this one is also undoubtedly a simple answer

my bike is leaking gas from the fuel filter, whatever you want to call it.  

tank mounted petcock:



i took the thing apart, and found that the single internal o-ring had dry-rotted

red arrow, cruddy cell phone pic:



question: is there somewhere i can find just that o-ring? or do i have to spring for the whole petcock/filter ($75)?

edit: second question: if i were to go to a plumbing supply or something like it and get another generic o-ring that happened to fit perfectly, would it be gas safe?

Here is the answer you editted question.  If the o-ring were made of nitrile it would be resistant to petroleum. BUT even if it was made of nitrile I wouldn't know if that particular o-ring would work for your application.  Nitrile is a copolymer of butadiene and acrylonitrile.  It is the acrylonitrile that brings the strong resistant to petroleum products to the combination. As more acrylonitrile is put into the mixture of the two components, the resistance of the nitrile o-rings to petroleum products increases. Unfortunately, at the same time, the low temperature flexibility of the nitrile o-rings decreases with more acrylonitrile. Thus, compounding nitrile (buna n) becomes a balancing act. If one wants good flexibility at low temperatures, some hydrocarbon fuel and petroleum oil resistance at high temperatures will have to be sacrificed.

It is up to you if you are willing to test it to see if it works.  You must weigh the risks.  Personally, I would try it but I've been know to make some bone head decissions.

-Porkchop

I just had this issue a month or so ago. I tried some fuel grade O rings from an auto parts and it just wasnt the right size. It was a close fit, but it still leaked a bit. If I had the tank was full it probably would have leaked more. I didnt want to deal with gas all over my garage or dripping into the crankcase so I bought a new one off of ebay. It was 35 or 55 bucks. Went on in just a few minutes and solved the problem and gave me some piece of mind. Its your call. Also, the one off ebay came with a new gasket that sits inbetween the fuel switch and the tank rather than re-using the old crusty one.
   
here is a link. $35 beans aint so bad. This guy was great to work with.

http://cgi.ebay.com/ebaymotors/97-02-GS500-PETCOCK_W0QQitemZ360160097228QQcmdZViewItemQQptZMotorcycles_Parts_Accessories?hash=item53db36f3cc&_trksid=p4506.c0.m245&_trkparms=72%3A1171%7C65%3A12%7C39%3A1%7C240%3A1318

well this thing has been leaking gas pretty much constantly since i got it, so i think its worth my time just to try and replace the o-ring first. if it still leaks too much i'm going to go the ebay way for sure.  

that said, the link you posted, xanthras, looks to be in much better condition than the only other one i found on ebay.

my new question, then, is whether or not the filter posted will fit an '04 gas tank. (i'm assuming it will...and it doesnt really matter to me either way, i'm just curious)

Someone here can correct me if I am wrong, but I am pretty sure the tank fuel tank switch is the same for all years. The other frame mounted petcock is the one that got the makeover. The ebay link says for a 97-02. I installed in on my 94 and it was perfect (exactly the same as the one I pulled in every way) I compared it to the diagrams and part numbers at bikebandit.com and I am pretty sure it was the same. Good luck whatever route you go.

thanks, i just checked the part numbers and its the same, im going to try and get an o-ring this afternoon after work, but if that doesnt work i'll get the petcock tonight   

Not even joking:

http://www.midwayautosupply.com/p-14397-permatex-the-right-stuff-for-imports-gasket-maker-75-oz-powerbead-can-85144.aspx

6k miles and still not leaking.

you used that as the O ring?? If so i am impressed. Or did you just smear it in there with the old O ring?