Continuing work on my New Holland GT18 garden tractor (basically the same as a Toro 518xi). On these tractors, the plastic fuel tank (HDPE, I assume?) is secured at its top and sides by several irregularly-shaped pieces of foam rubber sandwiched between the tank and the rear fender. The old foam rubber had deteriorated to what was essentially powder, which allowed the tank to flop around, and it's probably not in any better shape on my 522xi, which is the same vintage (2001).
I haven't (yet) found any available OEM or specific aftermarket replacements for the foam pieces, but if anyone knows of any, let me know. In the meantime, I'm going to experiment a bit and try fabricating the replacements.
I didn't know how thick the original foam was, since there wasn't any intact foam to measure, so I thought I would start by determining what the space is between the fender and the tank. To measure the space, I stuck little mounds of modeling clay at several locations on the tank, placed a little piece of polyethylene over each one to keep it from sticking to the fender, laid the fender in place with the mounting holes aligned with the corresponding captive nuts in the frame, and pushed straight down on the fender until it contacted the frame. When I lifted off the fender, the height of each mound of clay was mashed down to match the space. All of the mounds measured 9/16" or very close to it.
The foam rubber will need to be a little thicker than 9/16 inch, so that it compresses just enough against the tank to hold it securely without stressing anything. I'm starting with a semi-educated guess as to what thickness and compressibility would work best, based on a combination of intuition, logic, and black magic. I might have to try a few different foam rubber combinations before I find one that works to my satisfaction. At least I know what shapes the foam pieces will need to be, as I could still see the outlines of the adhesive backing before I cleaned it off.
For my first experiment, I just ordered some 3/4" foam rubber made of a blend of Buna-N, neoprene, and vinyl. It's listed as having good oil resistance, and it's considered ultra soft (rated at 4 psi to compress it 25%). This is a closed-cell foam, but the original foam 'might' have been open cell (I think there are pros and cons to each). I will need to be careful to place the foam pieces to allow drainage and not trap water. The foam should arrive in a day or two, and I'll post an update on how it goes.
I think I used a combination of scented candle remnants for the red disk when we did it this spring. 
