Black oxide bolts

[Bitsyncmaster]

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.

[eight8toy]

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.

[FABombjoy]

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.

[FABombjoy]

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.

[Drive Stainless]

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 (https://www.glaserbolt.com/materials-medium-carbon-steel-bolts-in-sae-j429-grades-2-5-8), grade 5 steel has a hardness between Rockwell C25-C34.

According to here (http://www.worldstainless.org/Files/issf/non-image-files/PDF/Atlas_Grade_datasheet_-_all_datasheets_rev_Aug_2013.pdf), 304 stainless has a max hardness of Rockwell B92.

According to here (http://www.carbidedepot.com/formulas-hardness.htm), 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.

[Bitsyncmaster]

That was me. :) I had three transmission mounts bolts fall out where I was using stainless steel fasteners and washers.

According to here (https://www.glaserbolt.com/materials-medium-carbon-steel-bolts-in-sae-j429-grades-2-5-8), grade 5 steel has a hardness between Rockwell C25-C34.

According to here (http://www.worldstainless.org/Files/issf/non-image-files/PDF/Atlas_Grade_datasheet_-_all_datasheets_rev_Aug_2013.pdf), 304 stainless has a max hardness of Rockwell B92.

According to here (http://www.carbidedepot.com/formulas-hardness.htm), 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.

[Bitsyncmaster]

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.

[Drive Stainless]

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?

[Bitsyncmaster]

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.

[DMCMW Dave]

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.

[FABombjoy]

Vibratory polishing technically, walnut media, plastic tub. Principally affected the AC compressor bolts. Should have been nice and gentle!

The 7mm hardware seemed to weather things ok. Amazingly the local Home Depot has a usable selection of 7mm zinc-plated bolts.

[David T]

S/S hardware is not graded like carbon steel and should not be used in a high stress location. Allen head bolts are high grade (not sure what it is exactly but it is not ordinary). You should not substitute fasteners unless you know what you are doing. Painted ones are not necessarily any good. Just the action of tightening it with a tool will nick the paint and it will rust, that is why they are plated. To some extent plating is self-healing til it eventually is degraded due to sacrificial corrosion. If you are into refurbing hardware check out Eastwood, they sell tumbling and plating stuff to refurb old, original hardware. Also look up "Hydrogen Embrittlement" before you decide to do any plating.

[hippieman9]

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.

I just re greased my C/V joints and installed new boots and put all new Stainless bolts (6pt hex head) and lock washers in. Weird thing was the bolts that were in the C/Vs were all 17mm heads (wench size) except one joint had all 16mm heads. Needless to say, I figured good time to replace them all so they match with stainless.

[dn010]

So I found zinc bolts at MaterCarr.

Do you have a part number for this? I'm about to do my clutch and would love to change those bolts to something that won't rust immediately.

[Bitsyncmaster]

Do you have a part number for this? I'm about to do my clutch and would love to change those bolts to something that won't rust immediately.

1 90128A292 Zinc-Plated Alloy Steel Socket Head Screw, M10 x
1.5 mm Thread, 60 mm Long, Packs of 10
3
Packs
3 0 8.30
Per Pack
24.90
2 94241A560 316 Stainless Steel Split Lock Washer for M10
Socket Head Screws, 10.7 mm ID, 16 mm OD,
Packs of 50
1
Pack
1 0 10.56
Per Pack
10.56

[dn010]

Great, thank you! I just placed my order. Cant wait to ditch the hex head bolts.


1 90128A292 Zinc-Plated Alloy Steel Socket Head Screw, M10 x
1.5 mm Thread, 60 mm Long, Packs of 10
3
Packs
3 0 8.30
Per Pack
24.90
2 94241A560 316 Stainless Steel Split Lock Washer for M10
Socket Head Screws, 10.7 mm ID, 16 mm OD,
Packs of 50
1
Pack
1 0 10.56
Per Pack
10.56

[dn010]

Wow! Ordered yesterday and arrived today. Guess I am close to their warehouse. Looks like a clutch job this weekend, fun times...

[Bitsyncmaster]

It seems the DeLorean torque spec of 30 ft/lbs is light. Recommended torque for a 10 mm 8.8 bolt is 40 ft/lbs. dry. I know some people are using locktite but that was not stock from factory. I think I remember one guy found his bolts were loose but he thought someone tried to steal his axles.

By the way I think I know what the plates used on the two bolts are for. I'm guessing they just work as flat washers but since we use a split lock washer which bites into the plate, single flat washers would just spin loose but with two bolts they can't spin.

[David T]

Actually, for bolts subject to rotation, they should be safety wired. At the least you can put inspection marks on them like a drop of nail polish. Yes, the plates can only act as fancy flat washers. Really doesn't seem to be much of a problem if they are torqued to spec and inspect the undersides of the car once in a while. If they all loosened up you would notice it right away I would think!

[DMC-81]

It seems the DeLorean torque spec of 30 ft/lbs is light. Recommended torque for a 10 mm 8.8 bolt is 40 ft/lbs. dry. I know some people are using locktite but that was not stock from factory. I think I remember one guy found his bolts were loose but he thought someone tried to steal his axles.

By the way I think I know what the plates used on the two bolts are for. I'm guessing they just work as flat washers but since we use a split lock washer which bites into the plate, single flat washers would just spin loose but with two bolts they can't spin.

That reminds me of my very first car (a 1982 Volkswagen that had very similar drive axles). I had to replace the engine short block. The mechanic tightened the drive shafts without Loctite and after a while, both shafts loosened enough to separate from the output flanges. After that, I always apply Loctite when working on these.


Actually, for bolts subject to rotation, they should be safety wired. At the least you can put inspection marks on them like a drop of nail polish. Yes, the plates can only act as fancy flat washers. Really doesn't seem to be much of a problem if they are torqued to spec and inspect the undersides of the car once in a while. If they all loosened up you would notice it right away I would think!

David, that may apply to aviation, marine, or racing applications, but I have never seen a bolt safety wired on a street car. For everyday use, we have Loctite. On the Delorean, I have seen Red Loctite specified/recommended for the flywheel, wheel hub nuts, transmission shaft nuts etc., and Blue for clutch pressure plate bolts , drive axles etc.