Metric coarse threads confusion

[choochoo_balooParticipant @choochoo_baloo]

[choochoo_balooParticipant @choochoo_baloo]

A beginners question:

I'm trying to identify a bolt that I bought a while back, that if I recall, was M6.0 x 1.0

The thread gauge confirms it's 1.0mm BUT I measure the major diameter as 5.70mm

This does not match any entry in my Presto data book (highlighted in picture). Is this due to poor tolerance on the bolt? Have also attached a photo of the bolt head in case it is useful.

Would appreciate clarification, thanks in advance.

 

Edited By choochoo_baloo on 11/09/2017 21:11:31

[duncan webster 1Participant @duncanwebster1]

Bolts often are a tad undersize on OD, if it fits into an M6 nut without rattling too much stop worrying

[EmgeeParticipant @emgee]

As Duncan says not unusual, could be a rolled thread as most are with poor quality control, if you want better fits search for a reputable stockist, preferably one local to you so you can check fits in store.

Emgee

[John ReeseParticipant @johnreese12848]

Even when making your own bolts using a die for threading it is customary to make the OD slightly undersized reduce the torque needed for threading. The critical diameter is the pitch diameter. Most of us are not equipped to measure PD.

[peak4Participant @peak4]

I've found it's often the case that cheap and cheerful stainless bolts are undersize.
For the price that folks like Toolstation charge, I've not let it bother me too much, though I'm a bit careful where I fit them on the motorbikes.

Bill

[Nick HulmeParticipant @nickhulme30114]

What grade was the bolt bought as?

[Keith LongParticipant @keithlong89920]

The table on this page shows the max and min diameters for NOMINAL metric bolt sizes. I believe that the reduction from nominal size is to allow a metric bolt to pass through a drilled hole of the same nominal size to ensure a closer fit than the often recommended clearance sizes which often tend to be a bit generous.

[not done it yetParticipant @notdoneityet]

One bolt, one cheap and nasty bolt? Not necessarily representative of bolts in general. But do remember that nuts and bolts are not completely interference fit, by any means.

Ridges and valleys of nuts and bolts must not interfere, so are cut/rolled appropriately. Yours appears to be outside the accepted standard, if measured to an apprropriate accuracy. Just check the thread form diagrams.

Cost (particularly buying cheap) is often a measure of of compliance with the accepted standards.

[Neil WyattModerator @neilwyatt]

It's nearly 4-thou out of spec for M6.

[jimmy bParticipant @jimmyb]

**LINK**

all the info you'll need for metric threads

[Clive Brown 1Participant @clivebrown1]

AFAIK, APL is a well regarded manufacturer of stainless steel fasteners, so the OP is maybe unfortunate in having a "rogue".

FWIW, I've just measured an M6, A2 set bolt at 5.9mm dia.

[roy entwistleParticipant @royentwistle24699]

Is it me or do other people find that the old Whit. BSF UNF etc nuts and bolts were better fits than metric ones ?

Roy

[HopperParticipant @hopper]

ISTR something in Tubal Cain's ME Handbook about thread depths etc and you can go down to something like 65 per cent thread depth engagement without significant loss of strength. So a few thou off the OD is nothing to worry about.

[HopperParticipant @hopper]

Posted by roy entwistle on 12/09/2017 09:12:20:

Is it me or do other people find that the old Whit. BSF UNF etc nuts and bolts were better fits than metric ones ?

Roy

Depends. If you compare the mid quality nuts and bolts used on old cars, motorbikes and machine tools with the cheap, low quality nuts and bolts in today's hardware stores, I'm sure there's a difference. Probably was back then too.

I've come across plenty of stripped out BSF, UNF etc fasteners on old motorbikes over the years so I don;t think they were anything too special.

[LambtonParticipant @lambton]

Out of interest I have just conducted a short survey of the various M6 sets and bolts that I have using a normal digital calliper.

Seven different types of good quality items including cap heads, HTs A2 stainless average thread diameter 5.84mm ranging from 5.78 to 5.89

Several non-engineering M6 bolts including gutter bolts, coach bolts and bog standard milt steel hex head all remarkably consistent at 5.7 mm diameter.

Eric

[MuzzerParticipant @muzzer]

You need to look into thread tolerances, rather than talk about "M6". For commercial applications, you'd specify a thread tolerance on both nut and bolt. For stuff we buy for hobby use, often the concept isn't even recognised.

You can get perfectly acceptable tolerances on rolled threads – hardly surprising, given that the vast, vast majority of fasteners are made this way.

Typically for a half decent combination you'd want a 6H for the nut and a 6g for the bolt.

Lots of stuff out there, although be aware that, as ever, the Mercans use a different nomenclature system to the rest of the world. We use ISO for metric of course.

Boltscience

Roymech

Murray

[JonParticipant @jon]

Posted by roy entwistle on 12/09/2017 09:12:20:

Is it me or do other people find that the old Whit. BSF UNF etc nuts and bolts were better fits than metric ones ?

Probably made in the days of when Britain ruled the world with far higher standards.

I do come across UNF X 20 often and quite alarming at whats produced the last 20 yrs from no go to ultra sloppy. All fasteners will have or should have a tolerance theyre made to and theres only one Euro company that produces screws, the rest are from china.

Good video on youtube with Unbrako, must just do specials now.

[Author]

Posts