Most manufacturers of fittings for high-pressure hydraulic applications agree that O-ring-sealed, straight thread fittings made according to SAE Standard J 514 are the most practical for tubing sizes to ¾-in. OD. Some experts may argue that these fittings are suitable for tubing up to l-in. OD. However, the consensus among manufacturers is that use of hydraulic flange fittings leads to more effective and efficient assembly of larger-diameter tubing.
Two compelling reasons build a strong case for switching to flanges when using larger-diameter tubing. First, tubing larger than l-in. OD would have to be made with very large hex heads, which, in turn, requires very large wrenches so that workers can apply the torque needed to tighten the fittings to specs. Second, designers would have to provide the necessary space to give workers enough room to swing these large wrenches when installing such large fittings. Also, wrench extensions (cheater bars) are often needed for workers to exert enough torque to tighten the hex nut. On top of that, worker fatigue may come into play.