I drilled out a 12mm bung and tapped it for 18mm
On my 2011 GSX-R750, I successfully drilled out the stock 12mm O2 sensor bung--while still mounted on the bike (didn't even loosen header)--and tapped it for 18mm x1.5 thread pitch. First, I removed the stock sensor and put a 5/8" dowel 6 inches long into the hole to simulate the drill bit and see exactly where "straight" is. I could then use a corner of the crankcase as a landmark. (Note: If I had loosened the header, this marker wouldn't mean anything.) I then drilled it with a 16.5mm bit with a 1/2" chuck. I had to get a drill with a 1/2" chuck (most drills are 3/8") and a 90 degree elbow to drill around corners. Power Commanders require you leave the stock O2 sensor in place and drill a new bung hole and Bazzaz requires that it no longer be connected, so drilling the stock bung only works if you're going with Bazzaz.
Last edited by Superbike Racer; 12-19-2018 at 09:34 AM.