What are the round parts attached to the Y-pipe just ahead of the O2 sensors? And when I replace the Y-pipe, do I need to reuse them or can they be bought new?

 

The "Y" pipe is post-after the O2 sensors.
https://www.lexussouthatlantaparts.com/showAssembly.aspx?ukey_assembly=758453&ukey_make=1082&ukey_model=15669&modelYear=1999&ukey_category=21750

https://www.clublexus.com/forums/ls-...ml#post5380363

Those replacing their system usually don't include these. I'm not aware of their exact function other than I suspect they are a damper of some sort for resonance-frequency tuning.

 

I bought new gaskets the first time I took my y-pipe off. I had a leak causing a P0420 and P0430 from both banks. Ultimately, I removed the y-pipe and ground down the flange ends and (reusing the old gaskets) used liberal amounts of Permatex Ultra Copper on both side of the gasket. That ultimately fixed the leak I'd been having in my flanges.

 

They help pull exhaust gas out of engine by opposite pulsing from the other side when plugs fire.Not sure that explains it well but for sure an old hotrodder mod to gain power.