Surface grinding removes material from the face and only straightens or restores the gasket face area to flat.Straightening a head refers to the process of physically straightening the head without grinding or milling. This process not only brings the face back to within grinding limits but also straightens the cam tunnel and the top face of the head. After straightening, the surface is skimmed to restore the gasket sealing area. When surface grinding a bent head without straightening, the head could end up with unequal amounts machined off the ends compared to the centre. This can effect compression's and as the head is still bent, the cam tunnel if OHC will still be bent causing possible cam seizure.