This is within MCT and outside SCT (Sensible Connection Time).

Yes, it is doable and yes, UA will handle a reroute if you misconnect. But, you are one thunderstorm, runway change due to wind direction, or incompetent jetway jockey, away from a misconnect.

The reroutes will likely be unpleasant or inconvenient. Particularly as to seating because it will be last minute. On the other hand, there is an earlier BOS-IAH which gives you a reasonable connection time.

Recommend considering the consequences of a misconnect rather than whether it is doable. If the onwards flight were to LAX, I would do this in a heartbeat.

Flying :
SYD-IAH on United (Business): Arrives 10:20AM
IAH-JFK on Delta (First) : Departs 11:58AM

These are on two separate tickets and I'm checking 2 bags. I have global entry and pre-check. Do you think it's a doable connection time?

Coming westbound you have a good chance of arriving early. You'll most likely arrive at Terminal E but possibly C. Your outbound is terminal A. Depending upon how quickly you can get off the inbound plane (where are you sitting?) and hussle to the arrival hall (a pretty good hike depending upon which gate you arrive at) You might have a pretty good shot. That's not the busiest time of day for international arrivals at IAH, but they probably don't have as many agents on duty at that time, either.

Of course, you can fly through Immigration and Customs and still wait an hour for your bags. Then you have to recheck them at the transfer desk just outside of Customs. THEN, you need to get to the nearest tram stop to grab the tram to terminal A. There is a tram reachable from the secure area so you don't have to go through security again. Having status with UA would get you Priority Bag tags but UA rampers are famous for ignoring those.

Do you have Global Entry? If not, be ready to do the Mobile Entry app thing. That'll at least get you through Customs and Immigration in pretty good time, assuming, of course, that you are a US citizen. If that's not the case, all bets are off. Global Entry also gets you into a priority line at Customs.

It's doable if all the planets align. A late connecting flight would help.

Thank you for the feedback. Most of what you asked is in my original comment but to clarify, I'm a US citizen, I have global entry and TSA-pre. I'll be flying on a business class seat from United (I'm also United Silver) and I'll be flying first class on my connecting Delta flight.

Somewhat tight.

But very doable. It would be safer without the bags.

You should be fine. Not sure what the above person was talking about regarding the tram inside security. Once you clear Customs, you'll need to go through security again. There's no TSA PRE at the baggage re-check right after Customs, so either take the landside train to Terminal A and go through PRE there, or go upstairs and use the PRE line in that same terminal (and then take the airside train to A).

The biggest risk I see is the bag dropoff requirement for DL. You will have to be rechecking the bags at the DL counter and be completed before their cutoff for bag checking. Which varied between 45 mins to 1 hour depending on the airport. Did not see IAH in their online list (call and ask DL)

Just adding my 2 cents... I won't schedule anything less than a 2-hour connection for an SFO->IAH-> international departure. There are just too many ATC/weather delays at SFO plus the possibility of summer t-storms at IAH. I fly SFO-IAH often - sometimes it arrives early but on more than one instance I've had a 3+ hour delay. As many of the international flights are 1x per day with few alternative options I have decided I would rather spend a couple of hours in the lounge or dining at Pappadeaux. Have never missed a connection though I did have to sprint 20 minutes from C->E once ….

Going the opposite direction IAH->SFO after 3pm I would say a 30-60 minute delay is par for the course. So if you are connecting to Asia I would give yourself a margin for error at SFO.

Now if I am going international ->domestic I don't worry at all because UA has so many flights at IAH you can always take the next one. So I am fine with a 60 minute connection esp. with Global Entry and precheck.

I agree with your assessment, but this person is arriving on United and departing on Delta.

On separate tickets with checked bags (which probably will not be checked-thru).

It's easy to make if you hurry and SYD-IAH is on-time or early. However, that's quite a bit of risk for an unprotected split-ticket connection. If you accept the cost of SDCing the DL flight if you are late, then I think it is fine though.

Have you already ticketed these flights and now asking to see if you should change them prior to traveling? It seems an hour and 38 minutes is too little unless you can through-check the bags, otherwise you're at the mercy of Delta's baggage cutoff time.

I wouldn't do that with checked bags. Bag delivery at IAH can be slow.