Or for those wuithout a handbook their 9th grade geometry textbook.
Fine threads have a longer helix length which offsets the reduced cross sectional area of an individual thread by putting more total cross sectional area in shear. In practice that principle, just like length of thread engagement, is limited by the rule of diminishining returns....
Dan
One of the great things about tractors is that they involve all manner of engineering principles. For many of us this results in a curiosity and we dig into things that we might have learned and/or forgotten a long time ago.
Anyway, to that end, I did dip into my Machinery's Handbook this morning and compared 1/4-20 to 1/4-28 bolts with similar and dissimilar materials, and looked at bolt/thread failure. The relevant section makes the point that failure can occur due to the bolt breaking, the bolt threads stripping or the internal threads stripping ... kind of obvious. It's not hard to calculate the amount of thread engagement required to "guarantee" that the bolt breaks rather than the external or internal threads shear.
I ran those calculations and for a 1/4-20 bolt there needs to be a minimum of 0.173" of thread engagement for the bolt to break. For a 1/4-28 bolt there needs to be 0.195" of thread engagement for the bolt to break first. The 1/4-28 bolt has a higher tensile strength than a 1/4-20 bolt of the same grade so there needs to be more thread engagement to allow more torque that breaks the bolt.
Going down this rabbit hole showed me that while a 1/4-28 bolt is stronger than a 1/4-20 having a higher tensile strength, the opposite is true for the threads in the mating component. I ran some more calculations with a Grade 2 bolt in 7075-T6 aluminum, and the shear area of internal threads for a 1/4-20 and 1/4-28 bolt, when compared at the same 0.173" of thread engagement, is 0.093 in^2 and 0.090 in^2 respectively.
In conclusion, fine thread fasteners are stronger than their coarse thread equivalents, but internal fine threads are weaker than their coarse thread equivalents
for the same thread engagement length. For this reason, and since the thread length is fixed for the drain plug, I will retract my suggestion to tap with fine threads.
Added in edit:
I think I need to add some clarification here.
So if you're designing something that bolts together, and you have to or want to use a bolt of a certain diameter, and you have enough thickness in the part with the internal threads, you can improve the strength of the assembly by using fine threads over coarse threads.
Years ago when I was in engineering school and some professor made the remark that fine threads were stronger than coarse threads, I took it at face value and never bothered to investigate the nuances involved.
