Do it backwards and save you some unneeded engineering aspects. Bore the
larger hole 1/4" bigger than the intended washer first. Make yourself a
countersink jig to maintain the same depth. Then drill the smaller hole to
run the lag bolt in. I'd go with a 1/4" bit for the 3/8 bolt, and 5/16" for
the 1/2" bolt. You don't need a particular bit type to make the hole, just
be sure its long enough. You might clamp the timbers in place while
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