i am buying a used downpipe and i just ordered a gasket for it ,does a o2 sensor plug into the down pipe?

 

Yes and use penetrant to losen nuts/bolts/studs and then on re install antiseize everywhere.

 

There should be a threaded opening (a bung) somewhere on the side of the downpipe where the O2 sensor will screw in. It is usually is toward the engine end of the downpipe. Don't use much anti-seize on the O2 sensor threads, and don't get any on the sensor tip itself.

I re-used the original metal gasket (between the DP and turbo outlet) without any problems, but got a new gasket for the DP to cat flange.

 

Just be really careful with those studs, like Les said use a TON of penetrant to loosen everything up. You don't really want to break those studs!

 

it's a good time to install a new O2 sensor if it's still the original.

 

Lightly sand off the old gasket and manifold face. Also torque to proper specs - overtorquing will break studs and tend to cause leaks over time. And don't final tighten anything at either end of the pipe until they are all threaded into place.

Dave