Ducati MONSTER 821 Workshop Manual page 946

If valve sealing test outcome is negative, remove valve, clean it and refit it making sure that it is properly
seated.
Then repeat sealing test; should test outcome be still negative, repeat the whole process.
Once sealing test has been repeated and outcome is still negative, identify the leaking duct and replace
valve.
Repeat the sealing test until outcome is positive.
Checking the rocker arms
Check for signs of wear, grooves or chrome flaking off.
Check the condition of rocker arm bore and shaft.
Clearance upon fitting: 0.025÷0.049 mm.
Wear limit: 0.08 mm.
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