2 times this week I when to startit up and it acted like the engine was locked up and I could not turn over the engine with a hand crank. I used a pipe wrench on the shaft on the front of engine and was able to get engine free by turning the engine in a back rotation when it popped loose. I then was able to use the starter and it stared up good. I used the tractor for 2 days on and off and then it locked up again. I did the same thing again by turning the engine in a back rotation until it popped loose, started good and run very smooth. What could make this locking up of the engine happen and what needs to done to correct this