Look at the teeth close, they should not be worn badly. They should have about the same shape as the gear ones next to them. Then shove the synchro ring over against the gear itself. It MUST have some clearance space between the gear and ring, that means there is still some brake angle left inside the ring. If ring is worn out, it will go all the way up against the gear to butt sides with it and no clearance. The synchro ring has an angle built into the center ID that matches a slight angle on the gear OD. Works like this: shift fork hub moves toward synchro ring and engages it, then those two move until the inside angle starts dragging on gear angle. That drag is the brake action that allows hub, ring and gear to all equalize speed so that they all engage. All parts MUST at some time be going the SAME SPEED or parts will NOT ENGAGE. You need that inside angle braking action to do that, synchro ring can look perfect but if it butts gear (wore out), will grind and jerky engagement just like clutch adjustment bad. You don't have to disassemble to check that.
