So on my 02 i35 the rear section of my exhaust/ the muffler pipe bolt on that connects to the mid pipe/ resinator has come apart right at that connection right behind the rear pass wheel. I was wanting to fix myself by just inserting a small diameter pipe inside. But i dont know what size that pipe is or what size i should buy to insert.

 

I have the same issue, my flange broke and is missing. I'm taking it in to an exhaust shop soon to get it fixed.

 

you may buy adjustable flange x 2 + regular flange @oreilly auto parts, then bolt them together.

2.25" is common, dont know if it is ID or OD (inside/outside diameter). But if yours is off, you can easily measure it to confirm.

 

That's what I did last year with my rusting Walker setup. I wished I did it when the same thing happened to my OEM exhaust. I would have saved myself some money.