Glossary
27 terms that turn up in these articles and on repair estimates, defined plainly and grouped by system. Every one links to the article that goes further.
Brakes
Most of these come up when a shop is explaining why a brake job costs what it costs.
Bedding inalso break-in, burnishing
A deliberate series of firm stops that lays an even film of pad material onto the disc. Done properly it is what makes brakes quiet and consistent; skipped, the film lands unevenly and produces the pulse people call warping.
Where this comes up: Warped Brake Rotors: One Word, Three Faults, Three Bills · Burning Smell After Driving: What It Costs, When to Stop
Blanchard grinding
A surface grinding process that can remove the hard spots an ordinary brake lathe cuts around. It's the only way to save a disc that has gone to cementite — and by the time you need it, a new disc almost always costs less than finding a shop that offers it.
Where this comes up: Warped Brake Rotors: One Word, Three Faults, Three Bills
Cementite
A hard, abrasive form of iron carbide that cast iron transforms into above roughly 650 °C. It forms under a hot spot on the disc, and it can't be polished, driven or machined out — a lathe cuts around it. A disc with cementite is finished.
Where this comes up: Warped Brake Rotors: One Word, Three Faults, Three Bills
Disc thickness variationalso DTV
A brake disc that is slightly thicker in some places around its circumference than others. Each thick patch passing between the pads pushes them apart, and that movement travels back up the brake fluid as a pulse in the pedal. It's what almost every complaint of a warped rotor turns out to be.
Where this comes up: Warped Brake Rotors: One Word, Three Faults, Three Bills · Resurface or Replace Brake Rotors? Measure First · Steering Wheel Shakes When Braking: 5 Real Causes, Cheapest First
Discard thickness
The dimension at which a disc must come off and be scrapped, machined or not. It sits only about 0.015 in. below the machine-to figure, so the working margin between can be cut and must be replaced is very small.
Where this comes up: Resurface or Replace Brake Rotors? Measure First
Hub face
The flat machined surface the brake disc bolts against. It corrodes on any car that has seen winters, and a disc fitted over that corrosion sits very slightly crooked. Whether a shop cleans it is the single thing that decides if a brake job lasts or comes back.
Where this comes up: Warped Brake Rotors: One Word, Three Faults, Three Bills · Brake Pad and Rotor Replacement Cost: Dealer vs Independent Shop · Steering Wheel Shakes When Braking: 5 Real Causes, Cheapest First
Lateral runoutalso LRO
Side-to-side wobble of the disc as it turns, measured with a dial indicator on a fitted disc. The limit on most vehicles is 0.002 in. Runout doesn't cause a pulse directly — it causes the pads to wear one band of the disc thinner, which then does.
Where this comes up: Warped Brake Rotors: One Word, Three Faults, Three Bills
Minimum thicknessalso machine-to
The thinnest a disc may be machined to and still go back on the car. It's cast or stamped into the disc itself, usually near the hat or the outer edge. A disc already below it can't be resurfaced at any price.
Where this comes up: Resurface or Replace Brake Rotors? Measure First · Brake Pad and Rotor Replacement Cost: Dealer vs Independent Shop · Steering Wheel Shakes When Braking: 5 Real Causes, Cheapest First · Warped Brake Rotors: One Word, Three Faults, Three Bills
Rotor micrometer
A micrometer with one flat anvil and one pointed anvil. The point reaches into the grooves worn by the pads; ordinary flat jaws bridge across them and report a thickness the disc doesn't have anywhere.
Where this comes up: Resurface or Replace Brake Rotors? Measure First
Transfer layer
The thin film of pad material that normally coats a brake disc. Brakes are designed to work through it, so it's not contamination. The trouble starts only when it's laid down unevenly.
Where this comes up: Warped Brake Rotors: One Word, Three Faults, Three Bills · Resurface or Replace Brake Rotors? Measure First · Steering Wheel Shakes When Braking: 5 Real Causes, Cheapest First
Oil and lubrication
The specifications printed on a bottle, and what they actually commit the oil to.
