We might be over-simplyfying the issue here.
The first one is composition; that is, most all metals used are some form of alloy. If you really got technical, I expect you would find that the aluminum in beer cans isn't quite exactly the same as the aluminum in bicycle frames. This is relevant, in that there WAS a change in the alloy used for aluminum wire around 1978.
The second relates to the connection method. Even in 'the bad old days,' I'm not aware of aluminum connections under lugs being the issue. Rather, I seem to recall the problems arising in connections in the following order:
- 'back stabbed 'spring loaded' types of devices;
- Conventional 'wrap it around the screw' connections,
- inside wire nuts (wire breaks)
Let's face it; in a conventional lug, that wire isn't going anywhere, no matter how it wants to expand and contract. The picture changes when the wire can 'squeeze out' the side of a clamp, or away from a screw head.
Naturally, out in the field we're not going to be performing a complete engineering study for every wire connection. Instead, we tend to 'keep it simple;' and one of the tried & true rules is 'stay away from aluminum wire!'
