There is a potential for getting drill cuttings into an oil passage when repairing the pulled or stripped cylinder stud on a BMW Airhead using Helicoil, Timesert or Bigsert thread repair inserts. (whew, that sentence takes care of all the keywords for the search engine) Thanks to WireWrkr for sending me a piece cut out of an R65 crankcase, this answered a long-standing question I had. When I did a Bigsert repair of a thread repair I did in the '80's on the left cylinder, upper left cylinder stud, to my surprise I saw what seemed to be an oil passage at the bottom of the drilled hole for the stud. I didn't recall the oil passage on the first repair, but that was "routine", drill to the same depth as the first hole, tap and repair type of operation. And this time I didn't drill any deeper (look at the attached image, you can still see the bottom of the original factory drilling) so I didn't think that I drilled into an oil passage. I figured that it might have been blind drill hole for the attaching bolts if the timing case cover. Nonetheless, I devised a vacuum device to suction oil from that oil passage into the drilled out stud and it came back with only a few small flakes of swarf. I packed the bottom of the drilling with a thick grease, tapped to hole and finished the repair. I asked online if there was an oil passage in that area, but didn't get an answer so I assumed that there was not a problem. But the crankcase cutoff answered my question immediately. There is a diagonal oil passage that comes up from the bore of the front main beariing cap and intersects a horizontal oil passage that feeds to two top cylinder studs that feed oil to the rocker arms. This oil passage is right at the bottom of the upper left cylinder stud thread, as designed and as it comes from the factory. Shown below is an annotated photo of a crankcase cutout from an R65 of the left cylinder in front of the oil dipstick. Refer to the oilsketch.jpg image for more info. Yellow-- oil supply passage from the front main bearing cap to the oil-supply studs for the rocker arm oil suply passages in Green. The area of concern is shown with the Red-Yellow splat near the top left cylinder stud. Red-- oil supply passage from the front main bearing cap to the rear main bearing and the oil pressure switch. Oil is supplied to the front main bearing cap from the oil filter and oil pump. Like I said, this repair is done frequently and you don't hear of a this oil passage or of this problem, but nonetheless it horrifies me as that you are drilling near an oil passage pointing downward to the front main bearing and generating a lot of drill and tap cuttings. I see the potential for scoring the mail bearing insert and/or crank journal. I would recomend that when this upper left, left cylinder stud is repaired that the bottom of the hole be packed with a thick grease to preclude any possibility of drill cuttings entering that diagonal drillway and making sure that the grease is removed from the passage before completing the repair. If you remove the oil pressure switch you can blow out this branch of the oil passage with compressed air or "backflush" it by pressurizing it with oil.