** Impro Precision Industries 1286.HK slides as much as 20.2% to HK$3.2; set for biggest one-day pct drop since April 7

** Stock hits lowest since August 5

** Chinese castings products and precision machining parts producer says additional 50% tariffs on steel and aluminium imports into U.S may impact co's business, operations

** Co, whose U.S. sales account for large chunk of total revenue, says it's still negotiating arrangement of additional tariffs with customers

** Adds additional tariffs will make it commercially impracticable for group to continue supplying products on existing terms, may even constitute force majeure event as extra tariffs far higher than group's gross profit margin

** YTD, stock up 92.3%

(Reporting by Donny Kwok)

((donny.kwok@thomsonreuters.com))