Theory:

     He then said that while he was about it he would see to the chain for me, and at once began taking off the gear-case. I did try to dissuade him from that. I told him what an experienced friend of mine once said to me solemnly: “If anything goes wrong with your gear-case, sell the machine and buy a new one; it comes cheaper.”
 
     He said, “People talk like that who understand nothing about machines. Nothing is easier than taking off a gear-case.”
Explanation:
 
The man suggested that he should look at the chain. He started to remove the gear-case. The narrator tried to discourage him again but to no avail. He told the man what one his friends had said about the gear-case. “If anything goes wrong with your gear-case, sell the machine and buy a new one; it comes cheaper.” The narrator was trying to tell him that the gear-case is quite expensive and that he should leave the gear-case alone.
 
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The man said that he should remove the gear-case

However, the man never listened to the narrator. He went on with his repair work. He said that people usually leave the gear-case alone because they are incapable of understanding how the machine works. His friend probably said that because he never knew anything about machines.

But things are different for the man, or so thinks he. He said that he finds it easier to remove the gear-case than anything else. Of course, the narrator never spoke anything about the difficulties in removing the gear-case, only the (monetary) difficulties in getting it replaced.
 
Meanings of difficult words from the paragraphs:
 
Sl. No.
Words
Meanings
1
Gear-case
A protective casing for the gear mechanism of a vehicle
2
SolemnlyIn a formal and dignified manner.
3
DissuadeDiscourage
Reference:
National Council of Educational Research and Training (2007). Honeycomb. A Bicycle in Good Repair (pp. 126-132). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.