> It's basically related to bolt size. Well, there's confounding factors: the type of bolt (such as stretch bolts), the material the bolt is going into (heads

> It's basically related to bolt size. Well, there's confounding factors: the type of bolt (such as stretch bolts), the material the bolt is going into (heads are aluminium, while some sleeves in aluminium parts are not), whether the bolt is lubricated before insertion (spark plugs really should be lubricated), etc. Bolt size is just another variable.