Why is 20mm such a very rare nut & bolt size..?

Rico. · May 23, 2012

I was having a little read through the Hanson socket tray thread and some of you guy's
were talking about how you have to fill in all the gaps even if you never use that particular
size of socket.

The thought occured that in my whole life I have never come accross a 20mm fastener.
I have seen a million 17mm, a few 18mm, loads of 19mm, loads of 21mm and a few 22mm
but not one 20mm.... Ever.

Now there will be a few of you that have seen 20mm fasteners here and there, but my
question isn't what uses 20mm fasteners.... rather why is it such a rare size when all the
other sizes around seem so common in automotive applications..?

Is it just one of those mysterious engineering things that has evolved like this over the
years and decades. or does anyone have a genuine answer.... Interesting none the less.

Chers Chaps.

shoturtle · May 23, 2012

What is even harder to find is 23mm and 25mm, 20mm more common of major construction then in automotive and diy stuff.

MBfreak · May 23, 2012

Hi.
If you work with old MB´s ( 50-60´s ) you will use 20 mm quite often for the banjo bolts in the cooling system- which is quite extensive on the fuel injected M127, M19 and M130 engines.
16 and 18 are the new " 17 and 19 " mm on modern ( well for me at least, ie 90´s) BMWs

Summer has hit mid Sweden, happy as clam! Hope it lasts for at least a week!

Ola

Fedwrench · May 23, 2012

I have never used a 20mm socket. 20mm wrench, yes, some GM fuel filters, ac lines, etc.
I personally feel it's a size built simply to satisfy some people's need not to skip a size.It's a true dust collector on that hansen peg. :wtf:

As for 23mm, my blue quick release prevost air fittings are 23mm.
I know to many Japanese made vehicles 15 and 18mm are also orphaned sizes.

shurik06_83 · May 23, 2012

I hit a few 20mm a few 23mm if u work in a shop that all kinda sttuff comes threw u want to be ready but if u work on ur own car then u don,t need it

I never in my life could have guessed i would ever need 30mm 12pt socket one day toyota or some **** came in 32mm 12pt vw audi

plinker · May 23, 2012

I've used a 20mm socket on screwed up/ rusted wheel nut's.

Harwinton · May 23, 2012

20mm probably isn't too common since it's real close to a very common size, 3/4" (19mm).

85FourEyedGT · May 23, 2012

when the chrome caps fell off my lug nuts, the nut went from being a 21mm to a 20mm lol kinda sucked

Super Mech · May 23, 2012

20mm on GM fuel filters not much else I can think of. How about that 9mm socket collecting dust wtf?

chris142 · May 23, 2012

The oil cooler lines on the newer Gm cars and trucks require a 20mm wrench. I use mine daily.

NC-Fordguy · May 23, 2012

Cross member bolts on 1990s F-250 and F-350 with ZF-5 transmission. At least 20 mm fit better than 3/4

nyrapscalion · May 24, 2012

I have a devil :evil:

of a time finding 21mm sockets in 6 pt. Both cars have (Nissan & Mazda) use 21 mm lug nuts. A lot of sets have 18, 19, 20, 22, 24.
Of course if you ask in Sears for a 6 pt 1/2" drive 21mm socket, she needs to go find her manager. No, I'm working on a global list of "to buy" and there's a spot for them.

Big Johnson · May 24, 2012

chris142 said:
The oil cooler lines on the newer Gm cars and trucks require a 20mm wrench. I use mine daily.

High-desert huh? Your work at the Gm dealer in Victorville? Im right next door in the decrepit city of Adelanto.

Big Johnson · May 24, 2012

nyrapscalion said:
I have a devil of a time finding 21mm sockets in 6 pt. Both cars have (Nissan & Mazda) use 21 mm lug nuts. A lot of sets have 18, 19, 20, 22, 24.
Of course if you ask in Sears for a 6 pt 1/2" drive 21mm socket, she needs to go find her manager. No, I'm working on a global list of "to buy" and there's a spot for them.

