How can one tell the amount of ware if you don’t know the original rotor thickness ?
Are rotors marked with its thickness ?

Front rotor wear limit is 9.5mm minimum; rear is 8.5mm.

What about vented rotors?

Review the application for which they were designed for the minimum wear limits.

If you see markings on a rotor that number will be the minimum thickness.

Most (but not all) rotors are marked somewhere. Often it is a light stamping on the outer diameter between the brake surfaces. These usually rust off pretty quickly and can be impossible to read. Sometimes wetting the outer diameter with WD/40 will reveal the numbers.

I've never seen the "new" thickness marked on a rotor.

On old rotors (914 qualifies as old) the difference between new and minimum could be as much as 2 mm. You could have those rotors turned a couple of times.

New modern stuff is built just over minimums (less than .5mm) and is designed to force you to purchase new rotors each brake job.

You should be able to find both the minimum thickness and the new thickness numbers on vendor sites or forums like this one.

My BIGGEST problem with having rotors turned is that very few true machinists are left over and the companies that have brake lathes commonly don't have people who can operate the machinery correctly.
I'd pay them the same amount if they would just let me use their equipment!

I have ONE machine shop that is a 45-minute drive that I will take VALUABLE stuff too.
It's a total of 3 hours of windshield time to get rotors done (2 trips) and they are not cheap anyways (30 per).

Rotors got to cost more than 200 to make this trip worthwhile.

I replace rotors all the time with new ones...

I hear this all the time, and I'm not sure who made it up first, but with very few exceptions, it couldn't be further from the actual truth.

Just about every rotor out there (again, there are a few exceptions) start out life 2mm thicker than the min thickness spec.

A 2018 Porsche 911 GT3 for example, has a 34mm thick rotor with a minimum thickness spec of 32mm.

Thx for the intel. Good to know, Pat.

I'm doing 100+ brake jobs a year at my business.
I've wrenched professionally for the last 4 decades.
This is what I'm seeing on a weekly basis at the sharp pointy end of the stick.

You're calling me a liar, Sir!

I've not had a GT3 in the shop ever. That might be a rotor that would be worth taking to my machinist.

I'm fixing appliance cars, truck, and SUVs.

New rotors are far more common to be only able to machine .010 on each side before being undersized. This is not something that I've read, this is something that I have experienced on a regular basis.

Any damage whatsoever and you have to replace.

I'm a customer of yours Pat.
Calling your customers liars is a poor business decision in my book.

I didn't call anyone a liar, I simply corrected a misconception. Get me a list of 10 vehicles you believe the 0.010" extra rule applies to, and I'll look up and post the specs.

I'm not putting 2 and 2 together on which customer/shop you are....but aside from getting parts from me, I like my customers to walk away with a bit more knowledge than they had before...without having spent 20+ years digging for it like me....another friendly service we provide lol.

What you're likely seeing, admittedly, is the result of abrasive friction material that the european manufacturers like to use, as opposed to adherent friction material used by most of the asian and domestic manufacturers. You get lots of rotor wear with abrasive friction...to the point where in some cases, the rotors are worn out before the pads. On an adherent system...the most notable being 2000ish to 2010ish GM truck, under ideal conditions the rotors will actually measure thicker than when they were new, because the friction transfer layer from the pads has dimension. They get replaced due to to rust, not wear.

Braking is over rated anyways

Just checked my new Zimmerman vented rotor.
The min thickness is stamped on it : 22mm

...And what is the thickness now (new)?

BTW the last time I had drilled rotors (like your new Zimmerman ones) turned, most every shop wouldn't do them. Said it ruins their cutters or something. Seems like a shame to replace the rotors if you're only looking at a very light cut that won't be problematic with the chamfers on the holes.

Maybe! Makes me wonder what I'm missing here. I turn square or octagon stock to round dimensions on my lathe without a 2nd thought. I can't see how the voids would be any different as far asa turning.

Current are 22.5

Odd...what's the part number?

A rotor with any surface breaks will TRASH the carbide tips on a brake lathe.

Drilled or slotted cannot be turned on a standard brake lathe.

All rotors can be Blanchard ground. This is how flywheels are turned and requires a single side to be machined, the rotor is flipped over, and then the other side is machined.

This DEMANDS excellence from the machine shop as both surfaces are not machined at the same time and a TINY deviation in how the rotor is chucked up will vastly affect your rotor run out.

It's also expensive as it amounts to two flywheel machining operations with a rechuck between the jobs that really needs to be verified with a dial indicator.



This is the main reason why I just replace most rotors now:

When you have your normal rotors turned at your flaps ask the guy doing the machining "when is the last time you changed your tips?"

Nowadays the answer is a confused look, and they say "The tips can be changed? I didn't know that."

The proper way to turn a rotor with a standard brake lathe is to use a fast feed speed when you're trueing it. Once you get a complete clean pass on each side you turn the feed speed to its lowest setting and take a final pass at .001 on both sides and pull the trigger.
This last cut takes a half an hour, you cannot see the machine actually feeding, you HAVE to walk away and listen to the hiss/whistle to tell when you are done.
Any change in the tone of this noise needs to be investigated/corrected.
Anti-vibration straps and pads should be employed and when the final cut is complete you take your 90 degree die grinder with a 200+ grit pad and sand the perfect almost mirror smooth finish creating surface finish that looks a lot like a Blanchard ground part.
This omni-directional pattern helps initial bed in like nothing else!

