* Crude unit expected back in operation Wednesday A.M.
* New unit was centerpiece of $10-billion expansion (Adds market reaction, details)
HOUSTON, June 5 (Reuters) - A newly commissioned 325,000 barrel per day (bpd) crude distillation unit at Motiva Enterprises' 600,000 bpd Port Arthur, Texas, refinery remained out of production on Tuesday for repairs to a leaking valve, according to sources familiar with refinery operations.
The unit was expected to return to production as early as Wednesday morning once the repairs are completed, the sources said. The CDU was placed on circulation meaning it has crude oil in it and remains at the high temperature needed for refining.
Gulf Coast conventional M2 gasoline differentials on Tuesday fell slightly in early trading, but later rebounded by a quarter cent per gallon to hold Monday's 2.00 cent gain at 9.50 cents under July RBOB futures on the New York Mercantile Exchange, traders said.
A Motiva spokeswoman declined to discuss refinery operations.
Motiva officially commissioned the CDU, the centerpiece of a $10-billion, five-year expansion project that more than doubled the refinery's capacity, in a ceremony last week attended by the chief executives of Motiva owners Royal Dutch Shell Plc and Saudi Aramco.
The addition of the 325,000 bpd crude unit made the Motiva Port Arthur refinery the largest in the United States.
A CDU does the initial refining of crude oil coming into a refinery and provides feedstock for all other units. (Additional reporting by Kristen Hays; Reporting by Erwin Seba; Editing by Marguerita Choy and Sofina Mirza-Reid)