Montgomery County faces a big bill to replace dozens of Ride On buses that were pulled off the roads this month after another bus caught fire.

The most recent fire, on July 18 in downtown Silver Spring, was the seventh involving a Ride On bus since 2009 and made the county accelerate its efforts to replace the troubled models that have accounted for the fires.

The county expects to pay a total of $12.3 million for 28 buses to replace 62 buses that were purchased five years ago and had key components made by Champion Bus of Imlay City, Mich., and Navistar of Lisle, Ill. But the new buses, from Gillig of Hayward, Calif., will not be delivered until early next year.

No one was injured in the July 18 fire, which occurred on an out-of-service bus. But Ride On service was disrupted for several days and has been returning to normal as transportation officials reorganize their depleted fleet.

The county was already planning on taking the Champion and Navistar buses off the road by the end of August and had acquired 30 refurbished Metrobuses at a cost of $5,000 per bus to help fill the expected gap in the Ride On fleet, said Bill Griffiths, Montgomery’s chief of fleet management services.

The first in the string of fires was in September 2009, and that led the county to take the buses out of service for several months. A federal safety probe was launched earlier this year after the fifth such fire.

“These buses have had problems for quite a while,” said Esther Bowring, a county spokeswoman. “They’ve just become very unreliable, and you can’t run a bus service without a reliable bus.”

The 50 diesel buses and 12 gas buses pulled off the road were purchased in 2007 at a combined cost of more than $10 million. The 28 new buses will each cost about $440,000.

Ride On has 278 buses in service on the regular weekday schedule. County officials said they wouldn’t know how many of Metro’s refurbished buses would be on the road on Monday until early Monday morning.

Montgomery is reviewing options for recouping some of the money lost by having to take 62 buses out of service five years into what is typically a 12-year life span, Bowring said.

In March, the National Highway Traffic Safety Administration opened an investigation into these buses, exploring buses built on a specific Navistar chassis between 2007 and 2010.

The investigation concerns three bus fires “that appear to have originated in the area of the transmission,” according to an April letter sent to Navistar from the office that investigates defects.

A response from Navistar dated May 18 noted that based on information available at the time, “Navistar does not believe there is a systemic defect in the parking brake system.”

The letter also mentioned that a fire related to the transmission parking brake occurred in 2007 as one of the vehicles was being delivered to Montgomery.

The previous bus fires appear to be isolated to Montgomery County, Navistar spokesman Steve Schrier said in an e-mail.

“Navistar is in communication with Montgomery County and will cooperate fully with their investigation into the cause of the incidents,” Schrier wrote.

Champion Bus did not respond to a call seeking comment.