MB engine help needed

["Ian J Haseltine" <Jim_Haseltine@classic.msn.com>]

OK, it fires and runs but - what the h*ll screws into the back of the head?

There is a threaded hole below and to the left of the temperature sender. It 
appears to be about 12 or 13mm diameter and as I have found goes though to the 
coolant passages. What is supposed to go there? I dont recall seeing it on my 
old head and I cant find anything that should go there - in fact to my 
amazement I havent got any bits left over.

Once again - help!

Jim Haseltine
88 Ur quattro (running but incontinent)