I have just bought a roof rack for my Volvo 850 saloon which fits on the door apertures. There is bolt holes there but cannot find out what size bolts I need to secure the roof rack. They were not supplied with the roof rack. Anybody got any idea of the size.

 

are you talking about the pin slots in the top of the door frames or are you looking to add the roof bars themselves? On my 850 my factory racks grab the top of the door frames with a spring clamp. The clamps have a nipple that fits into the hole to center/avoid any sliding but does not bolt in. I don't have the factory roof bars...