Undertray bolts... a suggestion - Page 2 - LotusTalk - The Lotus Cars Community
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post #21 of 26 (permalink) Old 09-24-2018, 08:03 AM
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Glen calls out the electrochemistry problem with stainless steel and zinc plating well. It's probably worth mentioning that there are two types of anti-seize compound and you really want to have both in your toolkit.

One is the silver kind most people think of, which was originally Aviation Permatex Anti-Seize compound (came in a pint paint can). It's specifically meant for use at the threaded interface of a ferrous fastener and a light alloy (aluminum or magnesium) part. Back in the day when Men were Real Men and Chevy and Ford engines were made of Real Cast Iron, pretty much the only place you could get this stuff for working on your bathtub Porsche, Alfa, Ferrari or Corvair was from Permatex, which had made it for aviation engines for decades.

If you have a ferrous fastener going into ferrous threads, you want a copper-based anti-seize compound. Yes, it's still made. This is what you really want on things like lug bolts or anywhere else where you've got a steel fastener going into iron or steel threads. The silver stuff will work, but tends to sacrifice itself rather than 'short out' electrochemistry at the thread interface, which is what you really want to happen.
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post #22 of 26 (permalink) Old 09-24-2018, 08:25 AM
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Anti seize is your friend. Having that little bottle and using it will save you, and the next owner of your car much aggravation.

I would never change any 'bolt' size fasteners with stainless unless I was really sure of the strength. Stainless bolts are marked grade 4 in the US[usually] and while I view them as an upgrade from a grade 2 regular bolt, only just. Standard bolts and never seize are your best bet there.
304 stainless is gummy stuff, and if you try to put much torque through it the threads can move and, yeah, gall up.

On my car I think the small M4 screws are threaded into little tinnerman style edge clips.

This is what galvanizers think about stainless and zinc:

https://galvanizeit.org/hot-dip-galv...h-other-metals

I tend to replace tiny fasteners that you don't take out much with stainless, because the next time you see them the head will be rusted away.

you are probably not driving in the winter in salt, and are probably taking your aero tray off for every oil change, so I doubt the steel bolts will rot away on you, so dealers choice there. I use stainless because I hate the look of rusty hardware. The next time a stainless fastener causes some kind of corrosion on my vehicle will be the first time, others experience may differ.
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post #23 of 26 (permalink) Old 09-24-2018, 10:50 AM
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Quote:
Originally Posted by steelypip View Post
. . .
, you want a copper-based anti-seize compound. Yes, it's still made. . .
At least here in the states, the Permatex brand is readily available at auto supply stores. They market it for exhaust systems and oxygen sensors.

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post #24 of 26 (permalink) Old 09-25-2018, 11:32 PM
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Did a bit of research on anti-seize and even there you have to watch out. I went with a nickel based compound from Permatex.

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post #25 of 26 (permalink) Old 09-26-2018, 08:19 AM
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Nickel anti-seize is best for stainless-stainless joints, but copper will do in a pinch. The big no-no is aluminum on a ferrous or stainless-ferrous joint, especially if it gets hot (like exhaust fasteners). It'll turn into aluminum oxide pretty quickly, which is not what you want there.

I wouldn't worry about the zinc flash coating on the clip nuts - it's sacrificial (and quickly wears off from the screw threads) anyway, and is much more electronegative than even the steel it's plated on. Treat the joint as carbon steel to the screw material and you're on the right track. Copper anti-seize would be a good choice for carbon steel screws. Nickel anti-seize would be a little better for stainless fasteners, but not required as the environment is pretty benign. The galvanic series table linked above is handy for this sort of question, especially if road salt is your concern.
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post #26 of 26 (permalink) Old 10-16-2018, 01:47 PM
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Hi all

Interesting thread this, as i've just removed the front under tray of my S3 Elise, and one of the bolts that screws into the captive thread on the under tray was seized. This is where the front bumper lower mount attaches to the under tray. On removal its snapped off the captive thread on the under tray, and i'm now left with a hole so the bumper mount will not tighten up. Does anyone have any ideas how to fix this?

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