Customs at ORD and domestic flight
 

British Airways flight from London Heathrow scheduled to land at Chicago O'Hare at 2:20pm on a Saturday. (terminal 5)

United flight from ORD to BNA leaves at 4pm that afternoon. (don't know what terminal)

I and my travel companion will have NO checked luggage. Just carryons.

Will an hour and 40 minutes be enough time to get through customs and change terminals to make the domestic flight?

You'll go through passport control and customs and then board a train to Terminal 1, where United's flights depart. It stops first at Terminals 3 and 2. You will have to go through security again when you arrive at Terminal 1. I assume you'll have been given your ORD-BNA boarding pass when you check in at LHR.

<i>Will an hour and 40 minutes be enough time to get through customs and change terminals to make the domestic flight?</i>

Some of the time yes, and some of the time no. What more info did you want?

Oh, by the way, that BA flight to Chicago is more than 15 mins late nearly half the time, according to the on-time statistics that the airlines provide to the FAA.

If both flights are on a single itinerary, then you're ok because United has a later (6:15-ish) flight to Nashville. But if they're separate tickets, United has no obligation to put you on the later flight and, ha-ha!, you could be out of luck!

The Chicago-Nashville flights are United Express flights operated by one of several regional carriers. Some of them leave from the F concourse of Terminal 2 (for example, four flights today leave from T2 and one leaves from T1). Check the United monitor as you are exiting the customs hall in T5 to find your gate info. Sometimes the security lines are shorter in T2.

If you don't already have your boarding pass, see if you can get it from the United transfer desk just past customs in T5, before you get on the train. Otherwise, United's main check-in facility is in T1. The website does say there are check-in machines in the T2 lobby, but I have never checked in there and can't verify.

I've only done this connection once, and we were able to do it in 90 minutes, but it was tight (and we also had only carry-on luggage). I found immigration a little slow, but we arrived in the evening.

The problem is that most of the flights from Europe arrive at ORD in the afternoon. If your flight is bunched in with others. it could mean long lines. There's no way to tell.

That 100 minutes is shorter a connection than I usually like in such circumstances, but that's me. It's also a drag to be tired and jet-lagged after a long flight from Europe and have to wait four hours for your next flight too. I've done that.

There's no ideal answer to this.

I've done this connection several times but my ongoing flight is to Evansville, Indiana even though we live in Kentucky. You should already have your boarding pass that will be issued at LHR.I try to always allow 2 hours but I usually have checked luggage that I must claim and than recheck.It's quick though. In 7 years I only missed my flight 1 time when my flight from LHR left 5 hours late.I had missed the last flight to Evansville. They put me up in the Hyatt at O'Hare and I got a flight out the next day after having a nice breakfast with my son who lives in Chicago.The train between terminals is very quick and although there is no time to dawdle if your flight is on time you should be fine.That is a big IF!!!!

Not only do flights from Europe arrive around that time in the afternoon at OHare, but so do 747s from Asia. Immigration can take a bit of time. Good for you that you do not have to stop for baggage collection before rushing on to your next flight. As others have said, watch the monitor to see if you are leaving from terminal 1 or terminal 2 (although the two are side-by-side, but it would take a little hike if you happened to exit the train at the wrong one)