So I have a 2004 a4 with one of the gearboxes made of dog excrement, but this is about something else, under the car today I find a massive 18mm bolt in the engine tray????!!!?!!?!!?? I had a look around but can't seem to find where it belongs, obviously this development is very worrying, can anyone identify this bolt? It looks fairly clean and has a built in washer.

It could be one of the bolts holding the front bumper mounting tubes and the lock carrier, or a service tool (300 mm bolt) used to temporarily support the lock carrier after the mounting tubes have been removed and it is moved forward. See Haynes Chap 11.7 or Bentley Manual.

Edit: You will have to remove the front bumper to verify the above.

 

I would agree with FrenchAvant...that is exactly what it looks like, and worth hanging onto for future use 

Hi, I've just joined the forum, that bolt is a front subframe bolt and I'd be really worried that you're driving around with one missing.There are four of these and they are stretch bolts. Have you had any recent work done, ie sump removal?