does anybody know the stock exhaust diameter on a 98 SE?

A tape measure and basic geometry will get you that figure.

http://www.geocities.com/n_dahi/

click on custom exhaust

Slartibartfast-
What is your problem? I really hate people that just go around the forum trying to start a fire. You don't even have anything to do with the Sentra SE forum, yet you come in here and all you do is try to start a war. Why do people do this? If a person has a question, they want an answer, not some remark from a smartass. If you are going to post, make it useful and answer the persons question, and if you don't have an answer, then don't post. It really can't get anymore simple.
If I could, I would be out there under my car measuring it way before I would come in here to take a chance on dealing with people like you. The reason why I can't do this right now is because my car is being repaired, it was involved in an accident.
The "search button partol" people really piss me off, all they do is surf the forum looking for some innocent question to unload on. And they don't even give them an answer, let alone a link to an answer. That being said...
Jacen-thank you very much for your help, that is exactly what I needed to know. I apologize if I offended anyone besides those I intended to, listed above.

Originally posted by B_baker03
Slartibartfast-
What is your problem? I really hate people that just go around the forum trying to start a fire. You don't even blah, blah, blah........

B_baker03, knock it off. You have your answer.

If you are going to post, make it useful and answer the persons question
I thought I did post a useful answer. To review, circumference = pi * d, where d = diameter. Measure the circumference of the exhaust pipe, that's the circumference. Divide circumference by pi (3.141 is close enough) and diamter is the result. Armed with this information, you can proactively obtain your answer instead of reactively waiting upon the largesse of a forum member, who may be a monkey and give you the wrong number.

I'm sorry your car is damaged. Had I known you did not have the car in your posession, I would not have answered. Feel better?