Wag-Aero had standard capacity tanks that worked well when I needed one. Dakota has one that is 24 gallon I believe. Their tank is STC’d if I remember correctly.
I lifted this off the J3 site, what it means for all the tanks they manufactured will have to be answered by him.
Good news, I have just acquired the STC’s from Biplane Investments for left and right J3 wing tanks (The STC is for left or right or left and right). I will move as quickly as possible through the PMA process to get them back into production and available again, however this will take some time (I’m hoping it can be done as quick as a few months). Please contact me for more information and I’ll keep you posted on the timeline for approval and production.
Might want research this option for short money..... from very knowledgeable 12 expert GP posted this.
"put PA-20 tanks in my 12 back in 1986 and got them field approved no problem. The 18 tank is identical except the 20 tank did not have the tube welded in for sight gages. I removed the drag/anti-drag wires in that bay and replaced them with a long 4130 strut through the tank and a shorter one to get back to the next set of drag wires. The rib outboard of the tank was moved in so the new cover would fit. Do a search on a 337 for 7889H and you will be in business." Just a thought??
I have done a few sets of original " turnplate tanks by simply removing them and completely resoldering the bottom seams ( less than an hours work) and had fantastic luck with the results! For some reason that option is frowned upon on here???? Ditto on replacing bungees on a 12......
Most common advice is to spend 5 grand on new tanks , or install Cub gear... oh boy......