If you ask for this at your flaps you will receive the "deer in headlights" look and MOST will tell you that they don't have any anti-vibration equipment and their machine does NOT have an adjustable feed speed. They will NOT have a die grinder set up for the final surface finish and they will not understand WHY this stuff is necessary.

You take your rotors to a proper machine shop and all this crap happens automatically...

No-one in Granbury does it properly. My machine shop Automotive Machine is a 45-minute drive away from me. They are busy and will not do a rotor while you wait. You drop off in the morning and pick up later that day or the next.

So, for me it's two 1.5 hour trips and 30-35 each for the work.

I replace 98% of the rotors I deal with now.

I'd love to have my own brake lathe but decent ones are 4K+ and all that machine does is rotors and drums. It takes up a "tool box" worth of space in the shop and is not a cost-effective tool for me to own.

Most new rotors (even the cheap shit ones) come Blanchard ground.

Last time I checked with AM on having a set of drilled rotors Blanchard ground it was 110 per rotor. The customer decided to just purchase new drilled rotors at those prices.

These are the guys I use. If they were close to me, I'd have more rotors machined over automatically changing them.

https://www.automotivemachine.com/

Most brake lathes use negative rake cutters which require a lot of tool pressure, which we can get away with without a lot of workpiece deflection in the case of a brake disc because we're cutting both sides at once, so there's no place for the workpiece to deflect...on a plain rotor. Interrupted cuts with negative rake tooling is a challenge.

Positive rake tooling with considerably less tool pressure can get the job done, but nobody is going to get set up with with special tooling just to run X-drilled rotors...not just the inserts, the holders, etc...

Most crossdrilled rotors are so grooved up by the end of the set of pads that cleaning them up will put them below the min spec anyway...

Replacing is the best option for many right now since very few left out there can operate a brake lathe correctly and actually make the rotor better than it was before. Most make it worse.

All this.

I'm old skool and used to work as a parts guy at a VW/PA dealership that cut rotors. The parts department was responsible for cutting the rotors and I was taught all of the above techniques. It worked great and produced nice cuts on, at the time, expensive rotors.

However, today it really makes no sense. Pretty much everything I drive now on the street uses rotors that cost less than $50 each shipped (I like the Centric 120 series for the street, 125 series for the race cars). I think even 914 front rotors are less than $100 each.

When you give it thought, and consider the few number of times in your life you're going to replace those rotors, it just doesn't make economic or time sense to refinish them. Just replace 'em.

And, by the way, drilling and slotting rotors on a small street car doesn't give you jack. Fight me.

GA

The previous posts explain why I see so many of these brake lathes for sale.
https://beta.govdeals.com/search?kWord=Brake%20Lathe

Should rotor thickness be measured by a special tool ?

A micrometer, so you can get past the outer ridge on a used rotor to see if it's worth cutting.

OH, I like the little baby digital Starrett mic.

You don't have to buy an EXPENSIVE baby mic to measure your rotor (but it's stupid nice!)

I'm using normal dial calipers (less than $100 for good ones) to measure rotors.

I keep my Starrett stuff cased and only use them for engine/tranny stuff.

Mine are not digital and some are as old as I am.

My wife found a 36" Starrett straight edge at a Garage sale for $10!!!!

This puppy had a list price over $400 for a simple piece of metal.

It's my "standard" and is awesome for checking stuff BUT, it will not fit in a normal engine compartment! It's limited to ""engine out" only.

Greg is also right about cross drilled/slotted rotors for street duty. Totally brain dead unless you have some type of directed air through the rotor.

Race cars will often use dedicated cooling hoses that are a "consumable" as the hoses take it in the ass during use. They get ripped off all the time.

Porsche uses plastic deflectors/scoops on most of their modern street cars and these work KICK ASS! It's the ticket for a streetcar. They do 50% of the duty of a dedicated hose and almost never get damaged. All of my street builds have scoops/deflectors for at least the front brakes. I DO NOT use the race car cooling hoses on my street cars.

Nice tool. Is there a vernier scale on the back of the shaft? I suppose it wouldn't be necessary with the digital read-out.

? It's right there on the front of the shaft...I suspect two decimal places of inches is more than sufficient for measuring brake rotors...

Or am I misunderstanding?

BTW, if you're looking for a good value in micrometers to toss into a race box (and you're not a 5-decimal-places fab shop) and as long as your rotors are not thicker than 25mm... then this works pretty good for $40. I keep one around.

https://www.amazon.com/gp/product/B0087TMUQ8

Vernier was a French engineer who invented the scale as a means of interpolating between the divisions of the main scales (sleeve & thimble).

Here you go:

https://www.wonkeedonkeetools.co.uk/media/w...M71/CM-71-1.jpg

I’ve been using this:

Oops…

Me too!

Jim, prob is, you can't use calipers if there's a raised ridge on the edge of a used rotor. Which there normally is.

windforfun, I'm not with you. I'm a mechanical engineer, I know what a vernier scale is (and I still have a slide ruler that I know how to use), but isn't that exactly what's on the dial of the handle in the photo I provided in post #22? Never mind, I see what you're talking about. Two digits - three, interpolated - is perfectly fine for measuring rotor thickness (and pretty much everything I do in the garage).

Regardless, to OP: just replace the rotors.

Both of those tools should work fairly well. FWIW, the calipers can't really measure parallelism. And don't store either tool with their "blades" closed.

I only see sleeve & thimble scales. Maybe I'm going blind. Open the link that I previously posted. Cheers.