Shock Proof, Glycerin filled dial to prevent vibration and internal damage. 1/8 in. male NPT centered back mount. If your using this liquid filled gauge in higher altitudes (above sea-level) and your not getting the correct pressure, you must equalize the inside pressure. To do this; turn gauge until the rubber side plug is pointing to the top and pop out the plug to equalize the pressure inside the gauge for a couple seconds and push the plug back in. The gauge will now read the correct pressure reading.