Ran many of these rods both with the extra I beam and the normal 600-4 solid caps. Motor will wind quicker than with the heavy ones. A round hole and good bolts are a must. a guy asked me one time many years ago if a Merc/ Quincy could turn 11,000 rpm's, I said yes but just once! A rod that has bolts receive a lot of stress at the parting line and the bolts will shear. That is why late type split rods are of the cracked cap design with good bolts that thread into the rod body. A bolt and nut type stretches and also has clearance for the bolt to pass through the beam. This allowes the cap to move or pinch the rollers. The cracked cap design hold the cap fro moving while the bolt holds it all together. If there were room enough a simple dowel at the bolt holes will greatly improve this shearing motion but then there is the rod deflection problem to solve as well. The shorter the rod length i think the worse the problem. I ran just normal 600-4 rods as long as I ran Flatheads until i bought my first Konig in 72 or so. The heavy rods look nice but really look at them, all they do is have the I beam the same hight as the big and little end, Big whoop, i'll take the weight savings any day and take some of the strain off of the bolts. Steve