* Swiss PMI 52.9 in Aug vs 54.3 in July
* Backlog of orders shrink slightly, stocks accelerate
* Suggests firms have overestimated current demand -PMI authors
ZURICH, Sept 1 (Reuters) - Swiss factory activity expanded at a slower pace in August as order books shrank, suggesting manufacturers may have taken too rosy a view of the economy's prospects.
Switzerland's economy is closely tied to that of the euro zone, its biggest trading partner and which stagnated in the second quarter as the impact of the crisis in Ukraine began to weigh.
The Swiss purchasing managers' index, a snapshot of the health of the manufacturing sector, fell to a seasonally adjusted 52.9 points from 54.3 points in July.
Monday's reading remained above the 50 mark that separates growth from contraction, but fell short of the 53.3 points forecast in a Reuters poll.
The PMI survey found that stocks of purchased and finished goods continued to rise, while production also grew and employers added to their headcount.
But the subindex tracking the backlog of orders fell 6.1 points to 49.9 points, dragging down the overall reading. A 3.1 point fall in "suppliers' delivery times" also weighed.
The survey's authors, Credit Suisse and procure.ch, said the acceleration in stocks combined with a lower backlog of orders suggested that companies had likely been too optimistic about current demand.
Private business activity in the euro zone expanded more slowly than expected in August.
A recent strengthening in the Swiss franc towards the 1.20 threshold the Swiss National Bank has set as a minimum exchange rate against the euro could also hurt exporters by making their goods more expensive.
In a weekend newspaper interview, SNB Chairman Thomas Jordan said the central bank may cut its economic outlook at its meeting in September given Switzerland faces increased macroeconomic and geopolitical risks. (Reporting by Caroline Copley, editing by John Stonestreet)