I agree with Pete. The bottom of the spark plug is in focus, but most of the other parts around it are out of focus. This due to a fairly shallow depth of field caused by a combination of a 105mm focal length and a wide aperture. It's also quite a 'busy' subject in terms of the number of potential points that the large central AF sensor could lock onto. This is a subject where using live view and zooming in to a precise focus point could have helped, or alternatively shooting in manual focus. If you intended to have more depth of field, then the aperture would have benefitted from being reduced to at least f/8 (ISO and shutter range permitting).
I very much doubt your camera has focus issues. Your perceived lack of sharpness compared to other images from other cameras is probably due to difference in the in-camera settings, or the PP adjustments made if shooting RAW.