Front derailleur compatibility
 

Sora FD-3304 has a broken spring, so I'm trying to find a compatible budget replacement. Clamp is 31.8 mm, what is the lowest diameter I can get or should it be precise? Also, can I use a 3x10 even though my bike is 3x8? I'm guessing yes. I see a lot of two speed derailleurs, but not a lot of 3s. Chainrings are 52-42-can't find the smallest number.

would ultegra FD-6500 or FD-6603 or tiagra FD4500 be ok replacement?

Check Microshift. They have something for most combos and different speed groupsets. Good performance and solid quality at reasonable prices but shop around for deals and I would say a step up from Sora quality. Front Derailleurs | microSHIFT

Rings are 52-42, can't find the lowest number, but I'm gathering that fd-6500 should work?

Since it's not for personal consumption, I'd like to pay as little as possible. The entire bike was $80. I'm looking at the following:

https://www.ebay.com/itm/31427678069...Bk9SR-qZx_njYQ
https://www.ebay.com/itm/15513162291...Bk9SR-qZx_njYQ
https://www.ebay.com/itm/204263309347
https://www.ebay.com/itm/175569662896

you have listed two MTB ders and two Road ders... the two MTB ders will not be compatible with road shifters.. The two road ders listed are priced suspiciously lower than normal... they should be avoided since there is a REASON they are discounted so heavily, IMO.

UPDATE: the "dura-ace" Der is a CLARIS der. !!! the seller has misrepresented his sale item and the Listing should be reported as Deception.

Head is spinning tbh, there are so many choices. I realized diore lx is for mtb, dura ace is a bit dated, but should be of great quality. Ultegra 6500 is missing a clamp bolt, but I just don't want to pay $25. Microshift may need an adapter, so additional costs. I may just stop by bike rescue, I may need cassette as well.

Yes, go to the Co-op.. they can find you a decent front Derailleur that will be correct for your bicycle... and that "Dura-Ace" you listed is NOT a Dura-ace.. it's a Claris Derailleur. the Ultegra der. looks corroded and may be thrashed too.

Go to the Bike Rescue. there's a good chance you will get a nice Derailleur for $10 or so.

The Tiagra triple is 4503 not 4500

Found this suntour in the junk drawer, seems like in good shape. If I match it with 9speed Tiagra RD, is this frown upon?
https://cimg4.ibsrv.net/gimg/bikefor...fdb8b24aa5.jpg
https://cimg5.ibsrv.net/gimg/bikefor...f99374c75c.jpg

I don't think it will index to your left shifters, if you're trying to do that. But I bet it'd work running friction, if the rear of that cage can clear the 42t cog when you're in high gear.

When road triples went from 52/42/30 to 52(50)/39/30 the FD needed a little more vertical depth on the inside to cover the
39 being smaller than the 42 in diameter. So an FD for 42t mid has at least 6mm less vertical metal on the inside of the
cage. This difference is apparent in the cages of your ebay FDs #1&2 compared with #3, which might possibly work.
ebay FD #4 is a double CW FD and won't have enough swing to cover the 3d CW. nick a refers to the FD inner cage
height differences when he mentions the rear of the cage clearing the 42t mid CW on upshift. And raising the FD on the
downtube will not be enough to compensate for the inner cage differences. I suspect the Suntour won't clear the 42t.

A friend, who liked 54/42/30 triples took a dremel to an FD meant for 39t mid CW and cut off about 0.25" off the inside
cage of the FD and it worked for him/

I was able to fix the original sora fd by finding a different contact spot for the spring to unsprung.