I was working on the motor for a couple hours this am and took the opportunity to measure the bolts...the knock sensor bolts were a perfect fit, so the flange takes an M8 and the hole in the block is 25 MM deep. I'll measure the flange depth when it comes in and buy an appropriate length. I suspect it will end up being an M8x25, or maybe even a 30 depending on the flange and washer depth.

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Current
Original owner 2002 E53 X5 4.4i to 4.6i swap
1993 Mopar 318 Jeep Grand Cherokee - original owner
2015 V6 Jeep Grand Cherokee - the nice car for my wife
Former
1972 Audi Fox
1986 Saab 900S
1996 BMW Z3
1998 BMW E36 M3 Sedan
2004 BMW E46 M3
2006 Audi A3 Quatro