Hi everyone after several prints of the Tiller Swing Arm and having them shear at the same location I discovered that the lower spigot (where the potentiometer inserts into kept shearing off where it joined the swingarm. It is always a clean shear, appears that the slice layers dont bond well here as the circular wall of the potentiometer spigot finishes and the wall is not stitching to the spigot wall on the face of the swing arm which is evident in the attached pics.
After trying various wall thicknesses and infill densities from 25% - 100% and various infill patterns found the 'Cubic' one had the best strength against shearing in all directions.
A quick redesign from the original arm so that the inner hole also reshaped to take the D shaft of the potentiometer and pass through the swingarm and into the top spigot so that the metal shaft on the Pot also provides further strength against possible shearing. An added hole on the flat for a grubscrew to secure againt the pot shaft to minimize any free-play of the shaft to the swingarm