Engine cover bolts...confusion!!!
 

Hi! My so-well-engineered 1994 e320 wagon's 104 engine is throwing me a curve! The engine cover -- by which I mean the cover for the well in which the spark plugs and coil elements live -- is totally random when it comes to screwing in the allen bolts which are supposed to secure it! Only one or two of the bolts can be screwed tight. The cover appears to be aluminum and the bolts (60 mm long ca 6 mm diameter) have very shallow threads! Even a very well-equipped machine shop supply house locally cannot tell me what thread was used. I want to tighten the cover to spec. but nothing aligns. Others must surely have had this problem. What gives?