greetings - i'm new to the forum - have owned this 1990 740 since 1993 (160k)
had a coolant leak - replaced radiator and water pump - ran well - let sit for 2 weeks - tried to start, would turn over but not catch. figured it was a good time to change the plugs - 3 came out ok - managed to mess some threads on the 4th - got a tap - tapped the hole, put sleeve on the plug and installed. Now, even with jumper cables, 1st key position dash lights fine - 2nd (start) and the battery dies instantly and the block will not budge - i'm baffled - where do I look? what do I look for?
Many thanks for any input.

 

I suppose if any metal bits from the tapping got into the cylinder they could have jammed between the piston rings and the cylinder walls, and jammed the block.... ugh.

or your battery is so dead the jumper cables can't supply enough current to crank the motor, I'd try fully charging the battery with a 6-12 amp car charger overnight.