The 4 x 6 timbers are pinned together with 1 1/4 inch dowels.  Because the timbers are somewhat prone to swelling and shrinking, it is important to make oversized holes for the bolts that will hold them together, if you ever have any hope of dis-assembling the engne for transport or maintenance.  The holes for these pins are drilled with only minimal clearance.


Because of the depth of the side rails, and the importance of drilling the holes straight, a special tool is needed.  We purchased a drill bit called a ship's auger, it is a single-flute bit several inches longer than the rails are high.  Choose a bit 1/8 inch bigger than the hardware that you will be using.  We used 1/2 x 12 carriage bolts, and used a 5/8 ships auger.

The hardware is recessed at the deepest parts of the frame, mainly for economics (getting 2 inch longer bolts cost $1 more each).