That 'thing' is what we call a dowel pin. They're tapered and go in from one side, usually the same direction as the adjacent bolts. You have to tap on the other (small) end to get them out. The A4 engine is basically 3 castings. The dowel pins maintain alignment critical stuff between the three pieces. The block is fit, doweled, and bolted to the transmission. That assembly gets cut smooth and fit to the oil pan, again doweled. Makes it tough to change one of the 3 parts, as the dowel holes don't line up and have to be recut or enlarged to the next size. Bolts generally have enough clearance to go in fine.
The corrosion on the side of your block came from the bolts holding the water jacket cover plate. Bolts have a clearance at the root, crest, and on the flanks of the threads - they will NOT make a seal. The fix for those bolts is to install studs into the block (sealed with epoxy) then use nuts to hold the cover plate.
Didn't see the second photo.
Are you currently heading toward repair or replacement? I know of an A4 that has been rebuilt but never run.
The corrosion on the side of your block came from the bolts holding the water jacket cover plate. Bolts have a clearance at the root, crest, and on the flanks of the threads - they will NOT make a seal. The fix for those bolts is to install studs into the block (sealed with epoxy) then use nuts to hold the cover plate.
Didn't see the second photo.
Are you currently heading toward repair or replacement? I know of an A4 that has been rebuilt but never run.
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