Lug nuts secure the wheels to a car, and are often replaced due to rust, stripped threads or stripped points from improper wrenching with a tire iron. A new lug nut has to fit the lug bolt threading and seat on the wheel hub. There are only a few wheel seat types that are easily identified visually, but the thread specifications must be matched precisely. Measure all specifications before ordering replacement lug nuts to ensure a proper fit on your wheel and lug bolts.
Identify the seat type where the nut fits into the wheel. The seat will be rounded, tapered at a 60-degree angle or offset with a smaller inner cylinder. Nut seats with a straight, 60-degree bevelled edge are called acorn, or tapered seats. Rounded seats are called rounded, or ball seats. Offset seats are known as mag seats, and typically have a washer between the ledge and the wheel seat.
Check the thread size for a lug nut. Refer to the vehicle owner's manual for thread dimensions; measure the width of threads on the lug bolt with measuring tape, or bring the nut to a hardware store and twist it around the thread pitch gauge or compare it to loose bolts with marked sizes.
Measure the thread pitch for the number of threads per inch on standard bolts, or measure the distance between the threads on metric bolts. Use the figure noted in the owner's manual, or determine the pitch from hardware store bolts or pitch gauge.