There must have been thousands of beams built - Stuart recommended it as an ideal engine for the beginner, so quality tends to be highly variable. They must turn over smoothly without any tight spots (otherwise they don't run at low speed and the whole point is lost). Most common faults we have to put right are inaccurately made motion work links (which bind up or cant the piston rod over), poor alignment of the crank bearings (the outrigger bearing pillar must be machined accurately to height - lots of engines have a great assortment of shims under the pillar) and tight beam bearings.