The $177 million renovation of St. Patrick's Cathedral -- the most extensive in its 136-year history -- is being pushed toward completion in time for Pope Francis' visit to New York City in September.

The four-year renovation was slated to be finished in December, but workers are ahead of schedule and most of the project likely will be done later next month, said Msgr. Robert T. Ritchie, St. Patrick's rector.

"I'm very pleased with the way the construction has been going," Ritchie said. "We're getting ourselves ready and my staff is looking forward to saying hello" to the pope if he comes to the cathedral.

The Vatican has not confirmed that Francis will go to St. Patrick's during his Sept. 24-25 visit. The only official activity so far is a speech at the United Nations, said Joseph Zwilling, a spokesman for the Archdiocese of New York.

Ritchie hopes the pope will come to the iconic cathedral, widely considered the symbolic seat of the Catholic Church in the United States.

"Every pope that has ever come to New York has always stopped at St. Patrick's -- at least for a prayer or a Mass or something," he said.

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Plans for the renovation date to 2006, when chunks of the cathedral's stone walls started falling off and workers erected scaffolding in front to protect visitors, Zwilling said.

The cathedral has had scaffolding around it and inside it since 2011, when full-scale repair work began. Now, most of the scaffolding is down. Some remains in the sanctuary and toward the rear of the building on the exterior.

Ritchie said all interior scaffolding should be removed by late July. He hopes the scaffolding outside will be taken down by late September, but that isn't certain.

Much of the renovation involves stone repair and mortar replacement. When engineers conducted a survey after the 2006 incidents, they found widespread potential problems.

"It's work that is necessary for the protection of the people who come in, for the preservation of the building for the future," Ritchie said. "There is almost nothing that's like a luxury item, or making things fancy."

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The project includes interior painting, cleaning and restoration of the cathedral's stained glass windows, bronze doors, pews and great organ, which has nearly 9,000 pipes.

One item that won't be completed by the pope's visit is a geothermal heating and air-conditioning system. Workers installing it are using a huge drill to bore 2,000 feet into ground beneath the cathedral, Ritchie said.

During the pope's visit, workers will do their best to cover up the drill, located outside the north side of the cathedral, he said.

Funding for the renovation has come through donations from individuals and the cathedral's trustees, Ritchie said. Church officials have not yet raised the entire $177 million.

New York is the second of three cities Francis is to visit during a five-day journey to the United States that begins in Washington, D.C., on Sept. 22, and ends in Philadelphia, where he is scheduled to celebrate an outdoor Mass on Sept. 27 on Benjamin Franklin Parkway outside the Philadelphia Art Museum; that mass is expected to attract at least 1 million people.

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Other possible activities for the pope in New York include a public Mass in Manhattan and a motorcade to give people a chance to see him, Zwilling said. No large outdoor Mass is planned because the pope does not want to overshadow the event in Philadelphia, he said.