Probably because harder rivets tend to crack if they are bucked to 1.5 diameter tails. Look at the specific materials and strengths. The differences are probably not random.
The main reason iam asking this question is ive always worked on a particular type of aircraft and have bucked them to 1.3d (fridge rivets L37) and was looking into other aircraft designs and discovered a lot of the US aircraft specs seem to quote 1.5d for there rivets like the AN470Ads so what you’ve said seems to put some logic behind what I am seeing I just wasn’t thinking about it like that thanks. sometimes it needs another head to break certain thought patterns. thanks again
Current technical standards for most US aircraft and the FAA specify 1.5D (width) X .5d (height)on the bucktails as standard. Years ago, 1.3 X .7, I believe, was used on relatively thin-skinned aircraft (.025 or less) because over-shooting the rivet buckled the skin. In reality, 100% hole-fill is the goal, with a slightly enlarged bucktail to prevent pull-thru.
