# Tool Time Simple Q: English & Metric Bolts
**URL:** https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220
**Category:** Factual Questions
**Created:** [September 2, 2014, 2:09am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220 "2014-09-02T02:09:12Z")
**Posts on this page:** 19
**Page:** 1
### Author:  [@Jinx](https://boards.straightdope.com/u/Jinx)
#### Post date: [September 2, 2014, 2:09am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/1 "2014-09-02T02:09:12Z")
I need to know what size metric bolt would be the next size up from a 5/16" bolt?
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [September 2, 2014, 2:13am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/2 "2014-09-02T02:13:54Z")
8mm
[Reference](http://www.boltdepot.com/fastener-information/US-Metric-Conversion-Table.aspx).
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### Author:  [@turner](https://boards.straightdope.com/u/turner)
#### Post date: [September 2, 2014, 2:14am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/3 "2014-09-02T02:14:21Z")
8 mm
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### Author:  [@Jinx](https://boards.straightdope.com/u/Jinx)
#### Post date: [September 2, 2014, 2:23am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/4 "2014-09-02T02:23:22Z")
Stupid question here: I follow the conversion, but is 8mm a common size for metric bolts?
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [September 2, 2014, 2:28am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/5 "2014-09-02T02:28:58Z")
> [@Jinx](#):
>
> Stupid question here: I follow the conversion, but is 8mm a common size for metric bolts?
Yes, very.
I actually bought 2 today, for my Hyundai Santa Fe.
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### Author:  [@Jinx](https://boards.straightdope.com/u/Jinx)
#### Post date: [September 2, 2014, 2:31am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/6 "2014-09-02T02:31:45Z")
Thanks! I never considered an auto supply store. I was hunting metric bolts at hardware stores (being a hardware project, not an auto repair project) which proved fruitless…hence I was clueless on common metric sizes.
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [September 2, 2014, 3:05am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/7 "2014-09-02T03:05:15Z")
> [@Jinx](#):
>
> Thanks! I never considered an auto supply store. I was hunting metric bolts at hardware stores (being a hardware project, not an auto repair project) which proved fruitless…hence I was clueless on common metric sizes.
FWIW, Ace (and Tru-value) have a much better faster selection than the “big box” hardware stores. The Ace around the corner has a full selection of English and Metric fasteners, in various strengths and in stainless steel, too.
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### Author:  [@Little\_Pig](https://boards.straightdope.com/u/Little_Pig)
#### Post date: [September 2, 2014, 3:30am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/8 "2014-09-02T03:30:00Z")
> [@beowulff](#):
>
> 8mm
>
> [Reference](http://www.boltdepot.com/fastener-information/US-Metric-Conversion-Table.aspx).
But your reference says 7.9mm . IANAN (I Am Not A Nut) but would .1mm difference in bolt size matter or is their sufficient clearance?
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [September 2, 2014, 3:34am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/9 "2014-09-02T03:34:42Z")
> [@Little\_Pig](#):
>
> But your reference says 7.9mm . IANAN (I Am Not A Nut) but would .1mm difference in bolt size matter or is their sufficient clearance?
Well, an 8mm bolt might not fit into a hole designed for a 5/16" one, but the converse will work.
I would never substitute metric for english unless it was a very non-critical application.
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### Author:  [@Isilder](https://boards.straightdope.com/u/Isilder)
#### Post date: [September 2, 2014, 11:16am UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/10 "2014-09-02T11:16:38Z")
> [@Little\_Pig](#):
>
> But your reference says 7.9mm . IANAN (I Am Not A Nut) but would .1mm difference in bolt size matter or is their sufficient clearance?
Yes it matters.
It would take that .1 mm off the threads grip.
Here’s what happened when they put the ALMOST correct size into an airplanes windshield.
> **[British Airways Flight 5390](https://en.wikipedia.org/wiki/British_Airways_Flight_5390)**
>
> British Airways Flight 5390 was a flight from Birmingham Airport in England for Málaga Airport in Spain. On 10 June 1990, the BAC One-Eleven 528FL suffered an explosive decompression. While the aircraft was flying over Didcot, England, an improperly installed windscreen panel separated from its frame, causing the captain to be partially ejected from the aircraft. He was held in place through the window frame for 20 minutes until the first officer landed at Southampton Airport.
> The County of S...
Well anyway the thread pitch may change. If its a different size it may well be a different thread TYPE… This can result in damaged threads.
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### Author:  [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)
#### Post date: [September 2, 2014, 12:24pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/11 "2014-09-02T12:24:31Z")
> [@Little\_Pig](#):
>
> But your reference says 7.9mm . IANAN (I Am Not A Nut) but would .1mm difference in bolt size matter or is their sufficient clearance?
They might fit in terms of diameter, but the thread pitch of SAE & metric are different enough that you can’t practically substitute them.
