On my 1990 190e 2.6 the upper timing cover bolt hole for [I Believe] the lower right side [intake side] was machined with a groove on the inside diameter of the bolt hole in the cover, maybe a 1/4 inch inside the hole, I used a small hook tool to replace the o-ring. The o-ring does not go between the bolt head and the cover or the cover and the block, rather it seals the shank of the bolt to prevent oil from migrating past the o-ring and leaking past the bolt head. There was a difference in the bolt shank for that particular bolt as compared to the other three bolts. The o-ring is not obvious unless you are looking for it. I looked for it because I noticed that three of the timing cover bolts came out dry and one bolt had engine oil on it when I removed it. I looked at my M/B service manuals and they do not mention the o-ring at all. If you start to develope a oil leak from your timing cover bolt head, then likely the o-ring is missing or damaged.