Fair enough, makes sense, but I have definately seen somewhere that the rocker cover gasket should be done as well as they wear with time, so if doing a full service that its just as handy to throw that on too while the plugs are being changed as they will only be taken out again to change the gasket if the gasket starts to leak in between services, thats my take on it anyway. Same reason as doing the waterpump same time as the belts, makes sense to do it then so you dont have to take off belts to change it when it does actually go.
I wouldnt have asked Mitsubishi to do it if I didnt see it somewhere cause I wouldnt have thought of it myself, I'm certain of that. But I'll take ye're knowledge on it, just saying I have seen it somewhere, so dont lash out now
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I'll be damned if I can find the blasted thing now though when I need it for back up!