I'm sure this is common knowledge but I'm not sure what is the diameter of my factory exhaust on my 1997 F-150 with a 5.4L? My passenger side rear catalytic converter is rattling and I want to replace it so I am going to buy a generic replacement cat but I'm not sure if I need 2 1/4" or 2 1/2". Any help on what size it is or where to get a replacement cat would be appreciated.

 

best measure it

My 99 had 2 1/4"
my sons 03 has 2 1/2"

I believe the tube diameter changes from cat area to muffler too.
best to measure it.

measure with one of these
6 in. Long Reach Digital Caliper


6 in. Digital Caliper with SAE and Metric Fractional Readings

 

The only ones I've seen for these trucks have been 2.5". I just replaced the driver's side cats on my 4.2 and those were 2.5" as well.

Of course that's just anecdotal, Guyina has the right idea.