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Thread: Black oxide bolts

  1. #1
    Senior Member Bitsyncmaster's Avatar
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    Black oxide bolts

    I had replaced my axle bolts with Allen cap black oxide bolts. But with less than 100 miles the heads of those bolts are completely rusted. So I found zinc bolts at MaterCarr. I also mostly use stainless flat washers and lock washers since there is no real strength needed for those. I did see where one owner had used SS washers on his transmission bolts and they fell out after use. I don't see how SS washers caused that since the harness is the same as carbon steel.

    Not sure is all black oxide bolts corrode like that but I'm not going to use them on any exposed locations anymore.
    Dave M vin 03572
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  2. #2
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    Don't know if anyone does this but I've sometimes taken some surface coated rusty bolts, cleaned them up with wire brush then hit the tops up with rustoleum black spray paint. Let dry then reinstall.

    Actually just did that to my stainless steel taillight screws as well last week.

  3. #3
    '82 T3 Turbo FABombjoy's Avatar
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    I've never had a black oxide bolt that didn't eventually rust. I even had them flash rust from condensation. Like you I stopped using them externally - hard platings seem to be the only way to go, and I just spray the heads with paint if I don't want silver.
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

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  4. #4
    '82 T3 Turbo FABombjoy's Avatar
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    Quote Originally Posted by eight8toy View Post
    Don't know if anyone does this but I've sometimes taken some surface coated rusty bolts, cleaned them up with wire brush then hit the tops up with rustoleum black spray paint.
    I tumble polished a bunch of engine fasteners once. They looked great until one winter where temperatures went from from 20F to 60F overnight. The resulting whole-car condensation turned all of the larger OEM bolts brown immediately - apparently they don't have a very durable plating. I just painted them too, and so far so good.
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

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  5. #5
    Matt Drive Stainless's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    I did see where one owner had used SS washers on his transmission bolts and they fell out after use. I don't see how SS washers caused that since the harness is the same as carbon steel.
    That was me. I had three transmission mounts bolts fall out where I was using stainless steel fasteners and washers.

    According to here, grade 5 steel has a hardness between Rockwell C25-C34.

    According to here, 304 stainless has a max hardness of Rockwell B92.

    According to here, Rockwell B92 has a tensile strength of 96,000 PSI, while Rockwell C34 has a tensile strength of 156,000 psi.


    Whether that makes the difference, I don't know. What I know is that when I removed the remaining transmission mount fasteners, all were loose, and the stainless washers were "dished." I do not see the same level of deformation where I've used plated steel washers, so my thought is that the stainless washers yielded over time and allowed the bolt/nut to loosen. I did not use spring washers.

    I reinstalled the transmission mounts with grade 8 washers and grade 8 bolts, so we'll see if they fall out again.
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  6. #6
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Drive Stainless View Post
    That was me. I had three transmission mounts bolts fall out where I was using stainless steel fasteners and washers.

    According to here, grade 5 steel has a hardness between Rockwell C25-C34.

    According to here, 304 stainless has a max hardness of Rockwell B92.

    According to here, Rockwell B92 has a tensile strength of 96,000 PSI, while Rockwell C34 has a tensile strength of 156,000 psi.


    Whether that makes the difference, I don't know. What I know is that when I removed the remaining transmission mount fasteners, all were loose, and the stainless washers were "dished." I do not see the same level of deformation where I've used plated steel washers, so my thought is that the stainless washers yielded over time and allowed the bolt/nut to loosen. I did not use spring washers.

    I reinstalled the transmission mounts with grade 8 washers and grade 8 bolts, so we'll see if they fall out again.
    I'm seeing carbon steel hardness of Rockwell B of 71 and 316 stainless of Rockwell B of 79. I also know stainless is a little harder to machine. I would like to know if your washers did cause the problem.
    Last edited by Bitsyncmaster; 09-15-2018 at 07:50 PM.
    Dave M vin 03572
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  7. #7
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by FABombjoy View Post
    I tumble polished a bunch of engine fasteners once. They looked great until one winter where temperatures went from from 20F to 60F overnight. The resulting whole-car condensation turned all of the larger OEM bolts brown immediately - apparently they don't have a very durable plating. I just painted them too, and so far so good.
    I have seen a few times when I open my garage door everything inside get wet from condensation. Those cold nights and humid summer days are the killers.
    Dave M vin 03572
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  8. #8
    Matt Drive Stainless's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    I'm seeing carbon steel hardness of Rockwell B of 71 and 316 stainless of Rockwell B of 79. I also know stainless is a little harder to machine. I would like to know if your washers did cause the problem.
    Carbon steel, but which grade?
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  9. #9
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Drive Stainless View Post
    Carbon steel, but which grade?
    Hardware store washers are not marked. I wonder what grade washers were used in production.

    If you used SS bolts maybe that was the problem.
    Last edited by Bitsyncmaster; 09-15-2018 at 08:50 PM.
    Dave M vin 03572
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  10. #10
    DMC Midwest - 815.459.6439 DMCMW Dave's Avatar
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    Quote Originally Posted by FABombjoy View Post
    I tumble polished a bunch of engine fasteners once. They looked great until one winter where temperatures went from from 20F to 60F overnight. The resulting whole-car condensation turned all of the larger OEM bolts brown immediately - apparently they don't have a very durable plating. I just painted them too, and so far so good.
    Tumble polishing would tend to thin or even remove the plating. You either need to buy new, high quality (hard to find especially the 7 mm ones!) bolts or find a good plater. Keep in mind that you've probably removed the plating from the threads as well, and eventually bare steel and aluminum will weld themselves together. The only thing worse is stainless into aluminum. You may as well weld things together.


    PS Black oxide is not really intended for use out in the weather. Zinc plating is what you want. Black oxide is typically used in parts that will be oily, for example inside an engine or machinery.
    Dave S
    DMC Midwest - retired but helping
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