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### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [September 2, 2014, 12:34pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/12 "2014-09-02T12:34:30Z")
> [@Isilder](#):
>
> Well anyway the thread pitch may change. If its a different size it may well be a different thread TYPE… This can result in damaged threads.
That’s the biggest potential issue - the difference in thread properties will cause the thing to bind after one turn and continued attempts to drive it home will strip or shear it.
That’s if it’s a bolt into a threaded hole. If this is just one of those applications where a nut and bolt clamps two flat plates together, then it’s less critical, as long as the nut and bolt match, and as long as they are not way too small for the hole.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [September 2, 2014, 2:56pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/13 "2014-09-02T14:56:40Z")
To be fair, the OP did ask for the next size _up_.
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [September 2, 2014, 3:27pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/14 "2014-09-02T15:27:55Z")
> [@Jinx](#):
>
> I need to know what size metric bolt would be the next size up from a 5/16" bolt?
What do you mean by next size up?
An 8mm bolt is for all practical purposes the same size as a 5/16" bolt – the difference is 2½ THOUSANDTHS of an inch, which is negligible in virtually any context. While 8mm (.315") is slightly larger than 5/16" (.3125"), it’s not considered the next size up.
In English/American sizes, the next size up from 5/16" is 3/8" (.375"). In metric sizes, the next one up from 8mm is 9mm (.354"), but it’s very rare, used only in a handful of specialty applications (i.e., you almost certainly won’t find it at a hardware store or auto parts store). The common next size up is 10mm (.394"), which is not too far different from 3/8".
If you want to replace a 5/16" nut and bolt with 8mm, 3/8", or 10mm, generally you’ll be fine. The 8mm will fit the hole the 5/16" bolt goes through; to use 3/8" or 10mm the hole may need to be enlarged to accept the larger size.
If you want something to screw into 5/16" female threads, don’t. Forcing an 8mm bolt in will open up a world of trouble due to differences in thread pitch (as would putting a fine thread bolt into a coarse thread hole, and _vice versa_). The bolt is likely to jam or even break off before it gets all the way in, and if you happened to get it in the threads would be significantly damaged. If you need to go to a larger size (e.g., the threads are stripped), you’ll need to drill and tap the hole for 3/8" or 10mm.
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### Author:  [@Shakester](https://boards.straightdope.com/u/Shakester)
#### Post date: [September 2, 2014, 3:56pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/15 "2014-09-02T15:56:34Z")
Why “English”? The UK uses the Metric system as standard, as does everywhere on planet Earth except the USA, Burma, and some other third world Hell-hole that escapes me right now.
Around here, the old system is often called “Imperial”, and occasionally called “American”, but calling it “English” is new to me. Why call it “English” when the English don’t use it\* but Americans do?
\*Yes I know some English people _do_ at times use it and that pints are called pints and so on, I’m talking about standards, not exceptions.
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [September 2, 2014, 4:12pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/16 "2014-09-02T16:12:20Z")
> [@Shakester](#):
>
> Why “English”?
Because the measurements originated in England and have been used there for centuries. England may have officially adopted metric units in the last few decades, but that doesn’t change history and doesn’t change the name of the measurement system.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [September 2, 2014, 4:18pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/17 "2014-09-02T16:18:43Z")
Except that, in many cases, the units which were used in England for centuries _aren’t_ the ones being referred to. The American system is not the same as the English system. It’s similar, and it shares many of the same names, but if you mean the American system, say American.
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### Author:  [@JacobSwan](https://boards.straightdope.com/u/JacobSwan)
#### Post date: [September 2, 2014, 4:49pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/18 "2014-09-02T16:49:18Z")
> [@Chronos](#):
>
> Except that, in many cases, the units which were used in England for centuries _aren’t_ the ones being referred to. The American system is not the same as the English system. It’s similar, and it shares many of the same names, but if you mean the American system, say American.
Plus we had multiple standards in the uk including different definitions of bolt size. A 5/8 whitworth spanner will not fit a 5/8 AF bolt. We like to confuse our engineers.
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [September 2, 2014, 5:09pm UTC](https://boards.straightdope.com/t/tool-time-simple-q-english-metric-bolts/697220/19 "2014-09-02T17:09:30Z")
> [@Chronos](#):
>
> Except that, in many cases, the units which were used in England for centuries _aren’t_ the ones being referred to. The American system is not the same as the English system. It’s similar, and it shares many of the same names, but if you mean the American system, say American.
I’m not talking about comprehensive measurement systems. I’m talking about the nomenclature for nut/bolt/machine screw diameters and thread pitches. They’re measured with inches, which have been the same for ages. The system can be called English, American, or Unified. It has been called English for long before anyone called it American or Unified, and English is still a proper and correct name for it, [as shown here](https://www.dot.ny.gov/main/business-center/engineering/specifications/english-spec-repository/bolt_table.pdf?nd=nysdot). Calling it English no more means it’s used in England than calling a food French toast means it came from France.