X-drilled & Slotted Rotors CAN be cut!

[CJ_Boxster]

Well as some of you know, i bought some x-drilled & slotted rotors by Zimmerman and about 2-3 months after installing them on my daily driver boxster, they warped. I torqued my wheels way too tight so they warped... stupid me, i went way over the 90 something odd lbs of torque that i should have had.

I asked on several forum if anyone knew if they can be cut and many mentioned that shops will not want to cause the holes and slots could break there rotor cutting tool

So, I went to my fathers shop to try and have the Rotors cut, Rather than cutting 5000th of an inch into the rotors like on any normal rotor, my father recommended going at 1000th of an inch at a time. We made about 4-5 passes on the rotor and the cut was perfect and smooth again!

So, if anyone wants to have there warped x-drilled & slotted rotors cut... you can on any run of the mill rotor cutting machine just as long as you make cuts at 1000th of an inch at a time till the entire surface is cut clean.

[Lil bastard]

It not a question of if you can machine the rotors, but whether you should. Porsche says you should not.

These rotors, because of their slots and holes require a greater thickness to maintain their overall strength than a solid rotor would. Cutting them down weakens them and makes them more likely to crack or shatter. Also, a rotor is basically a heat sink and the less material you have, the less efficient it is at absorbing and dissapating the heat energy from the system. It is exactly this heat, or more appropriately, energy transfer which stops the car.

On a warped rotor, the maximum allowable runout is 0.3mm (0.118"). And the max Peak to Valley roughness is 0.006mm (0.0002"). That means you don't get many passes to get it right and still maintain the required amount of material for strength and efficiency.

The other thing will be to find a shop willing to turn them on a lathe. This is not only because of worries about lessening their strength and making them prone to cracking and shattering, but also because the slots and holes really mess up the cutting bit on the lathe in a hurry. The shop will not make a profit replacing the cutting bits so quickly, unless they charge you quite a bit extra.

I wouldn't do it. Brake rotors are a consumable item just like the pads are. Gotta pay to play.

[CJ_Boxster]

I see your point and that infact if one of the reasons shops dont like to cut them, I went ahead and did it myself so if they fail, its my fault and ill let you all know but i did notice the Zimmerman rotors i purchased were alot thicker than my original OEM rotor. I dont know if i cut too much of material off the rotor by your measurments but I do know i cut 4000th of an inch from the surface of the rotor... is that to much or am i in the safe zone still ?

[Lil bastard]

Well, there's no way to tell if you did not measure the lateral runout or put a micrometer on the disc before you started.

The thickness of an OEM Boxster S disc is:

• front / rear
  28 mm / 24mm

The minimum allowable thickness of an OEM Boxster S disc is:

• front / rear
  26.0 mm / 22.0 mm

But, the OEM discs, while sometimes supplied by Zimmerman to Porsche for the 'S' model, are not the same discs that Zimmerman offers the public as aftermarket discs, so the allowances listed above may not be the same. But, the Zimmerman aftermarket x-drilled & slotted rotors have been reported to crack more readily by several owners who have installed them.

I'd measure your discs now w/ a micrometer and if inside the minimum allowance for the OEM disc, keep an eye on them for cracking, such as when you wash the wheels etc. Replace as soon as you see any cracks between holes forming. If outside the minimum allowance, I'd think about replacing them for safety's sake.

[CJ_Boxster]

Do you have thickness's for NON-S boxster, which is what i have?

[Lil bastard]

Base Boxster OEM non-drilled solid discs new:
front rear
24mm 20 mm

Brake discs Minimum thickness after machining*
front rear
22.6 mm 18.6 mm

* (from Porsche Workshop Manual) The brake disc may be reworked only symmetrically. ie. from both sides evenly - important point - don't just turn one side, take the material evenly from both sides!

Wear Limit
front rear
22.0 mm 18.0 mm

These are the specs for the Base solid discs. But, IMO, you should go by the 'S' specs as these are the ones the Zimmerman aftermarket discs are imitating.