I have an unmarked y pipe in place of the collector box. It makes the bike more responsive, makes gearbox oil changes better and reduces the baking of the gearbox and stand pivot bearings from the original exhaust collector.
In this exhaust the Guzzi Lambda probes (12mm thread) went into an aluminium adaptor in the exhaust with an 18mm thread.
Recently got a buzzing noise on low throttle over-run and after 800 miles of this one of lambda probes had fallen out - the thread in the adaptor was stripped. Now this failure may have been due to expansion/ contraction and vibration or due to me using the workshop manual torque figures when the aluminium couldn’t take it. Either way if you have this problem a stainless steel 18mm to 12mm adaptor is available on line as a Harley part! Ebay for 18mm to 12mm Lambda adaptors