BEIJING, Aug 1 (Reuters) - China's vast factory sector posted its strongest growth in 18 months in July as new orders surged to multi-month highs, a private survey showed on Friday, adding to signs the economy is regaining momentum after a flurry of government stimulus measures.

The HSBC/Markit China manufacturing Purchasing Managers' Index (PMI) climbed to 51.7 last month, up from June's 50.7 but slightly below a preliminary reading of 52.

A PMI reading above 50 indicates an expansion in activity on a monthly basis, while readings below that level point to a contraction.

Still, July's reading is the best outcome since January 2013.

Output and total new orders both rose at the strongest rates since March 2013, while new export work increased at the second-fastest pace in more than 3-1/2 years.

(Reporting by Koh Gui Qing; Editing by Kim Coghill)