard allows them to move more freely.

Although there may not be a huge difference, it's probably best to keep them all facing the same direction anyway.

The 15mm Test Plug

I began by assuming the GasGas 17.5mm dimension could be applied to the OSSA and made a plug 15mm in length and 25mm in diameter to check the uniformity of the finger height. 

The fingers are 1.5mm thick, so I figured 15mm + 1.5mm should give me at least 1mm of clearance between the plug and the inboard side of the fingers.  I assembled the clutch without the spring or discs after inserting the plug.  Only two screws were used to hold the control ring in place.

Some of the fingers had clearance to the plug and some were tight up against the plug.


15mm long plug installed as a first test

Clutch Release Arms (aka Fingers)

The 15mm test plug allowed me to determine that the clutch release arm height was not completely uniform. 

A Japanese repair video showed tweaking GG fingers with the tap of a hammer.  This looked like a good idea to me.  With each finger laying on a perfectly flat surface, I pressed on the wide end with my thumbnail.  If the narrow end moved up more than a very small amount, I clamped the finger in a vise and tweaked it with a tiny hammer.  I reasoned that if the narrow end did not move up at all, the finger was likely bowing up in the middle.

This process could ensure all eighteen fingers were as flat as possible.

Testing for flatness

Fine tweaking 

A Better Method

Although I tweaked my first set of 18 fingers using the process described above, I felt my thumbnail test was pretty subjective.  I wondered if there was a better way.

I then tried lightly sanding the inboard side against a granite surface plate covered in worn 100-grit silicon carbide paper.  

If the scratch marks were uniform, I felt the finger was flat.  If not, I used the vise and hammer shown above and sanded again.

This produced the result shown in the adjacent photo.

Bottom finger shows result of light sanding

Diamond Filing the Fingers

Still, the fingers did not all move perfectly freely.  I used a conical diamond knife sharpener to smooth the edges where they contacted the clutch hub.  This made a noticeable improvement.

The 18.5mm Test Plug

I decided to make a plug that was 18.5mm in length from the clutch hub.  To see how the fingers fit, the plug needed to be fairly large in diameter, the bigger the better (certainly more than the 21.45mm hole in the clutch hub that located the plug.  I settled on a diameter of 24.25mm.  But the only way to insert the plug is to allow gravity to pull all the fingers open simultaneously.  So, this is not a generally workable method as the gearbox/clutch assembly must be removed from the bike.

I'm not sure this test proved much or is worth the effort.  If the fingers are flat and their edges are smooth where they contact the clutch hub, that's about as good as it will get. 

I considered numbering the fingers with a vibrating engraver so I could put them back in the same orientation, but decided not to until I learned more.

18.5mm long test plug

Hydraulic Ratio

The clutch master cylinder has a 9.5mm bore.  The 2014 slave cylinder has a 14mm bore.  Hydraulic ratios scale on piston area, so the bore diameters must be squared to make a comparison.

2014 bikes: 14² / 9.5² = 2.17 (You can complicate this calculation by using pi and the radius, but the ratio works out the same.)

This means that the master piston multiplies the force applied to the slave piston by 2.17.  To do this, the slave piston must move 2.17 times farther than the master piston.  Thus force is traded for distance.  One way to remember which way the force multiplication works is to think of the large pistons on a disc brake caliper.  They move a small distance and produce the large force needed to scrub the pads against the disc.

But there is a limit as to how far this force multiplication can be taken.  The slave piston must move far enough to ensure the clutch discs can separate enough to completely release (slip). 

Although I don't have one to measure, the OSSA Factory models used a different diameter slave piston.  It was likely 15mm.

Factory bikes: 15² / 9.5² = 2.49  

This gives a greater force multiplication (making for an easier lever pull) but also a smaller separation of the discs in the pack.  This is probably good for a high-level rider who wants a more binary clutch operation, with a smaller slip zone.   The Factory model may use a different disc spring as well, but I don't have the part number to know for certain.

My notes are a bit unclear, but I think my 2011 TR280i probably used a 13.5mm slave piston.

2011 TR280i: 13.5² / 9.5² = 2.02

The 2011 bike definitely has a stiffer lever than the 2014/2015 bikes.  OSSA calls the slave piston a clutch pump piston.  The 2011 part number is 2620020211 versus 2620020214 for the 2014 models.

Lever Ratio

The clutch lever itself multiplies your finger force by the ratio of the X dimension divided by the Y dimension.  

We know this intuitively because the position of your finger along the lever changes the X dimension and thus the multiplication factor.  

The hydraulic ratio and lever ratio combine to create the overall multiplication of finger force that overcomes the clutch spring.

Credit: Adapted from Robinson's Motorcycle Tuning, Chassis

The lever ratio is defined as distance X divided by distance Y.

Disc Spring Engineering

The design equations for a disc spring are not as simple as those for a wound wire spring.  Typically, the manufacturers of such springs provide software tools to perform the calculations.  The screenshots below were created in about 2014 from a manufacturer's website.  Unfortunately, that website no longer exists.

Besides the elastic modulus of the material, four dimensions affect the spring's deflection versus force curve: material thickness, outside diameter, inside diameter, and overall height.  

In an effort to lighten the lever pull, I removed about 0.006" (0.15mm) from the inboard side of my 2011 disc spring.  I did this by sanding it on a surface plate.  A large cylinder of aluminum was machined to fit inside the spring's ID.  This gave me something to hold onto while sanding and assured even contact of the spring against the sandpaper. 

Removing this very small amount of metal effectively reduced the disc spring's clamping force by about 8 - 9%.

The standard clutch spring changed in 2104.  The old part number is 2420020211.  The new part number is 2420020214.

Dimension 2011 spring 2014 spring

Thickness 1.47mm 1.0mm

OD 120.8mm 120.5mm

ID 103.0mm 102.4mm

Overall Height 4.10mm 3.98mm

The main change was the material thickness.

Method used to reduce the OE spring's overall height by 0.15mm

OE 2011 clutch spring before modification

OE 2011 clutch spring after modification