How tap drill sizes are actually calculated, what thread engagement percentage means, and why the charts land near 75% instead of 100%.
ISO 68-1ASME B1.1ISO 273
Guides · 8 articles · free
Thread & fastener guides
The reference tables tell you what a thread measures. These guides explain how the numbers are arrived at and what to do with them - every worked figure is quoted from the same generated database that drives the size pages, and links back to the size it came from.
What actually changes when you go from a coarse thread to a fine one - stress area, helix angle, assembly speed - and which to pick for a real joint.
ISO 261ISO 262ASME B1.1ISO 898-1SAE J429
A repeatable method for identifying an unknown bolt or tapped hole: measure the diameter, measure the pitch, check the form, then match a standard size.
ISO 261ASME B1.1ASME B1.20.1ISO 228-1
What SAE grade marks and metric property classes mean, how strength changes with diameter, and the reasons two torque figures for one bolt disagree.
ISO 898-1ISO 898-2SAE J429
What the tolerance classes on a thread drawing mean, how Unified 1A/2A/3A compares with metric 6g/6H, and when a tighter class is worth specifying.
ASME B1.1ISO 965-1ISO 68-1
Why tapered pipe threads seal on the flanks and straight threads seal on a gasket, how NPT, BSPT and BSPP differ, and why they must never be mixed.
ASME B1.20.1ISO 7-1ISO 228-1
How stripped and galled threads actually fail, when a wire insert or solid bushing is right, and what stepping up to the next size really costs.
ISO 261ASME B1.1ISO 273
Why engagement length matters more than thread percentage, how material choice and bolt grade change it, and how deep a blind hole really has to be.
ISO 898-1ISO 4032SAE J429
Where to start
If you are sizing a hole, start with the tap-drill guide and then use the tap-drill calculator for your specific size. If you have a fastener in your hand and no idea what it is, read the identification guide and feed your measurements into the thread identifier. If you are chasing a torque figure, read the grades guide first - the reason charts disagree is almost always the friction assumption, which the torque calculator lets you change.
Every guide is free and needs no account. Torque figures quoted anywhere on this site are estimates that depend on a stated nut-factor assumption - always verify against your fastener manufacturer's data and the engineering specification for your joint.