Fitment: Use on ISX engine for brake adjustment, alternative to 4919693 Jake Brake Tool (3.8MM) feeler gauge. The 3.8mm area is precision ground to +/- .0005 inch Features: 1) Made of advance heavy duty steel process, its surface is treated with black oxidation, so that a layer of oxide film is formed on the metal surface to isolate the air and achieve the purpose of rust prevention 2) A fast, easy and accurate way to check the valve clearance with this feeler gauge. Getting the clearance adjusted is important for great performance, avoiding any unwanted failures or engine blows .