Air India Ltd.’s order for as many as 500 aircraft from Airbus SE and Boeing Co. is being held up by an impasse with engine makers powering the 737 Max, dragging out what stands to be one of the biggest single purchases in civil aviation history. 

CFM International, the General Electric Co. and Safran SA joint venture, is reluctant to offer big discounts on engines and maintenance that typically accompany mega orders, according to people familiar with the negotiations. The deadlock centers on the hourly rates Air India would pay for repairs, said one of the people, asking not to be identified as the deliberations are private. 

The engine venture and its rival, Raytheon Technologies Corp.’s Pratt & Whitney division, are grappling with earlier-than-expected repairs on the latest generation of turbofans for Boeing and Airbus workhorse jets. That’s made it tougher for the manufacturers to model revenue and costs over the l