Stainless fixings in UNF

scottonthefen · Nov 2, 2018

Is it my imagination, or is there a reason I can't find stainless fixings with a UNF thread? They all look UNC.. I've looked on accu, westfield fasteners, gs-components.. what am I missing? It's not a plating is it, stainless is the makeup of the steel right?

Discokids · Nov 2, 2018

It's not playing, you're correct, it is the makeup of the steel. A stainless bolt is made of stainless steel. Coated bolts are mild steel with zinc or whatever coating.
What are the bolts for? Stainless has different properties that affect stretch + shear strength, poor quality stainless bolts/nuts can lock up solid - I find this a lot on flanges at work on 12mm stuff.

scottonthefen · Nov 2, 2018

They are to hold some aly plates over a section of corrosion (rear fuel tank outrigger bracket to rear tub). I want them to be stainless so they don't react with the aluminium. Would UNC be ok? Is UNC more prone to coming undone? I think my grasp of when to use UNC/UNF is lacking..

scottonthefen · Nov 2, 2018

accu customer service say they only do fine pitch threads in metric from M10, so that would be why I couldn't find it. (I don't know if it's for technical reasons, or for lack of demand.)

Discokids · Nov 2, 2018

If they're being used in a fresh application (not screwed into an existing thread) why not just use metric? I've very little experience with funny threads like unf etc, obviously a finer thread means you can apply more torque to a fixing, put it under more load. Head bolts, suspension, tow bar and many other applications where the bolts need to be ****ing tight (technical term) use such threads.
I'm more interested in your application - sounds like a temporary bodge? If so, just use whatever you can find.

Turboman · Nov 2, 2018

Discokids said:
If they're being used in a fresh application (not screwed into an existing thread) why not just use metric? I've very little experience with funny threads like unf etc, obviously a finer thread means you can apply more torque to a fixing, put it under more load. Head bolts, suspension, tow bar and many other applications where the bolts need to be ****ing tight (technical term) use such threads.
I'm more interested in your application - sounds like a temporary bodge? If so, just use whatever you can find.

You definitely don't want to be using regular A2 stainless in high torque applications, it is quite weak.

As you say, I think I would just use mild steel Metric nuts and bolts to bodge a body support. I am not sure if stainless is any better in contact with aluminium than mild steel, and if it is a major issue he could always smear mastic all over the ali where the bolts will go through.

scottonthefen · Nov 2, 2018

Yeah.. I just learnt UNF is an imperial thread desingator, as in threads per inch, whereas metric uses thread pitch. Anyway, accu say they don't do fine thread pitch below M10.

I'm quite happy to use metric, and it seems like a coarse thread pitch would be fine for this.

Application thread: https://www.landyzone.co.uk/land-ro...l-tank-outrigger-bracket.336815/#post-4381427

Yes, I suppose it's a temporary bodge - but not because I don't have time or don't want to do something better. I'm just unclear what "better" would look like. If you have a better idea once you've read the thread I'm all ears.

Discokids · Nov 2, 2018

Turboman said:
You definitely don't want to be using regular A2 stainless in high torque applications, it is quite weak.

As you say, I think I would just use mild steel Metric nuts and bolts to bodge a body support. I am not sure if stainless is any better in contact with aluminium than mild steel, and if it is a major issue he could always smear mastic all over the ali where the bolts will go through.

I believe that's why fine thread ss is hard to come by.

scottonthefen · Nov 2, 2018

Turboman said:
I am not sure if stainless is any better in contact with aluminium than mild steel

I thought was generally true.. that mild steel reacts and corrodes where it touches birmabright, so people tend to replace steel fixings that touch the body with stainless ones?

Turboman · Nov 2, 2018

scottonthefen said:
I thought was generally true.. that mild steel reacts and corrodes where it touches birmabright, so people tend to replace steel fixings that touch the body with stainless ones?

I never used to, just plenty of Dum Dum between the steel and the alloy. Or you can make plastic gaskets from an old oil drum or similar.

How long are you expecting this to last, anyway? And can't you just get a new body support?

scottonthefen · Nov 2, 2018

I'm fitting a new outrigger and bracket. The tub has corroded where this contacts. I've never had a rear tub off, I don't know if I can easily replace that bit where it's corroded, or if it's part of a much larger section?

