I made this because i didn't have the money or a real pressure gauge so i just looked around the house and i had all the materials to make one so i went for it. 1st you're going to need a Pressure gauge that reads atleast 200PSI.(preferably with the relief valve on the side) 2nd an old spark plug. 3rd brass fitting to it inside hollowed out spark plug. 4th another fitting to fit inside brass fitting. 5th hose clamps & JB weld. Tools needed: Pliers, Adjustable wrench, Flat head screw driver, Dremel.
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Step 1: Hollowing Out the Spark Plug
the easiest way to do this is to 1st take a pair of pliers and bend the flat piece off of the bottom of the spark plug.2nd take a pair of pliers and twist the rod out that was under the metal flat piece. 3rd look or the lip near the top of the spark plug and use your dremel to grind or cut it off then take a screw driver or punch and knock the porcelain from the inside out. these might take a little time to do depending on the tools you use.
Step 2: The Fittings
I don't exactly know what size it was because it came with an oil pressure gauge installation kit but you can pick one up from your local hardware store be sure its threaded on the inside and outside. while you're there pick up another brass fitting that adapts from the inside of the first fitting to your hose size. Once you have the fittings you're fitting probably wont thread into the spark plug all the way to before you screw in the fitting put JB weld around the threads. with your second fitting just wrap the threads in plumbers tape and screw the two together.
Step 3: Attaching the Pressure Gauge
I just screwed the threads of my pressure gauge into my hose and used a hose clamp to seal it. do the same for your hose to brass fittings these lines will be under 100 psi and up. Thats it! now you have yourself a homemade Cylinder Pressure Gauge!