40 replies to this topic

[spiguy]

Hi folks,

 

I would like to use rivnuts to attach my arches, currently on with normal rivets. Reason being I want to be able to remove them periodically to clean / inspect under there.

 

I have googled and used the search on TMF, and found a fair bit of info, so I am currently thinking M4 rivnuts are the way to go.

 

What I am less clear on, is the type. I think I will opt for aluminum - easier to crush when installing, and I think stainless steel could cause a galvanic corrosion problem. However I am not sure if I should be going for the ones with splines on, or the smooth ones.

 

Seems to me splined ones might be less prone to spinning in the panel, but then again they will bite into the metal - I have seen people mention about painting the hole before fitting the rivnut, so I assume in those cases they have used smooth walled ones.

 

Just wonder if anyone can advise me on their views about material type, body type (splined / smooth) etc.

 

Many thanks!

 

Craig

[minidaves]

i use steel rivnuts, and paint first the fit rivnut with waxoyl

 

dave

[nicklouse]

the splines dont do much and even less on alloy rivnuts inserted into steel.

[spiguy]

Thanks folks. I think I will therfore go for M4 aluminium smooth walled rivnuts then. I will try to add paint / wax if possible, and also seal the back with sealant.

[sonikk4]

I would go for stainless steel over aluminium. Aluminium will corrode very quickly whereas if you use stainless and when fitting them apply paint just before you insert them there will be a good chance they will not cause galvanic corrosion.

Whatever you use you stand a chance of corrosion starting so being careful and fitting them wet ( paint) you minimise that happening.

[spiguy]

Thanks for the advice. Would it really corrode so bad with aluminium? I have currently and have had aluminium rivets holding my arches on in the past for years, and they have never showed any significant signs of corrosion.

 

In fact apart from the increased maleability of the alu being a factor for choosing it, I was thinking that it would be better than stainless, as stainless combined with the mild steel of the panel gives a much greater electropotential difference than combining alu with mild steel, and it would be the mild steel that would suffer. But do you think that alu rivnuts would quickly corrode? Why would these corrode more than alu rivets?

 

Good tip either way about inserting them wet with paint. Would you think that painting the rivnut then immediately fitting it would be the best way ?

 

Thanks again

Craig

[Daz1968]

Aluminium corrodes when in close proximity to steel

[sonikk4]

Yup dissimilar metal corrosion, as bad as galvanic. Once it starts its a pain. Something I have to deal with on Airbus aircraft.

[Steve G]

Yup dissimilar metal corrosion, as bad as galvanic. Once it starts its a pain. Something I have to deal with on Airbus aircraft.

Is there some sort of paste/paint use specially when attaching alloy rivets in steel ? I have a vague idea something special is available, perhaps it's more to stop corrosion starting between steel brackets and alloy skins etc. 

I'm considering using rivnuts to hold the alloy panels on my Scamp onto the chassis. I was thinking the best option might be decent quality steel rivnuts and stainless bolts of some sort.

[sonikk4]

There are several products that we use.

 

1:- Mastinox. A yellow Chromate Paste. We predominantly use this on bolts especially stainless steel to help prevent galling.

2:- PRC. This is a generic name for the numerous sealants in the range, for riveting especially on skin repairs in pressurised area we use PR870. I also have used this when we install Hi Lok fasteners.

 

Now either of these products will do the job nicely.

 

http://www.skygeek.c...l-catridge.html

 

http://www.skygeek.c...on-sealant.html

[Steve G]

Thanks. On the Scamp a lot of the pop rivets have long since given up so I wanted to avoid steel and alloy. I hadn't realised about the problems with stainless steel and steel though, hope that didn't stray too far off topic.

[spiguy]

Understood - aluminium corrodes in contact with steel, corrosion due to to dissimilar metals = galvanic corrosion, they aren't two different things - are they?

 

This is one reason I was planning on using aluminium over stainless steel, because as you know the same thing happens when stainless steel contacts mild steel, however with aluminium and mild steel, the aluminium rivnut will corrode rather than the panel (good), however it shouldn't corrode very much since the oxide layer will tend to inhibit further corrosion. This is the reason I asked why the rivnuts should corrode significantly more than aluminium rivets, which I have seen do not seem to corrode very much even after a long time.

 

On the face of it using stainless steel rivnuts in theory is 'worse' since in that combination the mild steel (panel) suffers, albeit it should be a very slow degradation as the panel is large and the rivet is small, but surely it would still be better to avoid this, since even a small amount of corrosion occurring at the panel is undesirable.

[sonikk4]

Both descriptions are effectively the same. Its an industry thing, we always refer to it as Dissimilar metal corrosion but that could also be to do with my ex RAF training. 

 

Now normally as you have mentioned aluminium will corrode quicker than mild steel but in turn it will also start the corrosion process in the steel as well if left for long periods of time. What you also have to take into consideration is the environment the rivnut is placed in.

 

In a wheel arch it will come into contact with everything that is on the road and as soon as the protective layer is damaged then bang corrosion will take hold.In an enclosed box section that has minimal contact with moisture etc it will last considerably longer. Now depending on what you use to protect the item plays a major part and of course when you install it.

 

Its like anything careful fitting and care plays a huge part in the longevity of anything. 

[nicklouse]

I know what I woyld using rather than rivnuts.

But not sure where you would get them. sonikk would do though.

I would call them floating captive nuts.

Just found some.

Edited by nicklouse, 03 March 2015 - 08:22 PM.

[spiguy]

Interesting Nicklouse, I also work in a related industry and we use something similar on some of our products mounting brackets. I think rivnuts will make for an easier quicker install though, and will do what I need of them.

 

Thanks for all the opinions folks. In any case  - aluminium or stainless steel, the whole point is for me to be able to get the arches off periodically and clean / inspect so that any slight corrosion can be attended to sooner rather than later. The area under the wheelarch is and will continue to be treated with dynax UC wax, (as is most of the car including in the boot, inside rear quarters etc!)

 

Cheers

Craig