Low head Cap Screws

[VicParticipant @vic]

[VicParticipant @vic]

Do all Low Head Cap Screws have the same size hex socket? I would have assumed so but looking on eBay some seem to have larger sockets?

**LINK**

[Ex contributorParticipant @mgnbuk]

Not sure what you mean by "larger sockets" ?

Looking at the the table of dimensions in the link supplied, the DIN screw sockets seem to be one size smaller than would be expected for a "normal" SHCS – an M5 "normal" socket is 4mm AF, where theDIN head M5 screw is shown as 3mm AF in the chart. The other sizes are the same.

Usual "rule of thumb" for a "normal" SHCS is the hex size is one down on the thread size & for button heads and CSK screws two sizes down. i.e a "normal" M6 SHCS is 5mm hex and an M6 buttonhead / CSK is 4mm hex. The DIN screws seem to follow the buttonhead / CSK format.

Nigel B.

[old martParticipant @oldmart]

I believe that the socket size is still related to the thread diameter, for hexagon or torx, of a reduced size like when a countersunk end is used. The depth of the socket is less than a normal screw, which calls for added care when tightening.

Edited By old mart on 26/03/2021 15:04:48

[JasonBModerator @jasonb]

may be larger so you don't strip out the shallow socket particularly in SS which can be a bit soft

[Bill PudneyParticipant @billpudney37759]

Google "Unbrako" and you can download the Unbrako Catalogue/Engineering Guide with more information than you will ever need to know about socket head fasteners.

cheers

Bill

[Neil LickfoldParticipant @neillickfold44316]

Some have metric sockets for metric keys, and some have sockets for imperial keys, even though it is a metric screw. Likewise some imperial screws come with metric sockets.

[VicParticipant @vic]

It seems the answer to this question is nobody knows.

In this instance I’m particularly interested in M5 socket screws. Standard cap head socket screws take a 4mm hex driver. Both low head and countersunk screws of the same size (M5) take a hex driver one size smaller – 3mm.

I just happened to notice in the pictures shown on eBay that some low head screws seem to have a socket for the standard size driver, or at least noticeably bigger than the others. I have two choices I suppose, buy a small quantity and see what arrives or ask the vendor and trust in the reply!

low head socket screws that accept standard drivers for their metric thread are an obvious advantage to me in that I don’t need to keep a separate hex driver to hand.

Thanks to all that replied.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Vic on 27/03/2021 14:19:50:

[…]

In this instance I’m particularly interested in M5 socket screws. Standard cap head socket screws take a 4mm hex driver. […]

I just happened to notice in the pictures shown on eBay that some low head screws seem to have a socket for the standard size driver, or at least noticeably bigger than the others. […]

.

It would be interesting to see one of those sectioned longitudinally … I suspect that either the sockets are not very deep, or the heads are at great risk of shearing off.

MichaelG.

[VicParticipant @vic]

I’ve done a Google image search for Low Head Socket Screws and out of dozens of images only one or two appear to have what you may expect to be an over size socket. I’m guessing the images on eBay are something of an anomaly. As Michael says they may not be that strong even if available. I’ll make do.

[Nigel BennettParticipant @nigelbennett69913]

We used them occasionally at work. They had smaller and shallower sockets than the standard ones and were also only available in grade 8.8 as opposed to 12.9 for standard.

[DMBParticipant @dmb]

Nigel,

What do you mean, please? Does grade 8.8, 12.9 refer to tensile strength?

John

[Jeff DaymanParticipant @jeffdayman43397]

Short explanation – yes the 8.8 and 12.9 numbers are the strength rating. 8.8 is a medium strength screw and 12.9 is a high strength screw.

Low head metric capscrews are often but not always made to DIN 7984 standard. DIN standards are the same worldwide, obviously, but fastener makers do sometimes deviate from them when it suits them, particularly some less virtuous Asian makers, so unless a particular fastener from a reputable distributor is stated as being made to DIN XXXX it may not be.

If I were OP I would stop looking at Ebay for fastener engineering info and instead look at manufacturer's or distributor's datasheets. A handy and free resource is McMaster Carr in the US. You don't have to buy from them but you can look at data on just about any fastener, in detail, on their website. If you click on a part number all specs and a product drawing appear.

Example at link below for low head socket head capscrew.

https://www.mcmaster.com/93070A123/

[Mike PooleParticipant @mikepoole82104]

Unbrako were on free issue in the tool room so we sparkies used them because they were handy. One thing you noticed was the quality of fit of a Unbrako key in the socket, the key was a nice sliding fit in the socket. Unbranded cheap cap screws often have a pretty indifferent fit and will soon damage key and socket if pushed to maximum torque for the screw. Many fastners have huge strength reserves for the application they are used for but some applications require a full spec fastner to perform safely. Big end bolts on a performance engine are usually close to the limit of their specification and the advice to replace during a rebuild is not to be ignored, unless you are good at jigsaws and like the challenge of bucket full of oily bits.

Mike

[DMBParticipant @dmb]

Thank you, Jeff.

John

[Steve AddyParticipant @steveaddy35670]

Thanks for mentioning these fasteners, I'd never heard of this head type before.

Steve

[Bill PudneyParticipant @billpudney37759]

Big end bolts and nuts. Mike Pooles comments about replacing them is right on the money. They are generally the most highly specified part in a motor. Material, heat treatment, surface finish, tolerance regime, post manufacture handling and storage etc etc

Socket head screws, "name" manufacturers like Unbrako, Holo Chrome etc are manufactured to very high specs and their fasteners are of uniformly high and reliable standards. Personally I wouldn't trust non branded fasteners as far as I could spit.

cheers

Bill

[MeunierParticipant @meunier]

Posted by Mike Poole on 28/03/2021 15:40:22:

/…. Big end bolts on a performance engine are usually close to the limit of their specification and the advice to replace during a rebuild is not to be ignored…./

Mike

Something which caught me out on the starter-motor on MrsD's mk1 Cortina in the early '70s. Go to start and Brrrrrp/Brrrrp/Brr-engage and start. I became quite adept at replacing the Bendix gear on the motor, 20mins job.
Wasn't until many years later I read a piece which mentioned that the motor-mounting bolts were 'stretch-fitted' on original assembly and after removing/re-using could not be relied upon to not stretch further in service.
DaveD

[Tim StevensParticipant @timstevens64731]

Low-head socket screws tend to be used in cases where the full strength is not required, and there are problems of access, appearance, etc. The same applies to hex button-head screws – pretty but can't be done up so tight. Remember that although the socket can be bigger, there is less around the edge for the key to drive.

Now a question: Have hex socket nuts disappeared completely?

Cheers, Tim

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