Bass Drum Os Brass 2" Bass Drum Rings. 2 Per Pack2" Brass Drum Os/Tom Ports (2 Pack)Features of Bass Drum Os Brass 2" Bass Drum Rings. 2 Per Pack When it comes to venting and porting, Drum Os gives you plenty of options Its easy to get creative with Drum Os because there are endless design possibilities. durable plastic rings that encircle the port..