Bolts - Why Are Some Black?

[ROOODBOY]

I'm doing some work on the car this week and I went out today to get some new nuts and bolts.

I've just been sitting here looking at them and I wondered why some of the bolts are metal (zinc coated?) coloured, and some of them are black.

Just to remind you all what bolts look like;




So, anyone know what this signifies?

I'm just curious, never really thought about it before!
I've Googled it but couldn't find an answer.

[snotrag]

http://en.wikipedia.org/wiki/Black_oxide

Its a basic, easy way of providing some level of corrosion resistance, and as described in that article it (much like anodising aluminium) has minimal effect of the dimensions of your part (ie its very thin - you wouldnt 'paint' a bolt as the paint would be too thick and the bolt wouldn't work).

[Roger Dodger]

Known as 'blacking'. Its a number of liquids you dip the stuff into (4 I think).

[ROOODBOY]

Aah, I see.

Thanks!

[WhoseGeneration]

ROOODBOY said:

I'm doing some work on the car this week and I went out today to get some new nuts and bolts.

I've just been sitting here looking at them and I wondered why some of the bolts are metal (zinc coated?) coloured, and some of them are black.

Just to remind you all what bolts look like;




So, anyone know what this signifies?

I'm just curious, never really thought about it before!
I've Googled it but couldn't find an answer.

Fasteners are more complicated than they, on the surface, appear. Fasteners need to be specified for their application. Importantly, this means strong enough for the use, taking into account the stresses and environment they will endure.
Your bolts, what are you using them for? and what is the tensile strength of the two types, because zinc plating can give problems on higher tensile fasteners. Google hydrogen embrittlement.
If your bolts are for a stress bearing situation on a car you need to ensure that their tensile strength is adequate.
The heads of the bolts should be stamped with a code letter indicating the strength.

Eta, if you want to find out more, a good read is "Nuts, bolts, fasteners and plumbing" by Carroll Smith. Interesting and all about this stuff in realation to cars and, in part, frightening when he discusses counterfeit fasteners.

Edited by WhoseGeneration on Tuesday 6th September 22:45

[robsco]

I knew you'd be a TVR owner before I even looked at your profile!

[Scottish Exile]

[ROOODBOY]

They are to hold my differential carrier to the chassis (TVR 350I).

The black bolts have 'L' on the head, the silver ones have 'JD'.

The receipt I have for them does state they are high tensile, and they've got to be better than the rusty, bent and snapped ones that came off!?

[Condi]

Generally, and it is only generally, the black ones will be 10.8 or above, whereas the zinc plated ones tend to be more usual 8.8s. Not a rule, but as someone above says the blacking can be used on stronger bolts than zinc coating.

[Mojooo]

Blacks are bigger.

[carl_w]

Mojooo said:

Blacks are bigger.

LOL, that's just a vicious rumour.

[perdu]

Condi said:

Generally, and it is only generally, the black ones will be 10.8 or above, whereas the zinc plated ones tend to be more usual 8.8s. Not a rule, but as someone above says the blacking can be used on stronger bolts than zinc coating.

is right

The actual metal used for the two types can vary as well as the heat treatment that they get

use the black ones for your car

more often not realised stuff about bolts

The threads arent "cut" into the bolts they are rolled onto the blanks by dropping the blanks between two pre formed, angled blocks.

Fascinating to watch the process.

Machine rumbles - stuff happens - bolts drop into metal boxes to be replaced every "not very many" minutes

Bloody noisy too, also has a definitely industrial aroma

I was told that if the threads are "cut" they are likely to fail (surfaces damaged and tensile strength lost) which is why you really shouldn't cut a new thread down an old bolt. Not if you want to put it under a reasonable load.

[Leptons]

The black ones are High tensile. The silver ones are fine pitch machine screws so don't get the nuts mixed up! They will strip much easier than the black ones too so go steady when torqing them.