My muffler fell off, so I decided to try to replace it with a straight pipe, similar to the Mufflerectomy by WannaWagon. Tried to use the 2.5" pipe suggested by the thread, felt too big and didn't form a tight seal when placed over the old pipe on the resonator. Went back to the store, and bought 2.25" ID sections instead. And...you guessed it, those seemed to small.

So, what is the right size? Maybe 2-3/8"? Anyone know? Or is there a way to measure the end of the resonator to ensure that i'm trying to fit on the correct size?

It is probably Metric sized, I seem to recall 70 mm OD, anyhow if you have a pipe stretcher it fits inside the smaller pipe and you put a wrench on it and it expands the pipe to fit, muffler shops use them.
Another iddea, if you have pipe that is too large you can slit it cross-ways on the end using a hacksaw. wrap some aluminum foil around the smaller pipe to make gasket then slide the pipes together and use a hose clamp, a T-bar clamp would be best, to hold it together.
Good luck!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!