For Cadillacs, It’s Fine to Skip Rotor Turning

Question: I recently took my 1998 Cadillac Deville in for a brake job. The dealer charged me for pads but not for turning the rotors. The service advisor said it wasn’t needed. I’ve never had a brake job in which rotors were not turned. Was I given a substandard brake job?

E.E.

Answer: Not necessarily. It is common for most repair shops to turn rotors, which means that the iron surfaces are ground smooth. But General Motors Corp. has issued a service bulletin for Cadillacs that specifically instructs mechanics not to turn rotors unless there is scoring or gouges that are deeper than 60 thousandths of an inch.

Indeed, turning a rotor when it is not in bad condition is a waste of money and will reduce the life of the rotor.

After a long period of wear and after a number of turnings, a rotor can become so thin that it does not meet the manufacturer’s specifications for structural soundness. As long as the overall rotor does not vary by more than 1 thousandth of an inch in thickness, it does not need to be machined smooth, according to the service bulletin as reported by Alldata, a publisher of automotive information.

At the same time, Cadillac has recognized that some of its brakes have a problem with excessive “grunt, groan, grind, crunch and squeal,” apparently related to a disc that does not have enough thickness in certain areas.

If you have a problem with noisy brakes, the recommended repairs, as outlined by another technical service bulletin, call for new pads and more substantial rotors.

Premium Gasoline Not Necessarily Required

Q: I just bought a Toyota Tundra pickup and am wondering about the correct gasoline to use. Should I follow Toyota’s recommendation for 87 octane-rated fuel or go up to a premium fuel? Didn’t all grades of gasoline used to be higher octane than now?

F.R.

A: As you suspect, not all premium lives up to its name, and octane ratings have been sliding downhill for a number of years. Premium octane in California used to be 92, but it slipped a notch to 91 during the last year.

Octane is a measure of gasoline’s ability to resist uncontrolled combustion, which causes engine knock.

When a cylinder is in its compression stroke, the gasoline is put under tremendous heat and pressure. If the octane rating is too low, it will begin to explode before the spark plug detonates the fuel-and-air mixture.

If the gasoline does detonate, typically when you hit the accelerator hard and the engine is under tremendous load, you can hear pinging from under the hood. That’s the sound of the explosion ramming the piston and cylinder walls at the wrong time--when the piston still is in the early part of its compression stroke.

Mild knock or ping isn’t anything to worry about, but a hard knock can cause severe engine damage. In some cases, it can knock a hole right through the piston, damage the rings or blow a head gasket.

So gasoline refiners boost the octane of their fuel to retard knock. Gasoline has a natural octane rating in the mid- to upper 80s, but depending on how it’s refined, conventional automotive gasoline’s octane ratings can be lifted all the way to the mid-90s.

The term “octane” refers to the component of gasoline, made of chains of eight carbon atoms, that best resists spontaneous combustion. If you were to put propane in your engine and compress it the same way you do gasoline, it would knock far more.

For decades, octane ratings were boosted by adding lead to gasoline, but that was discontinued in the early 1980s as more became known about lead’s health hazards.

Octane now is boosted using alcohol, though that’s expensive and has helped to contribute to high gasoline prices. At the same time, auto makers have redesigned engines to require lower-octane gasoline.

If your Toyota does not ping or knock during most of the driving you do with 87-octane gasoline, there is no reason to jump to a higher-octane fuel. It would not provide better gasoline mileage, though the premium fuel may contain a better additive package that will keep the fuel system clean.

It’s still possible to buy gas with higher octane. Most brands have a middle-octane fuel of 89. In addition, Unocal still sells a 100-octane fuel at many locations for high-performance vehicles.

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Ralph Vartabedian cannot answer mail personally. Write to Your Wheels, Business Section, Los Angeles Times, 202 W. 1st St., Los Angeles, CA 90012. E-mail: ralph.vartabedian@latimes.com.