Harbor freight has 21mm impacts for $1.97 You can buy the whole set with 21mm from them for less than $30

blacK20 · May 24, 2012

I use my 20mm socket when 21mm lugs start to round off. Keep one around. One day it'll get you out of a bind.

warpedredneck · May 24, 2012

20mm, headbolts on a C7 Cat

Rico. · May 24, 2012

blacK20 said:
I use my 20mm socket when 21mm lugs start to round off. Keep one around. One day it'll get you out of a bind.

I actually keep 4 around

3/8" drive 6 point and 12 point and the same for 1/2" drive.

I've always been a better to have it and not need it kind of chap.
It's the GJ way... :thumbup:

Hesketh · May 24, 2012

I did struggle trying to track down a 20mm deep impact socket to fill the tray. Still looking for a 23mm and a 25mm impact’s I know I shall likely never use these but as has been said above you like the tray to look nice and it is easy to spot when you have left a socket somewhere

I have as good as given up on the 25mm standard short socket and juts put in a 1” one

Rico. · May 24, 2012

Hesketh said:
I have as good as given up on the 25mm standard short socket and juts put in a 1” one

Hesketh, there are well over a hundred single 25mm sockets for sale on ebay uk right now.
Everything from your cheap but good Sealey and Draper stuff, to Teng, Elora and Stahlwille
at the upper end. You should find something to fit your needs (wants) :thumbup:

Skin · May 24, 2012

nyrapscalion said:
I have a devil of a time finding 21mm sockets in 6 pt. Both cars have (Nissan & Mazda) use 21 mm lug nuts. A lot of sets have 18, 19, 20, 22, 24.
Of course if you ask in Sears for a 6 pt 1/2" drive 21mm socket, she needs to go find her manager. No, I'm working on a global list of "to buy" and there's a spot for them.

21mm is a fairly common size found in wheel lug sockets...

Hesketh · May 25, 2012

Rico. said:
Hesketh, there are well over a hundred single 25mm sockets for sale on ebay uk right now.
Everything from your cheap but good Sealey and Draper stuff, to Teng, Elora and Stahlwille
at the upper end. You should find something to fit your needs (wants)

I have to put my hands up, and say that I did not think / look on e-bay. I have now found the 25mm & 23mm deep impact socket which were missing.

Happy Days and Thank you. :thumbup:

geologist · May 25, 2012

nyrapscalion said:
I have a devil of a time finding 21mm sockets in 6 pt. Both cars have (Nissan & Mazda) use 21 mm lug nuts. A lot of sets have 18, 19, 20, 22, 24.
Of course if you ask in Sears for a 6 pt 1/2" drive 21mm socket, she needs to go find her manager. No, I'm working on a global list of "to buy" and there's a spot for them.

Craftsman 1/2" drive 6 point metric sockets only go up to 22mm, and it took me 3 tries to find the 21 and 22mm to complete my set (9mm - 22mm).

vintagefan · May 25, 2012

Big Johnson said:
High-desert huh? Your work at the Gm dealer in Victorville? Im right next door in the decrepit city of Adelanto.

Strange, I'm in Adelanto too. I'm going to post a tutorial pretty soon on how to patch up bullet holes in your garage door (no I'm not kidding, about the bullet holes that is).

Anyways, on topic, I've never used or seen a 20mm, ever. Of course, I don't do auto, I'm in industrial, so maybe that's why?

edit: WTH, is this the high desert reunion thread or something?

chris142 said:
The oil cooler lines on the newer Gm cars and trucks require a 20mm wrench. I use mine daily.

Rico. · May 25, 2012

Hesketh said:
I have to put my hands up, and say that I did not think / look on e-bay. I have now found the 25mm & 23mm deep impact socket which were missing.

Happy Days and Thank you.

Splendid.... Nice one.

You can always rely on members here to help you spend your money on tools... :evil:

SMKS · May 25, 2012

I seem to remember using the a 20mm socket when I replaced the timing belt on my girlfriend's VW Beetle, but I can't remember what for.

Perhaps I'm misremembering or blocking out the memories because that job was such a PITA.

Why is 20mm such a very rare nut & bolt size..?

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