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Need a new front and rear derailleur
I'm fed up trying to rehab my old Shimano Altus A20 and C20 front and rear derailleurs. Can anyone recommend an inexpensive new set? It is for an 18 speed, Trek 800 Antelope. Also, I would prefer not to have to replace the thumb shifters if possible. I'm still holding out hope they can be reused.
The first question is why you are replacing them. It may be needed, but if you are having shifting problems, the derailleurs are not necessarily the problem. First check is that your limit screws and cable tension is right. Next check for alignment, bent derailleur hanger, etc, then cable issues.

Can you describe more what the problem is?

But if you do need derails, just about any low to mid range shimano ones should work, and there shouldn't be any reason to change shifters. Maybe someone else out there has specific recommendations.
I am replacing due to a lot of rust, the front derailleur does not move the chain enough so it only runs on 1 front gear, the rear derailleur's barrel adjuster is missing, i tried to replace it but the screw where the barrel goes on broke off and I an unable to remove it.

I may be looking for something that doesn't exist but I'm surprised I can't find anything that has bike specs and tells you what new equipment will fit the old bike. My bike is about 16 years old or so, I was told most of the equipment changed 5 years ago so they need to see the bike so they can measure, especially for the front derailleur.
I'm pretty sure any shimano rear derailleur that will work for 7 speed will work for you. There should be some cheap choice on that like the Acera model. There is also the Tourney line which lists as 6/7 speed compatible. One question on the rear is whether your derail screws into the frame or has a hook that goes under the axle bolt. For the first type you need something like this: http://www.amazon.com/Shimano-Tourney-7-Speed-Derailleur-Mount/dp/B001606M9G, for the second, something like this: http://www.amazon.com/Shimano-RD-TX51-Tourney-Derailleur-Hanger/dp/B000FFQ4LE/ref=sr_1_1?ie=UTF8&s=sporting-goods&qid=1267478955&sr=1-1

On the front, just about any mtn derailleur should work. But you need to match the "clamp diameter" (the size of the tube it clamps on to). You also need to get one that pulls the cable from the same direction. Most have the cable go down, but some have the cable go up.

It can get a bit confusing, but I think that covers everything you need to get the right part.
ok, i got the rear Shimano Tourney at my LBS but they didn't have any front derailleurs that would work for my bike. The inside of the front der. clamp said 28.0 28.6. Also, it is bottom pulling. Would this be a good one to use? http://www.amazon.com/Shimano-Fd-Tz31-Tourney-28-6-Trpl/dp/B002K2ER96/
That looks like the right thing (assuming the "28.6" in the name refers to clamp diameter).
If the derails are that bad, you might want to get new cables too. And while you're at it - brake pads if those haven't been replaced in a while. They get hard and don't work well if the bike's been sitting outside a lot.
My order has been placed! Thanks for the help. I have new cables and housings already. I haven't even started to look at the brakes yet but I know I will have to replace the pads.
Sounds like DaveM has you setup good. I if you need help on putting on the front deraileur this is the place to get help I think Alex has a video on this too if not just about any of us here can tell you how to set it up its pretty easy to do. Or you can do a search on how to do it lots of videos on this out here. Good luck with your bicycle, working on them can be as much fun as they are to ride them.
My dad always told me a Sledge a matic can fix any thing.

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