1955 – 1956 Dodge and Plymouth Fuel Tank Cleaning and 3/8″ Line Modification

(also applicable to Chrysler and DeSoto)


The factory 17-gallon fuel tank used on 1955 – 1956 Dodge and Plymouth coupes and sedans used a 5/16″ tube pickup located in the center-rear of the tank. The fuel pump sucks the fuel through the screen where it exits the front left (driver) side of the tank (Figure 1). The 5/16″ fuel line then runs down the left (driver) side of the chassis, crosses over to the right side at the front crossmember, and enters the mechanical fuel pump. For a factory engine with a 25 gallon-per-hour (GPH) mechanical fuel pump, this configuration works just fine; however, high-performance engines–especially those with multiple carburetors, blower, or fuel injection–require higher pump volume. The accepted calculation for Brake Specific Fuel Consumption (BSFC) on naturally aspirated four-stroke engines is 0.5 pounds of pure gasoline per 1 horsepower at wide-open throttle, which when converting weight to gallons comes out to 1 GPH per 12 horsepower. The gallons required increases the more alcohol is added to the mixture. While a stock poly 318 at 230 HP only needs 19 GPH of gasoline at wide-open throttle–well within the pump’s 25 GPH flow–a 456 HP engine needs 37 GPH and a 504 HP engine 42 GPH. For these engines, the factory pump would not keep up with the fuel demand.

Replacing the factory 25 GPH pump with an aftermarket 80 GPH or 110 GPH pump may solve the volume requirement of the engine, but feeding that high-volume pump with the factory 5/16″ fuel line and fittings can cause unintended problems from pressure (due to the narrow tubing and fitting passages) to pump cavitation (due to the line restricting the pump from sucking enough volume from the tank). When increasing pump volume, a larger-diameter line from the tank to the pump keeps the pump fed and reduces strain on the pump since the larger diameter line and fittings reduce resistance. The less bends, fittings, and length of the line from the tank to the pump also decreases resistance.

Unfortunately for those of us with 1955 – 1956 Mopars, no one makes a replacement fuel tank with a 3/8″ pickup tube. The most common modification I see is people replacing the factory fuel pump with a high-volume pump with 3/8″ NPT ports, the line and filter from the pump to the carburetor(s) with 3/8″ line, and the fuel line from the tank to the pump with 3/8″ line. The issue in this configuration is that by keeping the factory tank’s 5/16″ pickup tube, the 3/8″ line from the tank to the pump acts like a 5/16″ line since it is choked down to 5/16″ at the pickup point in the tank. Running the 3/8″ line along the same route as the factory line to mate up with the tank outlet also adds unnecessary length to the system. My 1956 Dodge coupe’s 390 stroker A-block with dual cross-ram AFB carburetors that is going to see track time requires a high-volume pump with 3/8″ NPT ports, so I wrote this article detailing how I built a 3/8″ pickup and modified the factory fuel tank.

1955 1956 Dodge Plymouth Chrysler Desoto Fuel Gas Tank
Figure 1: Factory 1955 – 1956 Dodge/Plymouth Fuel Tank 5/16″ Pickup
Fuel Tank Preparation

I hope it goes without saying that gasoline and any varnish in the bottom of the tank are highly flammable and when confined to the tank can be explosive. After pulling the tank, I drained the gas and a lot of flaked varnish into a container. I propped the tank up on its side with the filler neck facing down and left the tank in direct sun for an entire day to allow the fumes to evaporate. The next day, I plugged the fuel outlet fitting, filled the tank with one gallon of Pine-Sol–my go-to de-greaser parts cleaner–and a gallon of hot water, and worked the tank back and forth to agitate any varnish. I left the tank flat for two days to allow the Pine-Sol time to work on the varnish. I used a pressure washer attached to my water heater and cleaned out the tank. Between the Pine-Sol soaking and hot-water pressure wash, I got out all the varnish and found no rust damage inside the tank. I blew the tank dry with compressed air. In the past, for especially stubborn varnish, I’ve put pea gravel into the tank as an abrasive along with the Pine-Sol solution to break loose any varnish, but I didn’t need to use gravel for this tank. The Pine-Sol and hot water removed all remaining flammable residues, so I was ready to begin cutting and welding on the tank.

