are exhaust pipe diameters i.d. or o.d. for sizes. need to change my cat and don't want to order wrong size

 

ID (In Diamater) is the size of the pipe that's going into the cat, OD (Out Diameter) is the size of the pipe after that cat.

 

Hi,

ID is the inside diameter of the pipe, not the size of the pipe going into the converter. OD is the outside diameter. Fortunately, it does not matter much which you use. If the size of the cat and the pipe say the same thing, don't worry about ID or OD. You can bump up a pipe to make up for a ID/OD difference with a exhaust pipe expander.

 

All pipes are measured in O.D, it you order a 3" magnaflow cat, 3" pipe will slide into the sleeves on that cat.