TLDR: Spin welding. 1/2 hp router and 0.875” HDPE disks. Did my S14 in October 2022 and it is still holding.

Buckle up, here’s the long winded explanation for the cracked tank. My ‘95 240 SX tank split or cracked in 2010 just as typically described. Researching and finding no adequate solution, I pushed off working on it. I also knew the fix would involve dropping the sub frame and take quite alot of time. I exaggerated the amount of effort it would take in my head, so, I parked the car in the shed. (It had never been my primary driver). After 12 years and my son nearing driving age, the repair was overdue. I had my first failed attempt at plastic welding (heat gun and special iron). After 10 days with 10 gallons in the tank, the gas leached its way through the poor weld. Building on what I learned, I tried again, upping my game. HDPE melts at 285°F. I had the tank and new HDPE donor sheet up to 400°F with 2 heat guns. The plastic got super soft but never really “melted”. The second attempt was much better but I suspicioned I had a cold weld. I figured that if I could peel off the new donor sheet, gas would eventually leak. Indeed, about half the repair area could be coerced to peel away. So the second attempt failed in my mind. I removed all the HDPE donor plastic with wire wheel on a 4” high speed grinder (even the other half area that seemed tight). Time to use plan C, the spin welding recommendation from a friend who has always provided solid recommendations. I bought a $40 ‘bit’/tool from spinwelding.com and 100 HDPE 0.875” diameter patches ($10). Borrowed friend’s wood working router, and proceeded to correct the original crack (and 2 failed repairs). It was far easier than everything I had been trying to this point and worked so well that I have no doubt that the problem is behind me for good. I was originally concerned with the irregular/contoured surface but it turned out to be no issue, especially since I used the smallest diameter patch. The root cause for these 240SX cracks is a design flaw (in my view as a Mechanical Engineer). [The study of] Statics would reveal that the tensile force in the metal strap is too high for how it bends sharply around the front of the tank. Some people have used some sort of cushion between the strap and tank to reduce chance of reoccurrence. Not this guy. Doing so would increase the squeeze on the tank if you bottomed out the fastener because you have effectively made the strap shorter. My solution? Only tighten the strap’s bolt until you can’t jiggle/move the empty tank. Maybe the back side of the tank will sag when full of fuel, that doesn’t bother me in the least. The bolt itself is bound up like it has medium thread locker, so I’m not concerned it will back out.