Putting a Fuel Gauge in a Plastic Tank

by R. Preston McAfee, September, 2021

My favorite boat at the lake

has a plastic fuel tank. The tank sits under the rear transom, so it isn't that easy to see how much gas is left. After reading this site, I decided to see if I could install a sending unit, like this one purchased from Amazon,

in my tank, and make a few improvements along the way. Here is the tank I started with, after drilling the 38mm (1.5") hole and sanding around it.

Here is a closeup of the drilled and sanded hole.

It is important to measure at this point, before ordering the sending unit, to get the right length. When inserted, it should be at least half a inch short of the bottom so that it doesn't hit when the boat is bouncing around.

Here is the unit I ordered from Amazon.

It comes with a rubber gasket to seal against fuel leakage. I had the idea to put the head of bolts inside the tank, rather than use self-tapping screws, to get a tighter fit. I drilled the holes for the bolts just a hair too slender, so the threads hold the bolts in. I drilled the first one, inserted the bolt using a finger and needle nose pliers, and then put a nut on it to prevent it from falling into the tank while I worked on the others.

Using the gasket, I marked the other four.

To get the bolts started, I held them with needle nose pliers on a slight angle, and then stick a finger in to push up on the bolt.

Once all five are in, I removed the nuts and then very carefully inserted the sending unit on to the five bolts. This isn't too hard because the threads hold the bolts in place if you don't put too much pressure on them. I put the nuts on loosely until I had all five on, then hand-tightened them.

Once all five are on, I put a second round of nuts on, and then tightened the bottom nuts by holding the bolt thread at the top with vice grips. One this is very tight, compressing the gasket, I then tightened the second nuts, which serve as locknuts and prevent the lower nuts from working loose.

Here I am tightening the lower nut.

Here is the attached sending unit.

It is important that the gauge you purchase matches the sending unit. This is a matter of matching the ohm range. There are several different ohm ranges. My gauge has a 240 - 33 Ohm range, like my sending unit. The instructions are a bit inscrutable (they incorrectly show a nut at the top and random wire at the bottom) but the sending unit has a wire that goes to ground (negative terminal on the battery) and another wire that goes to the gauge. It doesn't matter which wire is connected to ground, the resistence is symmetric, at least on my sending unit. The gauge has a wire that goes to power (the positive terminal on the battery), a ground wire and the wire from the sending unit. My sending unit had two more wires to provide backlighting but the gauge works without those wires connected.

Here is the finished product, showing the correct amount of fuel in the tank. Because the fiberglass chipped slightly around the edge of the hole, I have 3-D printed a gasket to give it a better appearance but haven't installed that yet.