"O" ring position on oil filter
 

I am not sure of the placement of the large "O" ring when replacing the oil filter on a W203. Does the "O" ring go between the cap and the filter housing or inside?

Thanks for any help.

I will assume the W203 is the same as the W202. Do you have a black plastic cap that you unscrewed to get to the filter cartridge? If you look at the cap at about the mid point, at the point where the threads end, there is a groove that should have the O-ring in it. You just remove that one and replace it with the new one. Put a little oil on it so it seats when you tighten the cap.