[Terry M. Hunt]

Thinking of learning how to use more precision measuring instruments. Will be measuring front rotors soon, so thought this would be a good excercise for an outside mic. The shop manual for my car shows the maximum thickness for the front rotor as “up to 1.085” and a minimum thickness of .965. I see lots of tools for sale with a range of 0 to 1 inch. There are also many for sale with ranges of 1 to 2 inches, 2 to 3 inches, etc. I see very few for sale with a range of 0 to 1.25 inches or O to 1.50 inches. The ones that are available are like $900 to $1200, clearly not practical for a hobbyist like me. Is there something out there with a range of 0 to more than an inch that doesn’t cost a fortune?

[mgarblik]

If your goal is to just measure brake rotors specifically, there are digital caliper style "rotor mics", on Amazon for under $50.00. They measure 0-2" with a single tool. The advantage of a "rotor Mic" is they have one measuring point that is a sharp point and the opposing surface is flat. That way if the rotor is grooved and you want to see if it is worth the time and effort to machine it, you place the sharp point in the groove and get a minimum measurement before you bother to machine it. If you want a digital general measurement tool, a 0-6" digital caliper is much more versatile. They can measure inside, outside and depth and convert from thousandth of an inch to hundredths of a mm with one button push. Decent ones can be had for $30.00-50.00. 0-6" caliper is good enough for brake rotors also but can't measure a grooved one for minimum as mentioned.

[fyrffytr1]

This is the one I use when I worked for O'Reillys. One thing I might add to what mgarbkik said is to measure the rotor near the edge and as far in as you can go. Sometimes rotors wear differently and you might get a safe reading as far in as you can measure but the rotor could be thinner towards the outer edge. If there is a lip on the outer edge that will most likely be the true measurement of the original rotor size as the brake pads don't wear all the way to the edge of the rotor.

https://www.oreillyauto.com/detail/b...crometer&pos=0

[Stuart]

You can sometimes find good quality micrometers at garage sales or swap meets. If you want to buy new, one source is https://www.shars.com As mentioned above, calipers may do what you want to do and they can usually be found for much less money than a micrometer.

[lust4speed]

The other way to look at it is if it is a rotor that was on the vehicle then the maximum size doesn't matter and you are only interested in seeing if you are far enough away from minimum size that the shop can clean up the rotor within the .965" legal limit so the 0 to 1" works fine.

If the rotor is new and you are interested in seeing how thick it is, then a second micrometer with a range of 1" to 2" would be necessary. I only carry a set of calipers with me to the pick-a-parts because I only need a general idea of the size.

[Mr Anonymous]

Definitely hit the garage sales if you live in or near a large city with a decent manufacturing base. The good stuff often gets sold for pennies on the dollar because the family has no idea what the stuff is, or just doesnt care - they're just cleaning out grandpa's stuff as part of the estate. Here around Dayton, its quite common. I'm hoping to find a $50 Bridgeport this way, but nothing yet.....

[dataway]

Very simple .... take a 2" outside mic, mic a piece of something to use as a spacer, say a thick washer etc. Place the spacer on the rotor, mic both the rotor and the spacer at the same time ... then subtract the thickness of the spacer from your reading.

["QUICK-SILVER"]

Minimum thickness, for vented rotors, doesn't count if one side is thicker/thinner than the other.

One side could be safe while the other side is an accident looking for a place to happen.

Clay

[mgarblik]

Originally Posted by "QUICK-SILVER"

Minimum thickness, for vented rotors, doesn't count if one side is thicker/thinner than the other.

One side could be safe while the other side is an accident looking for a place to happen.

Clay

Correct. As a rule of thumb, measure the total thickness in a non-grooved area, then if you have a real rotor mic, measure in the worst grooved area. You have .030" per side to work with and material should be removed equally from both surfaces with a sharp cutter. Minimum thickness is just that, the minimum. You should machine to .030" of the minimum or more, leaving .030" for wear by the final set of pads. Generally, at my shop, we don't machine passenger car and light truck rotors anymore. They are so thin, low quality to begin with, it's more cost effective and makes for a better brake repair to just replace them. Exceptions are very expensive import luxury vehicles where each rotor is specific for the fin cooling angle, and they are very expensive. Lexus SUV rotors are $500-600 a set of 4. Same for BMW, Mercedes and others. Japanese, Korean, and USA vehicles are generally under $45.00 each for premium new rotors. We charge $25.00 each to machine them, so for $60.00 more on average, you get 4 new rotors.

[Terry M. Hunt]

I’ve read pros and cons regarding the need to turn/surface new rotors before installing them. What are your thoughts on this practice? Also read something advising that any turning done should be “non directional”. What is that and how is it avoided?

[RUDOLFSSON]

Look up the brand INSIZE... budget metrology instruments that might suit you..

Mitutoyo if you want more constant precision and a nicer feel... but it'll cost'ya !



Kris...

[mgarblik]

Originally Posted by Terry M. Hunt

I’ve read pros and cons regarding the need to turn/surface new rotors before installing them. What are your thoughts on this practice? Also read something advising that any turning done should be “non directional”. What is that and how is it avoided?

OK, to answer your questions . IF new rotors are worth a damn, they DO NOT need to be surfaced. They DO need to be washed in Hot soapy water to remove oil film and any grinding residue from manufacturer. Spray brake cleaner will work, but almost every manufacturer recommends hot soapy water. New rotors are parallel surface ground, flat and no directional pattern on the surface. When you resurface a rotor on a lathe, it is being cut with a carbide tool bit and no matter how sharp, it puts a tiny directional groove in the surface much like an old LP record album. When you install your new pads, the hard particles, either metal or ceramic can "play" an irritating "song" in the form of brake noise like a needle playing a record. A non directional finish is applied after the lathe machining usually with a rotary die grinder and a scotchbrite pad or sometimes 220-320 sandpaper on a rubber block. The sanding is done while turning in the lathe by hand moving from the outer edge of the rotor inward, back and forth to break up the tiny lines machined in by the lathe. It does reduce noise and improve break-in. I use the process when I machine rotors.