I am doing cylinder head gasket replacement on this engine. Was not observant when I removed the old bolts. Discovered there are two different sized cylinder head bolt heads: 13 and 15mm. Need to know where the 13 mm bolt heads go? Also would like to know why they are different. All bolts seem to be the same thread size and length and apparently have the same torque and angle, which is also confusing. Nothing found anywhere explaining it.