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Message: <blockquote data-quote="TriHonu" data-source="post: 59332" data-attributes="member: 897">3/8" is the bolt size and is equal to 0.375 inches.5/16" is the tap drill size for 75% thread engagement and is equal to 0.3125 inches.  Take it easy and use plenty of cutting oil and a sharp tap.  You will want to remove the tap and clear the chips frequently to keep from binding the tap and breaking it.The plan shows the hole center is located 1/2" down (middle of the 1" thick plate) and 3/8" from the two edges along the 4" length.Looking at the angle of the chains I would move the two holes closer together with the holes at least 1/2" in from the 4" dimension (the two holes will be 3" or less apart).  As long as you can get two chains bolted to the block without the chains interfering with each other you should be fine.  I don't like the plan dimensions with a 3/8" hole centered 3/8" away from the edge.  That only leaves 3/16" thickness to support the bolt that is being pulled toward that edge.Also use good thick washers to support the chains.  You don't want that much force on a piece of sheetmetal.</blockquote>