I got stuck talking to an old man at Home Depot today
 

One of those old farts cornered me and I couldn’t make a run for it.

However, he gave me a great piece of information that I would like to share.

How to figure out the wrench size of a bolt from the thread diameter.

Metric: Multiply the diameter of the threaded part of the bolt by 1.5 and add 1mm
Ex. 6mm bolt x 1.5 + 1mm = 10mm wrench

Standard: Multiply the diameter of the threaded part of the bolt by 1.5 and add 1/8
Ex. 3/8 bolt x 1.5 + 1/8 = 3/4in wrench

Thats cool.................first time I ever heard that before.

Don't rely on that too much....
 

Because the formula doesn't seem to work for standard bolts.

Examples - a 1/4 inch bolt takes a 7/16 wrench. The formula listed would tell you to use a 1/2 inch wrench.


A 3/8 bolt takes a 9/16 wrench, not a 3/4 inch wrench as you have listed. By the formula, a 3/8 bolt would need an 11/16 wrench, not exactly 3/4.

So much for free advice.

SteveM.

I think your arithmetic in your second example is flawed.

Aside from that, have you checked this guy's suggestion to see how often it comes up with the right answer?

(Edited to add: I see that Jetmugg has already pointed both of these out!)

I haven't verified that what they present is correct, but you might look at these charts for metric and fractional inch information.

Hey wait a minute!
What exactly constitutes an "old fart"? :eek:

Perhaps another formula is needed, eh?
 

This old fart has noticed 8mm bolts are most often 13mm across the flats.
So: 8mm x 1.5 = 12mm. Plus 1mm = 13mm.

That sure works on German tin but the Japanese use 8mm bolts that are 12mm between the flats. :eek:

So, by the formula>>>>> 10mm X 1.5 = 15mm. Plus 1mm = 16mm.

Oops. most 10mm bolts are either 15mm or 17mm (most common) between the flats.

Hunt the old fart down and inform him that his dog won't hunt. ;)

Well its too late for any serious thought......but I guess you guys explained why I haven't heard that one before....becasue it doesn't work.

Remember today because sooner than you think YOU will be the old fart. ;)

I'm 43.........to some people I am an old fart already.

thats funny
I didn't think to even test what he told me or check the math
I made the age old mistake of beliving a crapsman.

Yeeeesh..........

I'm 46 with 3 kids and 1.5 Grandkids...........

Just broke my left wrist snowmobiling trying to stay young.

So much for wrenching on the MB for a little while.

Stephen (aka Snuzzy Tumblelina)

Here is another formula for you:

If the difference between 2 numbers is one, the difference between what you get when you put them side by side and then reversing the order is 9

12 and 21
23 and 32
34 and 43

I may be wrong but wouldn't it be easier to use a caliper to determine the size of nuts and bolts? I am a maintenance mechanic in an alumina plant and come across all kinds of nuts and bolts. I have found out over the years that a caliper in my back pocket will save a lot of trip to the tool box.

This advice from an 'old fart' of 62.

I have a question.

If you have a bolt in one hand and a ruler in the other what part of the bolt are you going to measure to see what size wrench it takes??

Danny

The head across the flats...