UpNorth: See if any of the information below helps your trouble shooting. I input the symptoms you described above into an AI search. It sourced: OEM Service Manuals for excavators, LunchBox (hydraulic training courses), Parts Suppliers Guide, and this forum. I have no clue but was curious about your issue. I've seen this issue occur on other hydraulically operated machines and wasn't sure what was going on. Hope this helps. Why the Thumb Gets Stuck Fully Open Mechanical Over-Centering: When the thumb is opened 100%, the cylinder pin, main pivot pin, and thumb body can occasionally line up in a perfect, straight line (known as "dead center"). When you hit the rocker to close it, the cylinder pushes straight against the pivot pins instead of rotating the thumb. Jolting or "playing with it" vibrates the machine enough to break that straight alignment, allowing it to close. The Cylinder is Bottoming Out and Hydraulic Lockout: When the cylinder is completely retracted (thumb fully open), the internal piston can tight-seat against the gland. If your machine's secondary circuit relief valve is set too high or low, or if the pressure gets trapped in the return line, the oil cannot vent fast enough to start the reverse stroke. Sticking Control Valve Spool: The auxiliary control valve spool on the main block may be sticking only at its maximum stroke. If the electrical rocker switch sends a signal to a solenoid that controls pilot pressure, the pilot oil might not have enough force to push the main spool back out of its fully extended detent or seating position. How to Fix and Prevent It Install a Mechanical Stop (Shime/Spacer): The easiest and most common permanent fix is to welded a small steel block or bump-stop on the excavator arm. This physically prevents the thumb from opening that last 5% where it gets stuck. By stopping it just shy of full extension, it will work flawlessly every time, just like it does now when partially closed. Verify the Auxiliary Line Pressure: Check the relief valves on the auxiliary circuit. If the pressure is slightly out of spec, it may struggle to overcome the mechanical disadvantage of a fully open thumb. Inspect and Swap the Solenoids: Locate the auxiliary control solenoids on the main valve bank. Swap the "open" and "close" solenoid coils or valves if possible to see if the sticking problem reverses or disappears. If it does, a sticking valve spool or weak coil is your culprit.