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I made exactly the same modification ( only no O-ring ). As mentioned the back of the tensioner is rather thin so like Ronlander I tapped for a 5mm bolt. But as an additional safety I changed the 6mm lock bolt which is used during installation for a longer one which reach the ( large) flat on the push rod when tightened so holding the push rod in position. Now from time to time I loosen this bolt and tighten again, and at the same time adjust the 5 mm bolt.