My bike has a 68 x 118mm bb. Replacement cartridges are made in 68 x 126mm size. Does the larger dimension refer to the length of the spindle or the width of the shell on the frame that the cartridge mounts into?
In other words, can I replace my 68 x 118mm with a 68 x 126mm cartridge?

The 68 is the frame's bottom bracket shell width in mm and is the standard width for all English threaded road frames and most English threaded MTB frames.

The 118 is the spindle length in mm and determines where the crank arms are located. A 126 mm spindle used with a crank that requires a 118 mm spindle will place the crank arms further from the frame and will alter the chain line on the drive side. You should use the correct spindle length or very close to it.

And 103mm, 107mm, 110mm, 113mm, 118mm, 122.5mm, and I'm sure I've left out a few. If your LBS gave you the info repeated above NEVER go back.

Not to mention 115 mm and 127.5 mm. I agree if the 126 was all your LBS offered, you need a better shop.

I need to move the chainline out, but I'm not sure how much.
To make a long story short; this bike had chain suck problems from the get-go. I went back and forth with the lbs. After about 6 return visits they finally swapped out/upgraded the chainrings. That fixed the chain suck problem.
Now the chain rubs the inside of the derailler cage when in low gear - and YES I have adjusted the limit screw.
*When in low gear the chain rubs the cage. It rubs hard.
*When in low gear the chain is very close to the back tire. If I ride through any mud at all the chain scrapes it off the side of the tire.
*There is very little clearance between the large chainring and the chain stay.
I figure if I switch to the 126mm bb cartridge then the chainline will be moved outwards by 4mm. And yes, this is a wild a$$ guess.
How else can I tell where the chainline should be?
I thought that if I could measure the distance from the large chainring to the frame (seattube) on an unmodified bike, that would tell me how far the chainring should be from the frame and I could adjust accordingly. The bike is a 2011 Rockhopper with 26" wheels.
In case you are wondering, I am done messing with the LBS on this issue. I'll just eat the cost of the new bb cartridge. I think they are $16.

OK, if you need the extra clearance than a longer spindle will move the crank arm out further and 126 mm is as good a guess as any. We thought all the LBS offered was that length and you really needed 118.

[QUOTE=sknhgy;13258948]I need to move the chainline out...QUOTE]
This would have been a good piece of information to include in the first post. Another alternative to guessing would be to look to the manufacturer of the crankset to see what spindle length they recommend for your crankset.

If the chainline has changed that much the LBS replaced more than just the chainrings.
Whatever was done seems to be unacceptable.
Have you talked to the shop owner about this?

Yes. I have talked to the shop owner numerous times. They're lame. He told me all they did was replace the stock chainrings with Shimano. This was after about 4-6 return visits due to chain suck problems. They kept making adjustments. Then they told me the chain was going to rub the cage no matter what they did. The mechanic told me they were going to experiment with a longer BB and/or shims but they never did, and I wasn't about to keep taking it back to them to "experiment" with.
I even called the Specialized customer help line and they were supposed to get in contact with the LBS and get this fixed right but that never happened. Like I said, I'm tired of messing with them and I just want it fixed.