I have been running 1x10 for several years now with an 11-36T cassette and have argued that for most of the trails here in central Texas that 36T is enough. But as I have gotten older, I am beginning to think that there is a value in having a few more teeth in the back for steep climbs. My Shimano drivetrain tops out at 36T so to go beyond that limitation I needed to change some components and make some adjustments.
The problem with the Shimano setup is that the derailleur already has an extension to get it far enough out from the cassette so that there is not a chain mishap. The extra linkage has always bothered me, but I have never had a problem with it in terms of damage or adjustment.
Before the operation, you can see the derailleur alignment with the 11-36T cassette; clearly there was going to need to be an extra addition to get room for a bigger cog in the back:
In order to move the derailleur back I installed a Wolftooth Goatlink, which replaces the original linkage on the Shimano shadow derailleur. The Goatlink is not only longer than the Shimano linkage but it is also thicker. The Shimano link actually has a spacer where the Wolflink is the full width.
The first step was taking the Shimano link off of the derailleur. Remove the derailleur from the bike, remove the retaining clip with a small screwdriver and then unscrew the bolt that holds the link to the derailleur. This took a little more force than I anticipated because it has obviously never been removed and is a point of torque on the system. Once that is removed, just slip the Goatlink into the same slot (it is perfectly milled to fit right in with no spacer). Then reinstall the derailleur on the hanger. Now you have a slightly extended rear derailleur that will accommodate the larger cassette:
Now that the derailleur is extended, I can remove the old 36T cassette and pop on the 42T cassette. The Sunrace is not of the same quality as Shimano, so the feel is lighter, cheaper and does not appear to be as durable. The machining feels like a lower quality, but that is an acceptable tradeoff for this experiment because Shimano does not make a 1x10 with a 42T cog on it.
Install the cassette onto the wheel, lock it down with a cassette tool and then reinstall the wheel back onto the bike.
I cheated because I am lazy and can never get the shifting properly adjusted. I did all of the link installation without taking the derailleur cable off. I was banking on the spacing being the same for both cassettes. Despite the difference in machining, I was really lucky because both were the same. Besides the cable adjustments, I also have a fear of installing chains. I always end up with the chain too long because I don't want to overcoat the chain. I use a KMC chain because they are more durable, cheaper and have a linkage that can be opened and closed multiple times. Figuring that 42T was as large as a 10-speed would probably ever go, I risked just putting the full length of the chain onto the bike. Lo and behold, it fit perfectly! Or at least it was close enough for my experiment.
One note to remember on the chain is that the master link for a 10-speed chain is small and delicate. You can't just link it together by hand. You essentially line it up on the TOP of the chain loop and then mash the pedal forward to lock it in place. If you have never done this before you'll spend an hour trying to get the link to hold (I had forgotten this trick and wasted time trying to put it together link the old 9-speed chain.
However, even though the chain fits and it is shifting, you have to make one last adjustment. The increased size in the cassette means that you really need to watch the crossover between the largest cog and the top pulley on the cage:
To remedy this small gap, there is a "B screw" on the back of the derailleur. Tighten this up and that will push the cage back a little and give you a little more breathing room. But don't go too far or you will mess up the chain wrap on the smaller cogs. This is a bit of a trial and error.
Once you have done the B screw adjustment you should be good to go with the upgraded drive train.
