For example, if chain of our two wheeler got loosed, then we try to open the cover of chain. If we have the kit provided by company showroom we can remove easily by perfect wrench by trial and error method but if we lost that kit provided by company, first we need to determine perfect wrench size to remove the nut of particular bolt or else we need to consult technician. Or some people used to keep kit by their own. Then we need to know how to determine the perfect wrench size quickly. Before we going to know the solution we need to know some of terminology of bolt and nut.
Guys above image clearly explain the terminology of bolt. Now,
The perfect wrench size: How to determine it?
Guys now calculate the perfect wrench size by measuring it's major diameter. To measure the major diameter we can use Vernier caliper or you can measure it roughly by inch marks of your fingers.
Now the formulas is 1.5 times of major diameter.
Mathematically 1.5*d.
For example M16 is the bolt size. M stands for metric and 16 stands for 16mm major diameter or body diameter. Now calculate the perfect wrench size to remove the nut from bolt.
1.5*16=24mm. So wrench size is 24mm.
Whatever the wrench size we got by above expression is equals to hexagonal head diameter of bolt or nut.
Like this we can find out for Allen bolts also.
Like as wrench for Allen bolt we need to find out Allen key it's also in known as hex key.
How to determine perfect Allen key ?
Here is the formula, Allen key size is equals to major diameter - 2.
Mathematically (d-2). This is general formula, not suitable all Allen keys.
For example M16 bolt Allen key size is equals to 14mm. Like this M8=6mm, M10=8mm, M12=10mm,M12=10mm, M14=12mm, M16=14mm. But some of these are different these are M3= 2.5mm, M4=3mm, M5= 4mm, M6=5mm, M16=14mm, M18=14mm, M20=17mm, M12=17mm, M14=19mm. Why like this means, these are according to ISO standards.
Conclusion
By using above two methods, we can find the perfect wrench size and Allen key size quickly. It may help to reduce the time to repair or own mechanical things at home. This is not for professional technicians. They knew wrench size perfectly by their practice.

