Have a Zetor 6178 will not engage 5 gear just get grating sound ,have taken top off gearbox and can see a gap between 5 gear and syncro metal and brass ring of a least quarter of an inch ,any ideas? Think this gearbox has been apart before by all the blue sealant every ware ,can not see any dedent spacers in syncro ,This tractor has a cab so do I need to remove to strip gearbox in place to repair?