API service categoryalso SP, SN Plus
The letter code in the round API mark showing which performance standard the oil meets. Later letters supersede earlier ones, so an engine asking for an older category is generally satisfied by a newer oil — the reverse isn't true.
Where this comes up: How to Read an Oil Bottle: API, ILSAC and OEM Approvals Explained · 0W-20 vs 5W-30: What Actually Happens If You Use the Wrong One · Can You Mix Synthetic and Conventional Oil? · Synthetic vs Conventional Oil: What Actually Differs · and 2 other articles
Base oil group
API's five-way classification of the stock an oil is built from. Groups I and II are refined crude; Group III is heavily processed crude that is still sold as synthetic in the US; Group IV is built molecule by molecule. Two bottles both marked synthetic can be different groups.
Where this comes up: Synthetic vs Conventional Oil: What Actually Differs · Can You Mix Synthetic and Conventional Oil? · How to Read an Oil Bottle: API, ILSAC and OEM Approvals Explained
dexos
General Motors' own oil specification, stricter than the API category on several counts including volatility. A manual that asks for dexos has excluded conventional oil without using the word synthetic anywhere.
Where this comes up: Synthetic vs Conventional Oil: What Actually Differs · 0W-20 vs 5W-30: What Actually Happens If You Use the Wrong One · Can You Mix Synthetic and Conventional Oil? · How to Read an Oil Bottle: API, ILSAC and OEM Approvals Explained
HTHS viscosity
High-temperature high-shear viscosity: how thick the oil still is at 150 °C while being sheared between moving parts. It's the number that describes the film actually protecting a bearing, which is why thin grades are defined by it.
Where this comes up: What Can You Use Instead of 0W-20? · 0W-20 vs 5W-30: What Actually Happens If You Use the Wrong One · How to Read an Oil Bottle: API, ILSAC and OEM Approvals Explained · What Is Oil Dilution?
ILSAC GF-6
The starburst standard covering fuel economy and engine protection for gasoline engines. GF-6A replaces earlier grades directly; GF-6B exists only for 0W-16 and isn't interchangeable with anything else.
Where this comes up: How to Read an Oil Bottle: API, ILSAC and OEM Approvals Explained · 0W-20 vs 5W-30: What Actually Happens If You Use the Wrong One · Can You Mix Synthetic and Conventional Oil? · Synthetic vs Conventional Oil: What Actually Differs · and 1 other article
NOACK volatility
How much oil evaporates in an hour at 250 °C, as a percentage of weight. It's the mechanism behind most cars that seem to drink oil between changes, and the specification that quietly rules conventional oil out of many modern engines.
Where this comes up: Synthetic vs Conventional Oil: What Actually Differs
Oil dilution
Unburned fuel washing past the rings and collecting in the oil pan, thinning the oil and raising the level on the dipstick. Short cold trips are the classic cause. Laboratories generally treat about 4% fuel in oil as excessive.
Where this comes up: What Is Oil Dilution? · Can You Drive With White Smoke From the Exhaust? · Do High-Mileage Cars Burn Oil? What the Makers Actually Allow · Ford 1.0 EcoBoost Wet Belt: What the Recall and Investigation Actually Cover · and 5 other articles
Cooling
Coolant chemistry, which is the part of the system people get wrong most often.
HOAT coolant
Hybrid Organic Acid Technology: organic inhibitors with silicates or phosphates added, common in European and some Asian vehicles. Mixing it with a straight OAT product is where most coolant trouble starts.
Where this comes up: Can You Mix Coolant Colors? A Straight Answer
IAT coolant
Inorganic Acid Technology: the older green coolant with silicate inhibitors that deplete in about two years. Still correct for older vehicles, and wrong for almost anything modern.
