Your bag will be checked through to PIT at LHR. You will arrive at Terminal E. You will clear immigration, collect your checked bag, and clear customs. You are then legally in the U.S. The first thing you do is re-check your bag at a clearly marked desk right outside.

You then exit Terminal E and cross most of the airport to Terminal B, which AA uses. There is a shuttle bus, but since it goes in the wrong direction (counterclockwise, the longer way between those two) and you have no mobility issues, you don't want to use it. Walk out the front door, turn left, and go past Terminal C to Terminal B. Check the signs in B to make sure you go to the AA side: Terminal B is effectively two terminals separated by a parking garage. Clear security and board there as you would at any other U.S. airport for any other domestic flight.

If the weather is bad, an alternate route is to take the escalator up to the parking garage walkway in Terminal E and walk to Terminal B through the central parking garage. It's not much longer.

I would personally feel comfortable with 1:40 for the whole process, especially since LHR-BOS schedules are padded so you're likely to arrive early. If you encounter unusual headwinds (the usual ones are taken into account) the padding may go away, and if there are delays at Heathrow in addition, you might be late - so have a Plan B in mind, though you probably won't have to use it.

My error. Was pre coffee where I am and didn't think and misread the post. Edited for OP'S sake.

Should add that may be on a different carrier than you originally booked. Although your original carrier may not have any later flights, don't be shy about asking to be rerouted on a competitor that has flights the same day if you arrive late.

One should always have a Plan B in mind. But I took a look at the BOS-PIT schedules on an arbitrarily chosen day in mid-November (as OP did not specify date of travel, I don't think). The last American Eagle nonstop of the day departs at 740pm, and so far as I can tell the only later same-day connecting service departs only 20 minutes later, at 800p, and goes BOS-DCA-PIT. So that's not much leeway. In the event of a BA delay of 30 minutes or more, I would expect to stay over at BOS that night.

The good news is, there are at least three acceptable hotels right on Logan Airport grounds (Hyatt, Hilton, Embassy Suites), and if OP's flights are on a single PNR and he/she misses the connection, AA will provide lodging and an onward seat in the morning. So, not that big a worry.

Thank you all so much for your help, I'm flying on 20th November if this helps, I'll also be taking one checked bag and a carry on. I hope this helps answer any questions I was asked. Thanks again everyone, this is my first time flying and you've all helped a great deal

Right, so it looks as if you're on the BA203 arriving at 605pm, and intending to connect to the onward AA4588 at 740pm.

It's not an incredibly busy time to transit BOS customs, and your plane is a 777 which brings fewer passengers to the bottleneck than a 747 might do, and as it's mid-November you might hope for a low load and speedy transit. If you're on time I suspect you'll make it fine, and if there's no early-winter weather there's even a good chance you'll arrive early, but it is a fact that there's no way to depart BOS for PIT after 800p.

First rule of air travel is not to worry about things you can't control -- enjoy the trip regardless!

To all those suggesting GE: OP is a first time flyer from LHR. That strongly suggests they are a UK national and thus not eligible for GE. In any case, as later revealed, the flight is in five weeks, so there would be no chance of securing this in time.

To the OP: it's tight, but do-able. If BA is on time but you still miss the AA flight to PIT, AA will rebook you, but I'm not convinced they would spring for the hotel room. You may wish to take out travel insurance, in case you do not already have it, for this.

But if BA is late by more than 10 min (i.e. MCT is no longer met) and the OP misconnection at BOS, BA would have to provide care (meal and hotel) and perhabs even compensation according to EC Reg. 261/04.

Yes, but good luck fighting that battle with BA staff in BOS. I think it will be more likely for the OP to have to pay first, claim later.

I agree, that is very likely. But it is actually the eay I prefer it. Then I choose my own hotel, get full points/miles etc and I'm still able to explain when asking for reimbursement from the carrier later, that carrier refused to arrange meal and hotel and, hence, I was forced to do it myself.

Welcome to Flyertalk tjg1992,

As this is about a connection with two Oneworld carriers, we'll reroute your query to that forum.

~beckoa, co-moderator Information Desk

As this is really primarily about connection times and protocols, etc. at BOS, would the Destinations / USA / New England forum be a good landing spot? Not to mention a place the OP, new to FT, might locate.

Customs and Immigration time at BOS has been an issue.

Some recent stories from the Boston Globe, in chronological order. Story 1. Story 2. Story 3. Story 4. Latest data (Story 4, August 6, 2015) is that the average wait time for Customs and Immigration is 50 minutes.