A hat-trick from Birmingham striker Stern John saw the Premiership side squeeze past Leyton Orient and book a place in the third round of the Worthington Cup.
But City did not have things all their own way and the Third Division side pushed Birmingham to the limit.
It all looked so simple for Steve Bruce's side when Birmingham showed their class with some clinical finishing and raced into a two goal lead after just 27 minutes to put a damper on Orient's bright start.
John tucked away the first chance of the game on 15 minutes when he cashed in on some slack defending and 12 minutes later he added a second after Geoff Horsfield provided the cross.
But Orient fought back in the second half and veteran striker Kevin Nugent cut the deficit with a powerful header from a Matthew Brazier cross that hit the bar and bounced over the line just before the hour.
But John looked to have ended the game as a contest on 77 minutes when he crashed home his third from close range after Stan Lazaridis provided the left-wing cross.
But Orient refused to throw in the towel and substitute Jabo Ibehre who had only been on the pitch for five minutes, finished off a flowing move after Lee Thorpe set him up on 82 minutes.
That set up a grandstand finish but Birmingham held their nerve to edge through.
After the game Orient's manager, Paul Brush said: "I was satisfied and proud of the team in the second half because we gave it a real go.
"But in the first half we invited our own problems. I told the lads to go for it in the second half - and that's what they did," said the O's boss.
While Birmingham City boss Steve Bruce said "we're delighted to be in the next round: We knew we had to come here and perform and things looked very comfortable at half time.
"But fair play to Orient they kept plugging away and in the end made it tighter than it should have been."