Where this comes up: Can You Mix Coolant Colors? A Straight Answer
OAT coolant
Organic Acid Technology: long-life coolant using organic inhibitors, typically good for five years or more. Color tells you nothing reliable about which family is in the bottle — two makers use the same dye for different chemistry.
Where this comes up: Can You Mix Coolant Colors? A Straight Answer
Transmission
Fluid, service intervals and the vocabulary used to sell both.
Flush versus drain and fill
A drain and fill replaces the fluid the pan holds, usually a third to a half. A flush pumps fluid through the whole circuit and exchanges nearly all of it. They're different jobs at different prices and are frequently sold as if they were the same one.
Where this comes up: Transmission Flush: Needed, Upsell, or Actual Risk · Services Shops Recommend That Your Car Doesn't Need
Engine and belts
Drive belts, chains, and the failure paths that turn a cheap part into an expensive one.
Interference engine
An engine in which the valves and pistons occupy the same space at different moments. If the timing drive fails they meet, and a belt job becomes an engine rebuild. Most modern engines are of this type.
Where this comes up: Timing Belt Replacement Cost in 2026: What You Should Actually Pay · Wet Belt Engines: What They Are and What Happens When One Fails · Wet Belt vs Timing Chain vs Dry Belt: How to Tell What You Have
Oil pick-up screen
The mesh strainer guarding the oil pump inlet. It's what catches belt debris in a wet belt engine, and once it blinds over the oil pressure falls — which is how a worn belt becomes a bearing failure.
Where this comes up: Wet Belt Engines: What They Are and What Happens When One Fails · Ford 1.0 EcoBoost Wet Belt: What the Recall and Investigation Actually Cover · Timing Belt Replacement Cost in 2026: What You Should Actually Pay · Wet Belt vs Timing Chain vs Dry Belt: How to Tell What You Have
Wet belt
A toothed drive belt that runs submerged in engine oil rather than in dry air. It's quieter and more compact than a chain, and it sheds material into the oil for its entire life, which is where the failures come from.
Where this comes up: Wet Belt Engines: What They Are and What Happens When One Fails · 100k Mile Maintenance Cost: What Actually Starts Costing Money · Can You Mix Synthetic and Conventional Oil? · Ford 1.0 EcoBoost Wet Belt: What the Recall and Investigation Actually Cover · and 9 other articles
Shop and money
What the words on an estimate mean, and which of them are negotiable.
Book timealso flat rate
The published number of hours a repair is deemed to take, from an industry labor guide. Charging book time is normal and not a trick — but it's the line worth asking about when two quotes for the same job differ.
Where this comes up: How to Read a Repair Estimate Without Getting Overcharged · Brake Pad and Rotor Replacement Cost: Dealer vs Independent Shop · Steering Wheel Shakes When Braking: 5 Real Causes, Cheapest First · Timing Belt Replacement Cost in 2026: What You Should Actually Pay · and 1 other article
Goodwill
A manufacturer paying for a repair it's not contractually obliged to cover, usually to limit reputational damage from a known fault. Terms are discretionary and usually capped by age and mileage, so what you get depends partly on asking.
Where this comes up: PureTech 1.2 Wet Belt: What Stellantis Now Covers, and Which Years to Avoid · Brake Pad and Rotor Replacement Cost: Dealer vs Independent Shop · Ford 1.0 EcoBoost Wet Belt: What the Recall and Investigation Actually Cover · The Right Oil for a Wet Belt Engine — and Why the Wrong One Kills It
OE versus OEM
Nearly identical words for two different prices. An OEM part comes in the carmaker's box; an OE part is the very same component from the supplier who made it for them, in that supplier's own box, and usually costs less. Most people read the two as synonyms and pay the difference.
Where this comes up: How to Read a Repair Estimate Without Getting Overcharged · Can You Drive With White Smoke From the Exhaust? · Can You Mix Synthetic and Conventional Oil? · How to Read an Oil Bottle: API, ILSAC and OEM Approvals Explained · and 2 other articles