In a previous post I modified the stock fuel tank to convert it to a FIA-compliant fuel cell. The challenge is that to replace the bladder in a SL-C with a tub you must; remove the windshield, center body, cage, seats, tub and fuel tank close out panel. That’s a lot of work, especially given the amount of sealing that needs to be redone. For this reason, I designed the fuel cell so that the bladder can be removed from the bottom of the car. As can be seen in the picture below, this leaves the access plate and center section of the bladder unsupported.
The bladder is removed via a hole in the bottom of the car which requires the bladder to have removeable support structure
To support and protect the bladder I fabricated a bolt-in structure from rectangular tube and 1/4” plate. To get the fit perfect I took careful measurements and ordered a chipboard prototype from SendCutSend. Even though the piece was large, roughly 24” x 10”, it only cost $6.29. After laying it on the fuel cell I tweaked a few measurements and ordered the final aluminum part.
The center oval plate is lower that the rectangular plate to accommodate the thickness of the access panel and gasket. The oval ring provides clearance for the bolt heads on the access panel.
Bladder support structure installed. Note that only a few of the bolts are installed in the access panel.
The next step to add a thin cover plate to the bottom of car. I couldn’t figure out a good way to use quarter-turn fasteners, so I went with screws because the panel should only be removed every 5-7 years. Drilling and taping 22 holes on the underside of car was tedious.
Cover panel installed with Celco fasteners. The next step was to tap the holes.
While I was finishing up the fuel cell, I considered using Holley’s recently released LiDAR-based fuel level sender, but it can’t see through foam. That would have required me to cut a full-height hole in the foam and insert a porous tube to ensure that the foam didn’t obstruct the device. While this would have worked, the primary purpose of the foam is to prevent a flame front from forming. I assume that’s a small risk, but it felt antithetical to one of the key safety features, so I replaced the stock fuel-level sender with a high-quality one commonly used in aviation.
Although the bladder is tough and the fuel cell is very-well protected in a SL-C, there are two simple precautions to reduce the chances of the fuel-level sender puncturing the blader; (1) a bend was added to the tube in a location indicated by the manual and (2) a nylon foot was machined on the lathe and press fitted on the tip of the sender.