all the threads IN the heads are 5/16ths coarse , while the "end" studs have 5/16ths fine where the "nuts" go . the important thing on these is that they do go into the water "jacket" and will dump water when/IF removed . use of the convex thick washers as factory did allows the bolts/nuts to stay tight even when they heat/cool . I re- tighten them while hot/warm . careful not to snap one .... the lenght , you want about 3/4 ths inch protrusion . you can use a bottoming tap to clean/ check thread depth , or a notch filed bolt .
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