CHICAGO, Sept 2 - U.S. factory output likely expanded at a faster pace in August, its second straight month of growth after five months of decline, economists said. Economists surveyed by estimated the Institute for Supply Management's August manufacturing gauge at 53.8, up from 51.8 in July. The 24 estimates ranged from 52.0 to 56.0. The ISM will issue its monthly manufacturing report on Tuesday at 10 a.m. EDT (1400 GMT) on Tuesday.//