Question: What happens to the diameter of the hole when we heat a metal block with a hole?

Answer: The hole expands.

 

Question: Will a nut loosen or tighten if the whole assembly (bolt and nut together) is heated evenly?

Answer: The assembly will tighten more as hole expands less as compared to expansion of bolt. In order to loosen the nut, you will have to heat the nut more and allow less heating of bolt. 

 

Question: Do we need to leave some play (space) between two gears as heat is produced due to friction which may result in expansion of gears?

 Answer: Yes, as the gears will expand upon heating. An already perfect fit assembly will over tighten after expansion as the result of heating.

 

Question: In a gear box the shafts are made of solid metal and gear box enclosure (containing holes that hold shafts) is made of a metal sheet. Do you think that after heating up, due to friction, the shafts and hole assembly will tighten?

Answer: Holes expand less as compared to the shaft and tightens the assembly.