rb1957...
"...Sure, if in overhaul the screw is removed and the hole checked (for wear, corrosion, etc)... ...but if it is within tolerance why not reinstall the original size fastener ? ".
Generally speaking, I have no-problem with the concept of 'reinstalling the original size fastener' for a joint that is normally intended to be removed/disassembled for maintenance [typically class 1 or 2 fit already]... or is associated with a mechanical installation or assembly. Where this get unbelievably out-of-whack is when You realize that the 'same bolt' has probably been removed and reinstalled so many times that the head recesses and/or the threads are worn-out... as well as the mating nutplate/nut threads... and self-locking torque is fundamentally non-existent... and no one cares.
The practice of 'reinstalling the original size fastener' becomes 'sketchy' for fastener installations intended to remain as permanently assembled structure [close-tolerance, transition fit or interference fit fastener-to-hole]; especially when the joint is disassembled to facilitate other maintenance or repairs; and is ill-advised if new parts are mated to the old parts using the same fastener pattern. When 'hard fasteners' [bolts, lock-bolts, blind-bolts etc] are removed from these holes, damage to a significant percentage of these precision holes is inevitable and would be hard to inspect for; and then attempts to match-drill/ream new parts to these old hole, same diameter, WILL ABSOLUTELY blow-them out... or at least that is what my experience and various durability studies have proven. Not to mention that if the NDI lower limit of crack detection is, say 0.050, and the hole NDI's clean; then durability guys will tell you that the MUST assume a 0.049 flaw still exists. However if You oversize a hole, say 0.016 [1OS], then the flaw size can be 'presumed' to be 0.041 [0.016/2 smaller size] which is somewhat better for crack growth.
BUT...
I have violated this practice on a few rare occasions where NDI/DI of the hole 'looked good'...and oversize's were simply unavailable and next size larger was inadvisable. On the other hand I have jumped thru hoops a few times for 'non-standard oversize's' such as: (a) manufacturing 2OS solid sleeves that are shrink fit on fastener shank, then finish lathe-turned to precision [sometimes odd] ODs for a tight-match with OS-reamed-out hole; or (b) using the ACRES Fastener sleeves [MIL-S-85069/* and MIL-HDBK-271/105]. Also, I have removed and reinstalled the same Taper-Lok bolts in the same tapered holes for similar reasons and necessities [except TLs are worse]... but with TL unique special 'practices/criteria'. In all these cases I 'knew the rules so I could safely break the rules'.
BTW I prefer different definitions for Bolts and Screws.
Bolts and Screws can have any head style and any size or thread type.
Bolts have bare/precision shanks with minimized thread length and are generally intended for shear and tension loads. Because they have smooth shanks the shank Dia can be anything the manufacturer intends, Nominal, 1OS, 2OS, 3OS and stepped/shouldered; and tapered shanks [Nominal, 1OS and 2OS, what a nightmare].
Screws generally have fully-threaded, or mostly-threaded, shanks and are intended for tension loads with incidental shear thru the threads [not enough shank anywhere to transfer significant shear loads]. Screws NEVER have any oversize's associated with them [standard thread sizes only].
OK there are special bolts/screws, such and 'Cap-Bolts', Cap-screws and the family of tension bolts with necked-down shanks but... these have special mechanical [not structural] uses.
Regards, Wil Taylor
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