Are you sure they are monel? This seems a very unlikely material to have been used.
To quote Wikipedia "It is a very expensive alloy, with cost ranging from 5 to 10 times the cost of copper and nickel, hence its use is limited to those applications where it cannot be replaced with cheaper alternatives."
It seems to me very unlikely that it would have been used by Rover. I suggest that they are almost certainly aluminium alloy rivets that originated in the aviation industry. It is even possible that I have some - I have a box of disposals rivets of a variety of types that I picked up at a clearing sale years ago, all still in sealed packages.
Monel would have been a very unsuitable material as well because it would result in electrolytic corrosion of all the other metals it was in contact with - birmabright, steel, and galvanising.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
Bookmarks