Bicycle Mechanics - what derailleur will work for me?

The rear derailleur on my old nishiki custom 12speed needs to be replaced and I need some help. I've been looking on ebay for some new derailleurs but how do I know which ones will work? My bike has old shimano sis shifters and the rear derailleur is a shimano rd-l523 "light action". I was looking at a short cage shimano 105 and some ultegras on ebay but will any of these work with a 12 speed or are there specifics I need to look for? And will they work with my sis indexed shifters or do i need to replace those as well? Any advice will be greatly appreciated.

Three factors to look for when acquiring a rear derailleur:

The first is indexing. Any Shimano indexing rear derailleur will work. That is to say, any modern Shimano rear derailleur.

The second is large cog size. Count the teeth on your biggest rear cog. Road bike rear derailleurs, like the 105 you mentioned are rated for 27 tooth cogs or smaller. If the rear cog is too big, the upper derailleur pulley might rub on it.

The third is chain wrap. Subtract the number of teeth on your smallest rear cog from the largest. Do the same for the front chainrings. Now add those two numbers together. That's the amount of chain wrap that your rear derailleur should be able to handle. A short cage 105 will handle 29 teeth. A long cage 105 will do 37 teeth.

The catalogues often list rear derailleurs as 7-speed, 8-speed etc. Since the rear derailleur guides the chain from the middle, in real life that's usually a non-issue.

Since your bike is a 12-speed you should also look at how it bolts onto the frame. All but the cheapest modern derailleurs are designed to bolt onto an arm that hangs down from the bottom of the frame. If your frame doesn't have that you can either reuse a separate bolt-on arm, if your bike has one or you will have to acquire a Tourney or Altus derailleur that comes with it's own hanger arm attached.

The first is indexing. Any Shimano indexing rear derailleur will work. That is to say, any modern Shimano rear derailleur.

The exception being the pre 9 speed DA incompatibality.

Thanks. On the rear the largest is 28 teeth and the smallest is 14. Front is 52/40. So chain wrap is 26. I guess my only concern then is having 28 teeth on the largest rear cog.

The only thing im still unsure about is if I have 6 cogs on the rear wheel do I need A derailleur made for six speeds or are they universal between 5,6,7,8,etc.

Here is an example of a derailleur on ebay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=58094&item=7157954740&rd=1&ssPageName=WDVW
It doesnt state the number of speeds it willl work for. I just dont understand how a derailleur made to work with 9 rear cogs won't go into the spokes on a bike with 6 rear cogs.

Thanks. On the rear the largest is 28 teeth and the smallest is 14. Front is 52/40. So chain wrap is 26. I guess my only concern then is having 28 teeth on the largest rear cog.

The only thing im still unsure about is if I have 6 cogs on the rear wheel do I need A derailleur made for six speeds or are they universal between 5,6,7,8,etc.

Here is an example of a derailleur on ebay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=58094&item=7157954740&rd=1&ssPageName=WDVW
It doesnt state the number of speeds it willl work for. I just dont understand how a derailleur made to work with 9 rear cogs won't go into the spokes on a bike with 6 rear cogs.It'll work.It'll handle 28 teeth.Youll likely need to adjust the B tension screw.... Limit screws are wondrful things, for keeping chain or inner and outer cogs. www.parktool.com repair section has the drill on them and adjusting a RD.

Thanks. On the rear the largest is 28 teeth and the smallest is 14. Front is 52/40. So chain wrap is 26. I guess my only concern then is having 28 teeth on the largest rear cog.

The only thing im still unsure about is if I have 6 cogs on the rear wheel do I need A derailleur made for six speeds or are they universal between 5,6,7,8,etc.

Here is an example of a derailleur on ebay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=58094&item=7157954740&rd=1&ssPageName=WDVW
It doesnt state the number of speeds it willl work for. I just dont understand how a derailleur made to work with 9 rear cogs won't go into the spokes on a bike with 6 rear cogs.

Short cage will work. Shimano road rear derailers will work - even though the new ones are spec'd at 27T max, they will run on a 28T. I've even run mine on a 30T.

Speeds don't matter. You can use any Shimano SIS rear derailer EXCEPT Dura-ace 740x

sweet...thanks for the help

I, too, am interested in rear derailleur compatibility....I need to know if I've got this straight....if you've got a 15 speed or 18 speed bike with 5 cogs on rear and 6 cogs on rear and you need a rear derailleur, then is the derailleur is designated based on the number of cogs on the rear cassette only?
thanks...

ps....both of the bikes i mention have long cages...is this a basic necessity for triple crank bikes?

I, too, am interested in rear derailleur compatibility....I need to know if I've got this straight....if you've got a 15 speed or 18 speed bike with 5 cogs on rear and 6 cogs on rear and you need a rear derailleur, then is the derailleur is designated based on the number of cogs on the rear cassette only?
thanks...

Generally speaking, the number of cogs is a non-issue. The rear derailleur guides the chain from the middle of the chain. All chains have essentially the same inner width so, from that point of view, any rear derailleur will work with any number of cogs.

ps....both of the bikes i mention have long cages...is this a basic necessity for triple crank bikes?

The issue regarding long or short cage is chain wrap up. As you change into the various gear combinations the derailleur has to take up the different amounts of slack in the chain. Typically bikes with triple front chainrings have a lot of slack variance to account for and, consequently, require a long cage rear derailleur.