Cutting Drilled-and-Slotted Rotors

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VStar650CL
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Car: 2013 Nissan Altima 2.5 SL
2004 Nissan Altima 2.5 S

Cutting Drilled-and-Slotted Rotors

Postby VStar650CL » Mon Aug 30, 2021 11:06 am

This is another Blue Light Special in the Live-and-Learn Department. Early last month I was "gifted" a set of slotted rotors by way of another tech who replaced them on an Altima, having told the customer that they couldn't be resurfaced. I've always believed the same thing and would have offered the customer the same advice. However, these were some very expensive racing slots with dissipation sinks and the whole nine yards. The runout on them wasn't terrible (about 0.005") and my '04 needed front pads anyway, so I decided to do some experimentation. To protect the lathe bits, I tested for the highest spots at 3 places across each rotor face before actually starting a cut. Then I cut in tiny steps, first a coarse cut of about 0.0015", then successive fine cuts of 0.001" until they were smooth. All cuts used a "spring and stone" type silencer plus a radial elastic silencer to keep bit-bouncing to an absolute minimum. It took 4 passes to smooth out the worst side of the worst rotor. I was worried anyway that there might be tiny ridges at the edges of the slots and holes, but I'm happy to report that after a month of use, she still stops smoothly and I see no evidence of striated wear in the pads that would indicate high spots.

So the lesson is, the only reason D&S rotors can't be cut is because your machinist needs extreme patience. You can't just whack them down 0.005" in one cut like you would with plain rotors, and you can't just start at the hub and hope the high spots don't get higher toward the rim. You have to measure the latter and take your time. But don't toss your D&S's if they run out. Just find a patient machinist, or find a lathe and do it yourself -- patiently.
:biggrin:


macgiver
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Re: Cutting Drilled-and-Slotted Rotors

Postby macgiver » Mon Aug 30, 2021 2:14 pm

Back in the day , "everyone " would think , cut the rotors , and do so. Like I owned GM then and people had the thought and the push to machine rotors . Then I read a TSB from GM stating some common sense approach to disc brake pad R&R . Which I totally adhere to these days , and works .

GM said " DONT MACHINE ROTORS IF PEDAL HAS NO VIBRATIONS" . Even if there are slight grooves from when a few miles into the rivets , yet rotor is still very thick. No more rivets today so that second consideration is moot now.

And you don't want to get dragged down the rabbit - hole some clowns will try to argue to the DEATH of whether warpage happens , or if it's "really deposited material " - duh , or whatever some troll will talk.
IF the pedal is still smooth - your damn pads R&R will be fine , yet some clowns act like they own a f'n Maserati or something!
I've replaced drilled / slotted / combo etc. last 20 years and myth #1) that drilled "cheesegrates" your pads - never seen it . Yes , drilled may be compromised under racing - no brainer , streets OK.
Myth #2 ) Not really a myth but I NEVER had performance ( non-flat/stock rotors) EVER need machining , even hard street driving. And of course they'll wear with metallic - even semi-metallic pads . My best experience is use a Ceramic that bites or stops hard - or both.

p.s. Just seen your experience with your beloved dog , we had Jack Russell w/ baseball tumor on jaw and feel your pain - we do everything for our little ones - God knows you did.

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VStar650CL
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Car: 2013 Nissan Altima 2.5 SL
2004 Nissan Altima 2.5 S

Re: Cutting Drilled-and-Slotted Rotors

Postby VStar650CL » Mon Aug 30, 2021 4:31 pm

macgiver wrote:
Mon Aug 30, 2021 2:14 pm
p.s. Just seen your experience with your beloved dog , we had Jack Russell w/ baseball tumor on jaw and feel your pain - we do everything for our little ones - God knows you did.
Appreciate the good wishes very much. Funny how ours brains hang onto the littlest things. Every morning when the alarm goes off, the back of my brain insists on doing a double-take when the foot of the bed is empty. I wonder how long it will be before my hindbrain gives up. I'm thinking it'll take awhile.

GM is right, if it ain't broke then don't fix it. Whether you ever get warpage depends greatly on the quality of the rotor casting and the metallurgy, but also on the individual flaws in a given casting. Castings all have flaws just like diamonds, it's a matter of how hard you have to look to find them. There's no such thing as a flawless one, and whether they cause problems depends on where and how big they are. They also have carbon nodules just like an engine block, and once the nodules are reduced to powder, if it hasn't already warped then it never will. My deceased hoopty Hyundai went through about 6 sets of front pads and never had any warpage at all. There was no point in cutting them so I never did. We cut fronts routinely at the shop, but probably 90% of the people who come in with a brake complaint already have some shudder. We only ever cut rear discs or drums if there's an issue. When fronts get cut, I'd say probably 8~10% will be shuddering again inside of 10K miles, and that means a serious flaw that's distorting under heat stress. It will only get worse as more metal is removed, so cutting them again is pointless, they have to be replaced.

Back to the topic, every brake "expert" I ever talked to told me you can't resurface D&S rotors because the bits will jump at the far edge of the hole or slot and the resulting ridge will striate the pads and cause premature wear. I can now tell you definitively that the "experts" were full of baloney. My month-old pads are flat as pancakes and don't show the slightest evidence of any problems. I can only assume the ridging they talked about was an artifact of trying to take off too much metal and finish the cut quickly.

macgiver
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Re: Cutting Drilled-and-Slotted Rotors

Postby macgiver » Mon Aug 30, 2021 10:07 pm

One funny thing the stock / plain rotors did which never happened with slotted / drilled / etc. was how the pedal vibes could come and go , according to ambient & how hard I would brake ( usually f-way off ramp 70-95mph), was especially bad and unpredictable with 114+ temps , and or rush hour . We're HOT here and made stock rotors performance waaay too finicky ! Like Gump said "you never know what your gonna get ".

We found a puppy after several months , no replacement of course , filled the void and same breed but much his "own dog" , similar instincts / attitude . May you be able to move on , yet keep the little-one memory just as close as ever VStar . Respect , G .

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Re: Cutting Drilled-and-Slotted Rotors

Postby PapaSmurf2k3 » Wed Sep 08, 2021 2:56 pm

I went to 4 or 5 local places to turn my D&S rotors after hearing mixed stories on NICO and other places about not being able to do it. Every one of the places said "it can't be done" until the final place I went.
I fully expected to come back and have the guy tell me "something went horribly wrong and I'm sorry but your rotors are destroyed and so is our lathe. The new guy you talked to has been fired". Instead I was greeted with "ALL SET! That'll be $8 per rotor. Do you need anything else?"
I asked him if he had any issues and he said "no, why?". Explaining drilled and slotted he just said "oh, that old wive's tale? Yeah, its BS... you just have to go a little slower and not take off as much in 1 bite" and ever since then, that's what I've been telling people haha.


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