Mast Sai tattoo pen has an interchangeable stroke length 3.2mm and 3.7mm stroke, has a soft and hard adjust knob. Mast Sai rotary pen tattoo machine strong power, super stable,Pretty quiet - low noise, it is quite easy to master. Carved by aircraft aluminium alloy enclosure. Compatible with all stardard cartridges needles. This machine features a powerful motor and exquisite appearance,Light weight, super stable. This rotary tattoo machine simple, classic, and the most suitable machine for precise line work, black and grey shading, solid color packing. Operating Voltage:5-10V, The max operating voltage is 10V. Don't run it higher than that or you'll overheat the motor and cause mechanical wear. Specifications: Length: 122mm Working Voltage: 5-10V Rpm Range Stitches: 8V/9000rpm Frequency: 55-165Hz Needle Protrusion: 0-4.5mm Stroke: 3.2mm/3.7mm