Optical. It is not as deep as where the red hole is. Recall from an earlier measurement of mine, the 13/64" hole is about 7/8" in deeper than where the 1/2" bit bottomed out. Being that drill bits are conical at the tip and a 1/2" bit is relatively large, the red hole is probably 1" deeper than the deepest part of the large gaping hole.

Hope that make sense.

__________________
Mike Mass
Freehold, NJ

1999 E300 TD 178,000 miles
2007 E320 Bluetec - Sold

1998 E300 TD - R.I.P.