Clutch Plate "Stacking"

This procedure is for standard 851, 888, 996 models and can be used for overall guidance for the S, SP and SPS variants.

The term "stacking" is actually "thickness", but thickness in a special way.  For  this article it means an arrangement of plates in a particular way to a specific depth or thickness.  For clarity, the term "outboard" means farthest away from the engine and "inboard" meaning closest to the engine.

 Ducati Clutches are to be stacked to 38mm plus (+) or minus (-) 2mm.  This is all the plain steel plates (9 for the 851/888) and all the friction plates (7 fiber coated  plates for 851/888)........but NOT the outboard end pressure plate.  The pressure plate is the one with the deep holes that the clutch springs reside.  I know.........I didn't mention the 916/996.  Not to worry.

 Note: There is one 1.5mm thick steel plate amongst your nine (9)  1.5mm & 2.0mm flat steel plates that is slightly arched (curved).  This curved plate is barely discernable to the naked eye and is marked with a  single "dot" (aka punch mark) near one of it's inner circumference serrations.  This "dot" indicates 2 things.  #1. You have found the curved plate and #2 "this" is the concave side of this curved plate.

You can confirm this by placing the plate on a flat piece of glass and pressing your  finger tip on the outer circumference of the plate and pressing down.  The opposite side (180 degrees) will rise off the surface of the glass.

 Friction plates (usually 7) are the ones with the "tangs" on the outer
circumference, these tangs fit into the outer clutch basket.  The 9 steel plates have inner circumference serrations that fit into the inner clutch hub.

 Ok...........now that you have the basics, here's the stacking order of the clutch, starting with the first plate that you should put into the clutch basket (inboard).

 1.5 mm steel plate with "dot" facing outboard
 2.0 mm  steel plate
 3.0 mm friction plate
 2.0 mm steel plate
 3.0 mm friction plate
 2.0 mm  steel plate
 3.0 mm friction plate
 2.0 mm steel plate
 3.0 mm friction plate
 2.0 mm  steel plate
 3.0 mm friction plate
 2.0 mm steel plate
 3.0 mm friction plate
 2.0 mm steel plate
 3.0 mm friction plate
 2.0 mm steel plate


 Overall guidance:

 You will need 9 steel plates with a combined thickness of 16mm to 18mm.
 You will need 7 friction plates with a combined thickness of 18mm to 20mm

 Bruce Myers (BCM Motorsports) note:
 This last 2.0mm steel plate can be interchanged with a 1.5mm "dot" (curved) plate  facing INBOARD (not outboard like the first 1.5 mm plate) ----IF :

 1. You need to adjust the stack to get your 38mm stack height (thickness) --OR--

 2. If you want a very progressive (soft) clutch engagement as opposed to a clutch  that can/will engage at a definite "now" feel.  Your choice.

 Also..........any of the steel 2.0mm plates can be exchanged with 1.5mm steel plates  along the procedure to reduce stack thickness or increase stack thickness as needed  to achieve your 38mm stack. You will see the need for this as friction plates wear.
 New 3.0mm friction plate service limit (minimum) is 2.5mm.  2.0mm and 1.5mm steel  plates only need replacing if scored or warped.

 I realize that many of you will not have spare plates of any kind, but when/if you  change out your clutch (now that you know the stacking procedure) you will have spare  (used good) plates left over from previous clutch change-outs. Not to mention that  you can buy individual plates from people like Barnett and others so that you can  adjust your clutch stack.

 Final footnote: the bolts that hold the caps on the clutch springs.  5 ft lbs. (60  inch lbs) is the proper torque.  The metal the screws are threading into is cast  aluminum and extremely soft material.  These bolts (10mm hex heads) are extremely  easy to strip.  We Americans have a tendency to use large wrenches and large 1/2 inch  drive ratchets that have long handles.  This scenario encourages excessive tightening  of bolts and screws (over torqueing).  If there is a single bolt on the whole motorcycle that you need to be very careful with.........this is the one.  Do not use  locktite on these bolts as it can easily remove the threads the next time the bolt is  removed.  Soft aluminum and steel is not a good combination for locktite.  Easy  way......or hard way..........your call.

Why's my Clutch so Stiff?

The force required to pull the clutch lever is ultimately controlled by the engine's power output. A Ducati superbike with a maximum torque of 65 ft-lbs. being transmitted though it's clutch needs to have around 430 pounds of preload in the clutch spring(s) to prevent the plates from slipping.

This means that the hydraulic pressure on a typical 28mm slave cylinder needs to be about 425 psi to overcome the 430 lb. spring preload and disengage the clutch. The distance that the slave cylinder needs to move (the pushrod that in turn separates the plates) has to be at least the thickness of the 2mm dished plate in the clutch pack. Say 3mm tops.

About 94 lbs. of force is needed to be applied to a typical OEM 13mm diameter master cylinder piston to create 425 psi of pressure in the incompressible hydraulic fluid that in turn moves the slave cylinder. For every 1mm that the slave cylinder moves the pushrod, the OEM master cylinder has to move about 4.6mm. This requires the displacement of about 1.2 cc of hydraulic fluid. But only in the ideal world.

In the real world, the master and slave cylinders are connected by a rubber clutch actuation line that expands a little under this 425 psi pressure. This undesirable expansion can be reduced in two ways. The rubber can be reinforced with Kevlar fiber or steel braid, or the line internal diameter can be reduced (by choosing a 2 or 2.5 mm I.D. line rather than a regular 3mm) thereby increasing the line wall thickness (and it's strength.) The net result of having a real-world line is that it increases somewhat the volume of fluid that the master cylinder has to displace to get the same internal line pressure. This is often described as a "spongy" lever.

Continuing on. In order to apply the required 94 lbs. of force to the clutch master cylinder piston, the clutch control lever needs to be pulled, and here's where the lever's mechanical advantage comes into play.

The human hand can't repeatedly (without fatigue) apply 94 lbs. of force to the lever so the master cylinder lever is designed to provide a mechanical advantage - to amplify the hand's force. The OEM design for example provides between a 4:1 (two finger) to a 9:1 force reduction (end of lever.)  This means that you have to normally apply between 23 and 11 lbs. respectively with your hand to release the clutch.

The lever's mechanical advantage has it's consequences, however. Instead of having to move the master cylinder only 9mm to disengage the clutch 2mm, the end of the clutch lever now has to move more than 3 inches. In a racing situation this is undesirable so there's replacement radial master cylinders offered that reduce this movement, but at the sacrifice of higher clutch pull forces. For the street it's desirable, since the longer lever travel makes it easier to launch the bike from a stop by increasing the range of the "friction zone."

That said, the only practical way to reduce the clutch lever pull force is to change the diameter of the slave cylinder.  The force-reduction aftermarket slave cylinder replacements offer around a 20% reduction but at the penalty of needing a longer clutch lever pull to get full disengagement. Another solutions would be to just reduce the distance between the clutch lever pivot point and the master cylinder in combination with decreasing the diameter of the master cylinder. These options are not currently offered in the aftermarket.

Larry Kelly