Replacement OE discs on my Brembo brake system are silly money and, although I don't need them changing just yet, I was interested to see on last night's Wheeler Dealer programme that you can refurbish semi-worn discs with a special lathe.

The results on the Porsche Boxster on the tv looked good, but has anyone tried this on their Jag? Cost? Availability?

 

In-situ brake rotor turning is nothing new, it's been around since the '80s IIRC. There is no need to turn rotors (on car or off) as a routine part of a brake job.

 

Agreed. Watch your brake pads (best time to inspect them is when you're rotating your tires), and once they get down to an eighth of an inch of material left, replace 'em. No point damaging your rotors by trying to wring every last millimeter out of the pads....

 

Wouldn't it make sense to have the discs skimmed when the pads are replaced to get rid of the slight grooving on the discs?

 

No, typical grooves do no harm. The new pads soon wear themselves to match the contour of the rotors.

 

That's your call. Many brake experts these days say no, you'll just be shortening the life of your brake disks. Of course, most brake shops want to machine your disks whether they need it or not so they can lighten your wallet....

 

A good point to keep in mind is that when your disks get down to the min. thickness spec. (either by wear or machining) it's time to replace. A lot of folks who should know better will machine down to the min. spec. and send you on your way. Big mistake.

Brake dicks operated at min spec will warp probably sooner than later.

 

According to vendor recommendations the stamped minimum thickness is a "machine-to" number that takes into account the wear from one further set of pads.

In other words, if the machined rotor is at the specified thickness it is safe to use it for one more set of pads by design because there is an allowance included for the wear associated with the newly installed pads.

Second guessing the engineered number only lightens wallets needlessly.

 

Well, I usually like to go with the OEM recommendations too but I have experienced 2 instances where the disks warped in heavy stop and go traffic after a Min. Spec. brake job. Call me crazy but seeing is believing.

Once with my 05 XJR and another with a 79 Olds Toronado. This within 200 miles of the work. Since then, I have had an experience brake tech tell me that "you are just asking for trouble with any brake disk operated below the stamped spec.

 

I've been tempted by the idea of skimming a few times over the years but in the end have always gone for replacements.

What swings it for me is if the discs are so badly grooved or warped as to justify skimming, the amount of metal that has to be removed to bring them back to true and flat is bringing them too close to the minimum thickness.

Graham

 

Barring unusual circumstances, such as particularly heavy glazing, wear or scoring, I usually simply opt for new rotors evey other pad change.

I can definitely understand your hestiation on the costly Brembos though.

 

That's right ... below spec not at spec.

Even above spec, if the rotor has formed hard spots causing brake pulsation, the hard spots may return after machining.

There is also the skill and patience of the machinist to consider. Fast, deep cuts make money faster. Slow, shallow multiple skims produce the best result. Most production shops won't do it that way. You have to be running the lathe yourself or talk the operator into it. If you can get fresh cutters put on for the job .. bonus!

Then, the rotors need to be washed in dish soap and *hot* water. If not, the metal shavings will transfer to the new brake pads and be embedded. Pretty soon, the problems resurface.

When you can get a set of rotors done properly as above, the vehicle can probably be serviced with pad slaps for the rest of its remaining life as long as the pads never get down to nothing.

 

There are three things to consider ..

- the stamped marking is a working thickness

- some aftermarket rotors meet thickness specs, but reduce *mass* by reducing plate thickness and increasing vent channel size. it is mass that yields heat resistance

- GM now has a TSB out that forbids cutting rotors only for cosmetic scoring when there is no other problem such as excessive runout or lack of parallelism. Their number is .030 - .060 inches .. about as big as a sparkplug gap. Seems pretty deep .. but that's what it is.

Of course, new parts are like new shoes ... always nice to have.

BTW, well machined "seasoned" rotors have the advantage of being dimensionally stable.

 

I am aware that some makers now recommend against maching discs. However, most modern vehicles will usually accept a maching without problem. I will say that I believe it to be a huge mistake to simply "slap a set of pads" onto a used and worn disc. Since I do many brake repairs, I see that "Jaguar" discs, in particular, wear very unevenly. To the naked eye, they look pretty good, but when take to a machine, they sometimes require 3-4 passes on the lathe to clean up. By then, you are either "at minimum" or beyond. If you just change the pads, I would suspect it would take a very, very long time for the two to seat in properly. Might as well buy replacements, which are pretty reasonable in the aftermarket ($100). Brembos are, of course, another story! When they do clean up within specs, I have had very, very few problems with warping and such with normal driving. In fact, the discs on my personal XK8 are resurfaced.

 

A used disc is not necessarily a worn disc. There is a great variation depending on driver habits. For example avoiding the brakes to begin with, "rolling out" on hard stops and never applying the parking brakes while the brakes are hot.

BTW, when I do skim cuts it might take me 4 passes just to get to 360 degree contact on the cutter. I adjust the cutter to just touch on each pass and use the absolute slowest feed I can get on the particular lathe.