[BADSUV]

I measured my turbo exhaust pipe and came up with 4-1/4in,would this be considered 4in or 4-1/2 inch when ordered new?Thanks

 

[RAACCR]

Pipe is measured on its ID (inner diameter).
Tubing is measured on its OD (outer diameter).

Most exhaust systems use tubing, which is measured on its OD. How are you taking your measurement?

 

[TubingJon]

Most of the small frame GT47 stuff or twins will step down to a 4" while the big frame stuff steps up to a single 5".

 

[XJHEAD]

Pipe is measured on its ID (inner diameter).

Not always.

 

[twistedstang]

^ true. Pipe is a nominal measurement. The id changes per the schedule (wall thickness) and the od stays the same so the same fittings can be used.
6 inch pipe measures around 6 3/4 o.d. sch 10 has a thinner wall than sch 40 therefore had a larger id.

tubing can go either way- take copper for example. 2 inch copper tubing is actually 2 1/8 od and 2 id. You have to specify "od" on tubing to get an od measurement.