As Massachusetts officials gear up for the final stretch of repairs to the Longfellow Bridge, they still do not know how much the job will cost, nearly five years after work began.

One thing they do promise: the final price tag will not go above the upper limit officials have set, $303 million.

During a tour Wednesday to discuss some of the last work being done on the bridge over the Charles, officials said they are still disputing some costs with the contractor, a joint venture of J.F. White, Skanska USA, and Consigli Construction.

“As with any large scale construction project, there’s a lot of ups and downs with the contractor,’’ said John McInerney, who oversees the Boston-area district of the state’s highway division. “We’re working through them. They’re complex.’’

The Longfellow job will be about 20 months overdue by the time it is expected to reopen to traffic in May 2018.

The base cost for the rebuild was estimated at $255 million, but the state has reserved another $48 million in potential payments for unexpected delays or additional repair work.

For commuters, the long-running repair to the distinctive ‘salt-and-pepper-shaker’ bridge has caused lane closures and traffic disruptions, most notably by detouring westbound traffic from Boston to Cambridge.

The MBTA’s Red Line is running at reduced speeds over the bridge.

Additionally, it has forced partial closures on the Red Line during certain weekends, requiring passengers to take shuttle buses between Kendall Square and Park Street.

Officials said the job has encountered numerous complications associated with preserving the historic nature of the century-old structure.

Earlier in September, Braintree Mayor Joseph Sullivan, who sits on the board of directors for the state’s transportation department, said he had misgivings about awarding J.F. White another big contract, to repair the Tobin Bridge, because of the Longfellow job.

McInerney declined to detail the disagreements with the contractor.

He called the negotiations “a very long, iterative process,’’ but said they are not uncommon for major projects.

Chris Keough, a manager for J.F. White overseeing the joint venture, declined to comment on the contract negotiations.

However, he noted that the state required contractors to use historic construction materials and techniques, such as riveting, rather than bolting, steel connections, to maintain the character of the Longfellow.

“It’s a method of connection that was used 100 years ago. They asked us to do it as part of the contract,’’ Keough said of the process. “It’s not done regularly. It’s done typically for this type of historic replication.’’

McInerney agreed, citing the riveting as an example of a skill that was difficult for the workers to learn.

“We have an old adage: looks good on paper, until you get out here and start doing it,’’ he said.

The state is beginning another round of suspending weekend Red Line service over the bridge, to do work around the railway section and to replace train tracks.

The closures, which also includes all passenger cars, begin this Friday evening and will continue nearly every weekend until December 17.

The exception will be the weekends of the Head of the Charles rowing race, and following Thanksgiving, when the Red Line will continue to run over the bridge.

When the bridge is completed, there will be two lanes of traffic into Boston and one to Cambridge, as well as bike lanes on either side.

The Red Line, which has been forced to travel 25 miles an hour over the bridge during the project, will be able to travel 40 miles an hour.

The repair work should give the Longfellow another 75 years if life.

The iconic stone towers that line the bridge were dismantled and restored off-site and have already been returned.

Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter at @adamtvaccaro.