“If we did discover a problem, we could resume containment operations,” he said.

If containment were resumed, either at the end of the test period or later, it would continue until the company could complete the relief well work  by the end of July or August at the earliest. Mr. Suttles said that even if the new cap was kept closed, the relief well work would continue “ultimately to make sure this well can never flow to surface again.”

There were signs that BP’s latest subsea engineering effort was proceeding smoothly, with few of the hitches that marred some earlier attempts. Removal of the old, loose-fitting cap went quickly, and clearing the way for the new cap by removing six 50-pound bolts that held a stub of riser pipe was straightforward.

On Monday evening, video from the seafloor showed the cap being lowered onto a connector pipe that had been installed the day before. The cap’s latching mechanism had a sticker on the side that read, “THINK twice, act once!!”

Perhaps learning from previous frustrations, engineers had made plenty of contingency plans, including having another loose-fitting cap on standby in case there were significant setbacks with the tighter-fitting one. Backup tools were available to help get the pipe stub off if the first one, called an overshot tool, did not work. The additional tools were not needed.

Engineers had performed dry runs, on land, of the installation of the cap, a 75-ton assemblage of forged steel, with three hydraulic valves, or rams, that are much like those on the blowout preventer that failed when the blowout occurred April 20. An animated video was produced to show technicians at the well site, 40 miles off the coast of Louisiana, how the work would proceed, to help coordinate the movement of vessels and remotely operated submersibles.

The new cap will be attached to the connecting pipe with a hydraulic latching device. Antifreeze will be pumped around the latch in an effort to avoid the formation of hydrates, icelike crystals of methane and water that could affect the latching mechanism and that scuttled an earlier containment attempt.

The work crews did encounter minor delays overnight in starting up a new collection system that could divert up to 25,000 barrels of oil a day to a surface ship, the Helix Producer, Mr. Suttles said. That system began operating on Monday, he said, and was expected to reach full capacity over several days.