what i mean is, i never used to really see the point of allen key bolts, but they really do look better than normal hex bolts, so i've started using them wherever i can, but should i be using them for things like engine mounts?

silly question perhaps, but is there any right or wrong time to use them?

Most allen bolts are 10.9 standard IIRC, therefore often better than most standard hex as they tend to be 8.8's unless otherwise specified.

Steve

Sometimes a particular head type makes access for tools easier (e.g. when the head is in a counterbore you need allen or Torx, if there's a restriction above the head preventing access with a socket or allen key then hex may be better as you can still get a spanner on), but otherwise as long as they're the correct grade (or better) then it doesn't really matter.

cool. allen bolts it is then. there's some nice 12.9 grade zinc plated steel bolts on ebay. i'll get some of them. i guess i should use spring washers on them too?

When I used work back in the UK designing newspaper printing equipment we would get bollocked for arbritarlily calling out hex socket (Allen) fasteners when regular hex heads would work fine...it seems there was quite a cost difference when multiplied by several hundred.

Not that this helps you at all...I'm just lonely today and felt like joining in....

12.9 zinc plated bolts can't exists. I've been told that it is technically almost impossible to guarantee the 12.9 strength when it is zinc plated. I've seen 12.9 bolts that were Boromite plate (?) and they had a bit of vage grey color, and weren't smooth finished. These were also very expensive!

quote:

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Originally posted by AlphaX
12.9 zinc plated bolts can't exists. I've been told that it is technically almost impossible to guarantee the 12.9 strength when it is zinc plated. I've seen 12.9 bolts that were Boromite plate (?) and they had a bit of vage grey color, and weren't smooth finished. These were also very expensive!

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The problem with zinc plating hi-tensile bolt's is hydrogen embrittlement - bubbles form under the plating , causing stress points that seriously weaken the bolts and do cause sudden failure ( I found this out because I used to have all my bike bolts chromed , and I had a lot of engine mount bolts fail...)
The cure is low temp heat treatment over a long time - low enough not to damage the bolts or plating )
That's how I understand it , anyway

i got the steel ones that are black in colour. don't know why they're black. but they are stamped 12.9 so should be plenty strong enough

can i use them for bolting manifolds too

i suddenly love them for some reason

what you have got to watch is the fasteners your replacing - the original spec will have a torque value - dont forget to torque it up with a allen socket
might be something you already know but bolts when torqued correctly are under elastic tension - IE they are stretched - this stretch will 'clamp' the part its holding so it aint coming loose - you will find bolts such as 12.9's will be used for really high and vital fasteners - such as flywheel bolts, prop bolts, driveshaft flange bolts.. that sort of thing.. suspension bolts can be 8.8 with no problem - its a matter of using the right torque for the right application.. very important to not have stuff like wishbone bolts falling oot..

interesting question.

flakmonkey should know the answer?

one instance that I can think of where you would want to use a cap head is if you are fixing a box section - the hole in the outer section needs to be just big enough for the cap head to pass through whereas for a hex head, it needs to be much bigger so that the socket can get past.

Just be aware the black ones will rust in a moment.

I use black, but have them zinc plated once any shortening is done.

I know that that is not correct for ultimate strenght but most bolts in a chassis are so overkill for the application is doesn't seem to be a problem

Cheers

Fred W B

quote:

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Originally posted by blakep82
i got the steel ones that are black in colour. don't know why they're black. but they are stamped 12.9 so should be plenty strong enough

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^^^^ self-coloured.

painting them stops rust.

got some nice gloss black paint waiting
was going to paint the block today. engine mounts need painting anyway

12.9 cap heads are overkill for most applications in our vehicles, with the exception of perhaps flywheel bolts, they do look nice though. (Even then these are normally 10.9).

There is however no problem using them.

With regards to torquing fasteners correctly. The quoted max torques for fasteners arent always suitable to use. If they are into cast iron or mild steel you will rip the threads out of the part. However if they are into the appropriate nuts then you can torque them up to the max if the application calls for that much clamping force.

The max torque is based on stretching the bolt to approx 70% of its yeild strength. At this point you are getting massive clamping force (ideal for suspension pivots etc) which is very easy to calculate. At torques lower than this you are getting steadily less clamping force. Not all applications need the highest force a bolt can give (such as gasketed joints etc).

In essence use whatever bolts you like, as long as they are at least the same tensile strength as the ones they are replacing, and torque them to the manufacturers recommended specification for that application.

Stainless bolts are not suitable in highly loaded applications, not due to their tensile strength, but because stainless steel suffers heavuily from fatigue at loads well below its yeild point. Steel bolts do not suffer the same problems.

As for colouring. Self colour (the black ones) rust quickly as all the black is is burnt on oil from the heat treatment process. Zinc (BZP) are best as they wont rust but as others have mentioned there is the embrittlement issue. However most bolts on our cars are so over specced this is never going to be an issue.

Lastly, dont be tempted to put antisieze on bolts before torquing them up, unless it is specifically requested, as this increases the preload you are giving which could cause issues in some cases.

David