Since I couldn’t remove the 5/16″ pickup since the screen is larger than the sending unit hole, I planned on leaving the pickup intact and plugging the outlet with a 1/2″-20 inverted flare plug. I planned to install the new 3/8″ pickup on the right (passenger) side of the tank and run the line down the right frame rail since the pump is on the right side. This route will remove about 4′ of line compared to the factory routing. I spent time online piecing together parts and ended up getting a 3/8″ NPT bung and a 3/8″ inlet fuel pickup sock OER-K405 for a 1962 – 1981 GM from Jegs. I already had the 3/8 steel line and a 5/16″ washer (Figure 2).

I sanded the front right of the tank wall to bare metal and drilled a 1/2″ hole. Working with a tubing bender and my knee, I put a sweeping bend into the new 3/8″ line until I got the inlet placed where I wanted it in the tank. The 3/8″ line fit under the original 5/16″ line in the tank, but I knew the filter sock wouldn’t clear. I inserted a carpenter’s “cat’s paw” through the sending unit hole, caught the underside of the factory 5/16″ line, and pulled it up to create a larger gap between the line and floor of the tank. My goal is to pin the new 3/8″ line with the old 5/16″ line to keep it from bouncing around and fatiguing where it welds to the tank.

I used a bench wire wheel to clean off the zinc plating from the outlet end of the 3/8″ tube and the 5/16″ washer. At the outlet side of the 3/8″ tube, I secured the 5/16″ washer by pinching it against the 3/8 tube in a bench vice (Figure 3). This washer will act as a collar both against the tank wall and the 3/8″ NPT bung. While a TIG would be the best tool for this type of fine work, I don’t own one. I used my 130A MIG with argon to weld the backside (tank side) of the washer to the 3/8″ tube and used a hand file to clean up the weld where the washer sat flush against the tank wall when installed. I clamped the 3/8″ NPT bung against the washer in a vice, welded around the bung, and ground the weld smooth (Figure 4).

After sliding the 3/8″ tube into the tank, I positioned the inlet of the tube up through the sending unit hole. I slipped the filter sock onto the end of the tube and used a small machinist’s vice to very slightly pinch the end of the tube to where it belled out enough to where the sock could not slip off even though it was already crimped on nicely. I slid the 3/8″ tube and sock under the factory 5/16″ line and then took it back out to adjust the sock filter where it lay flat against the floor. Once I had the 3/8″ tube and sock positioned properly, I tack-welded the bung to the tank wall, triple-checked the positioning, and finish welded around the bung ensuring I didn’t have any gaps where it would leak (Figure 5). After the material cooled, I took the cat’s paw and gently pressed the factory 5/16″ line down onto the new 3/8″ tube/sock to firmly pinch them against the floor (Figure 6). When I bent up the 3/8″ line, I planned on this step and was sure to bend the last 5″ of the inlet parallel with the floor of the tank to where the very tip of the line would not pinch and cut through the sock filter. Figure 7 shows the front of the tank finished with an adapter to go from 3/8″ MPT to female 5/8″-18 inverted flare. From this point, the 3/8″ line with a male nut attaches and runs to the mechanical pump.

1955 1956 Dodge Plymouth Chrysler Desoto Fuel Gas Tank
Figure 2: 3/8″ Pickup Parts
1955 1956 Dodge Plymouth Chrysler Desoto Fuel Gas Tank
Figure 3: 5/16″ Washer Ready to Weld to 3/8″ Tube
1955 1956 Dodge Plymouth Chrysler Desoto Fuel Gas Tank
Figure 4: 3/8″ Pickup Welded for Installation
1955 1956 Dodge Plymouth Chrysler Desoto Fuel Gas Tank
Figure 5: 3/8″ NPT Bung Welded to Tank
1955 1956 Dodge Plymouth Chrysler Desoto Fuel Gas Tank
Figure 6: New 3/8″ Tube and Sock Filter Pinched in Place by the 5/16″ Line (the discoloration is staining and not varnish or rust)

1955 1956 Dodge Plymouth Chrysler Desoto Fuel Gas Tank
Figure 7: Finished Tank Front
Next Up: 3/8″ Chassis Line, Pump Connection, and Carburetor Connection (in progress)