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I’m new to changing cassettes so I need some expert advice on doing it.
I currently have a Sram 12-26 cassette and I want to change it for one that will help me in the low end for climbing hills. (I’m not currently in shape)
I have a Shimano 105 rear derailler. How many more teeth can I increase before I have problems with the derailler?
 

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Standard Shimano road goes up to 27t. Your derailleur can probably shift on to a 30t cog. Even though Shimano specs 27t as being the largest, it seems that most Shimano derailleurs can handle a bit more. But it may not be able to take up the extra chain if it is a standard (non-triple) derailleur. The derailleur needs to be able to take the slack (or "wrap") between the chain being on the largest chainring and largest cog, and smallest chainring and smallest cog. A derailleur made for triple chainring setups can handle a lot more wrap. If you have a standard (52/39) double and standard road derailleur you may be able to make a 12-30 work. I'm running a compact setup with the same amout of wrap as that, and with the chain exactly the right length, it works, even though it is outside Shimano's spec.

Besides the wrap, you may need a longer chain. Even though you're not supposed to use it, you should be able to shift into the big chainring/big cog combo without breaking anything. Otherwise a simple brain-fade mistake could be ride-ending.

IRD makes some 12-28 cassettes. Sheldon Brown sells some custom 12-30 or 13-30 cassettes, and solo cogs to make your own. You need a steel freehub body for that.
To get the cluster off you'll need a chain whip and a shimano freehub lockring removal tool.
 
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