9-U.S. crude breaks below $40 as oil ends down 4 pct on glut

* U.S. crude below $40 for first time since April


* Market tumbles on stop-loss liquidation, fundamental selling


* Marathon's 212,000 bpd Ill. refinery shut down unexpectedly


* Worries of record high OPEC output, U.S. oil rigs weigh


* U.S. crude stocks seen down last week, after rise in previous week


By Barani Krishnan


NEW YORK, Aug 1 (Reuters) - U.S. crude tumbled below $40 per barrel on Monday for the first time since April, as oil prices settled down nearly 4 percent on heightened worries of a crude glut despite peak summer fuel demand.


A nearly 15-percent slump in U.S. crude prices in July, the worst monthly loss in a year, also triggered liquidation as trading began for August.