Turboman · Nov 2, 2018

scottonthefen said:
I'm fitting a new outrigger and bracket. The tub has corroded where this contacts. I've never had a rear tub off, I don't know if I can easily replace that bit where it's corroded, or if it's part of a much larger section?

Probably much easier to weld the outrigger nicely if you take the tub off anyway.

Wimblowdriver · Nov 2, 2018

Turboman said:
I am not sure if stainless is any better in contact with aluminium than mild steel,

Stainless steel in contact with aluminium will lead to corrosion. This can get really bad in winter road conditions - wet and salty.
If you really wanted UNC / UNF threads, search for triumph motorcycle parts (classic / meriden built but not the modern hinckley built) they used those threads from late 1960's until mid 1980's. They are available in stainless if you need them.

Turboman · Nov 2, 2018

Wimblowdriver said:
Stainless steel in contact with aluminium will lead to corrosion. This can get really bad in winter road conditions - wet and salty.
If you really wanted UNC / UNF threads, search for triumph motorcycle parts (classic / meriden built but not the modern hinckley built) they used those threads from late 1960's until mid 1980's. They are available in stainless if you need them.

I didn't think stainless and alloy went well together.

Thanks for the tip, but I don't, it is the OP that wants them.

mick 1986 · Nov 2, 2018

scottonthefen said:
Is it my imagination, or is there a reason I can't find stainless fixings with a UNF thread? They all look UNC.. I've looked on accu, westfield fasteners, gs-components.. what am I missing? It's not a plating is it, stainless is the makeup of the steel right?

Westfield fasteners do stock both UNC and UNF in A2 and A4 Stainless steel.

What length and diameter are you looking for?

scottonthefen · Nov 3, 2018

40mm length, diameter around M8.

I've had it wrong about stainless for a while it seems. I thought the school of thought was to use stainless fixings whereever they touch the birmabright body, I honestly don't know where I picked that up from!

scottonthefen · Nov 3, 2018

mick 1986 said:
Westfield fasteners do stock both UNC and UNF in A2 and A4 Stainless steel.

You're right, cheers:

https://www.westfieldfasteners.co.uk/A2_ScrewBolt_HexHd_UNFine_THRfull_0.3125.html

Turboman · Nov 4, 2018

scottonthefen said:
40mm length, diameter around M8.

I've had it wrong about stainless for a while it seems. I thought the school of thought was to use stainless fixings whereever they touch the birmabright body, I honestly don't know where I picked that up from!

I wouldn't worry about it. It is a common misconception that using stainless steel is a cure all for any problem with corrosion.

It isn't, and often using stainless will lead to other problems, especially where high tensile strength is required.
The people who design vehicles are highly qualified and experienced, and there will usually be a reason they specify a particular metal for a particular application. Often, it may be best to go with what the designer specified.

The Mad Hat Man · Nov 4, 2018

Turboman said:
I wouldn't worry about it. It is a common misconception that using stainless steel is a cure all for any problem with corrosion.

It isn't, and often using stainless will lead to other problems, especially where high tensile strength is required..

That depends whether you use A2 or A4 stainless. HT stainless is available.

Turboman said:
The people who design vehicles are highly qualified and experienced, and there will usually be a reason they specify a particular metal for a particular application. Often, it may be best to go with what the designer specified.

Yes - cost is the overriding consideration. Using stainless on non structural fittings has no detrimental affect and can be of considerable benefit.

Turboman · Nov 4, 2018

The Mad Hat Man said:
That depends whether you use A2 or A4 stainless. HT stainless is available.

Yes - cost is the overriding consideration. Using stainless on non structural fittings has no detrimental affect and can be of considerable benefit.

I do sometimes use it for non structural stuff, but never for big bolts holding the chassis together, things like that.

Cost is a consideration for most people, and often it is more cost effective to use high tensile bolts, and use coatings and mastic to protect them. Vehicles have a design life, and at the end of the day you can't make a silk purse out of a sow's ear.

Stainless fixings in UNF

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