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Ex-Biker 28th December 2003 09:22

Securing the floorpan
What method is best to fit the floorpan? :confused:

I have a number of options
  • 1. Rivnut
    2. Pop rivet
    3. Self tapper
    4. Panel bond
Which ever I decide, I am going to use panel bond. This is firstly to create a watertight seal between chassis and floorpan and secondly for strength.

So I suppose the question is more to do with whether I need anything else and if so what? :doubt:

I was happy to use widely spaced self tappers initially, until I saw a picture where rivets where used. I do see a problem with rivets, which is the need to seal the holes after. :rolleyes:

Any and all suggestions welcome.

Merry Christmas and Happy New Year!

mudhut 1st January 2004 23:50

Securing the floorpan
Hi Ex-biker

IANE but FWIW, here's my take on this.

Chassis strength through panelling is only increased if the bond between panel and chassis is effectively continuous. So right off the top of my head, here goes...
  • Rivnuts: need relatively large hole in chassis and may weaken it if placed close enough together to be effective in fastening the panel. Since rivnuts stand proud of the chassis member, the panel to chassis gap is large and will require a considerable thickness of adhesive/sealant which may or may not be OK. Never used a panel fixing adhesive on a car.

    Pop Rivets: cheap and easy to get a close fit if holes deburred. A smear of sealant "under" the head prior to fixing plus a little dab in the hole ought to do the trick. IME they are cheap enough to use close enough to make a proper join but can work loose with vibration and other movement..

    Self Tapper: As good as pop rivets if holes deburred. You already have steel chassis and aluminium panels. Are you proposing to use stainless self tappers or zinc plate or plain steel? There might be complications due to potential electrolytic corrosion but I don't know enough about this except that the fewer dissimilar metals in contact, the better. Suspect that the aluminium panel may be vulnerable especially in contact with stainless.

    Panel Bond: I don't think this is a good idea if you were proposing to use this on its own with no mechanical fixing. The strength is then dependent upon the bond of the powder coating to the chassis as well as sealant to aluminium and sealant to powder coating.

I think I'd go for the pop rivet approach with the panel adhesive as a sealant.

If I remember correctly member vojx has a background in finishes and might be able to shed more light on the adhesion of powder coating and panel bond.

Happy new year to all


Ex-Biker 2nd January 2004 17:47

Many thanks Mudhut.

I've had feedback from elsewhere too and everything seems to point to rivets being the best idea.

Anyone got an opinion on the best spacing between rivets. Chassis is steel, floorpan galvanised, side panels and bulkheads aluminium (if any of that makes a difference).

vojx 6th January 2004 11:07

IMHO look before you leap. The floor of my stratos is aluminium, the chassis steel. Sealed (rather than bonded) and rivetted (as wide-headed as I could find) about every four inches, wax-painted inside where I could reach and that black underseal-stuff (?!). The strength is fine, but with time (and therefore scratches, salt, radiator fluid leaks) corrosion starts, and where dissimilar metals touch the corrosion is accelerated (an electrochemical cell, i.e. a battery) - hence the rear floor in the engine bay has given way, the rivet-holes enlarged by corrosion (and the rivet heads broke off). Where they contact, zinc will dissolve in preference to steel, and aluminium in preference to steel and zinc (I think).
Not all is lost - the accelerated corrosion will not occur if water (and contaminants) are excluded. Powder coating will not be sufficient as it does not penetrate tight areas. Applying bitumous (outside) / waxy (inside) materials on all contacting surfaces is paramount to prevent water ingress, but that means you can not subsequently powdercoat. Wet painting is fine, provided it will adhere to the bitumen (some underseals are no problem, but impossible to paint over waxes - use them for sealing after paint).
None of this is going to look pretty, but then how many cars have visible joints? If a rivet or screw is going to show, accept bulbous corrosion protection or bulbous corrosion blisters.
As they say, its all in the preparation.

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