WHAT  IS FLYWHEEL STEP HEIGHT?

Flywheel step is the difference between the friction surface and the upper step as shown here in the picture. You can also click on the photos for better viewing.

In order to have your new clutch work properly the flywheel must have it's friction surface machined flat.  The step height is critical to proper clutch operation. Whenever a stepped flywheel is resurfaced, how ever much material is removed from the friction surface must also be removed from the upper step. Most automotive machine shops use a special flywheel resurfacing machines for this task and know how to do the job but here are some things you can do to make sure your flywheel is resurfaced properly.

When it comes time to have your flywheel resurfaced at the machine shop, make sure to ask that they "true" their cutters. This will eliminate the possibility of the friction surface being machined with taper. Understand that as the cutters wear down, they will become tapered. If a flywheel is machined with worn cutters, the friction surface will be soup bowl shaped and  flywheel  will read good on the outer most diameter of the flywheel but has too much material removed from the inner most edge.  This causes all kinds of trouble because the clutch disc is only grabbing on the outer edge and the clutch won't hold the power because the all of the friction material isn't making contact. Good machine shops will true their cutters often. However, sometimes they forget, so make sure to request them to do so as it will ensure that your flywheel is indeed flat and has the same flywheel step height at the inner most edge of the friction surface and at the outer most edge.  The photos below show a correct way to check step height.

With the dial indicator up on the step, the tool is set at zero.

The measuring tool is then dropped down to the friction surface. Each rotation on the dial is .100" 6 turns are counted as the tool is dropped down. That is .600" with an additional .010" showing for a total of .610" shown here.

 If the flywheel has a tapered friction surface the clutch will not be able to hold as much torque before it slips and the clutch disc will wear out at its outer most edge of the friction material.  In summary, request that they "true" their cutters.

The ideal step height for most performance clutches that are available for the DSM is .608" to .610"  Do not attempt to use a step height greater than .612" for this will cause the pressure plate's spring plate geometry to be less than ideal. Flywheel step height is critical and a couple thousandths can have a drastic affect on the spring plate geometry and how the clutch works. (More about spring plate geometry)  If the flywheel step height is machined deeper than .612" the end result will be a clutch pedal that feels mushy & vague, as if there is air in the hydraulic circuit, and the pedal pressure will be heavy and not have that breakaway point where pedal pressure becomes less as the clutch pedal nears the floor.

Simply put, if the spring plate geometry is fuct up because the flywheel step height is not correct the clutch won't work as good as it should and you'll tear up your synchronizers, so get that flywheel machined correctly.  Far too many times people are such a hurry that they end up not machining the flywheel, only too end up with a clutch that doesn't perform as good as it could have and end up tearing up the synchronizers in their transmission.

Rule of thumb:  Any time that you replace the clutch disc and/or pressure plate you need to have your flywheel resurfaced. The clutch will hold more torque when the flywheel is flat and this will help eliminate chatter during engagement. Ask the machine shop questions and have them measure it right in front of you to make sure it's done correctly. If you are considering an aftermarket flywheel I highly recommend purchasing a Chromalloy flywheel, especially if the vehicle is AWD.... and make sure the flywheel step height is machined to the specifications listed above.

* EVO 4-9 uses a flat flywheel with no step. They are dead flat and anyone should be able to resurface one correctly.

Photos & revised text were barrowed with permission from our good friends at http://www.roadraceengineering.com

RRE's original full in depth clutch tech page http://www.roadraceengineering.com/clutchandflywheeltech.htm

 

  2009 TRE