MP 6,1 Why does the MacPro cylinder housing have this weird rectangular marking?

[timidhermit]

I am in mid of purchasing an used Mac Pro 2013. The Mac Pro has been well kept (at least what I think) by the original owner. However, I notice something very odd about the MacPro's cylinder housing. There is a thin rectangular area near the top of the housing for which the shining finish is dulled. Please see photos. Note in the photos a piece of yellow felt is covering the top of the cylinder to minimize light reflection in order to see this bizarre mark

They are very difficult to capture on photos. They are not residues, such as from adhesives. The surface is not etched in any way. It is smooth and is indistinguishable from other surfaces of the housing. Tried to clean it without effect with isopropyl alcohol, water, and even Mr. Clean sponge.

What is it? Please help me to solve this mystery.

 

[Wando64]

It looks as if there was a length of tape that has been removed.
The whole unit has slightly changed colour due to UV light, but the bit under the tape has not.

 

[timidhermit]

Is there a history that the coating on the MacPro's cylinder housing is prone to UV discoloration. I was not aware of this and cannot find any info online. Is this a known issue? Any way to fix this?

 

[mikas]

One of my tubes does have it, but it's almost invisible to me. It's to the right of the Apple logo, shape is the same as yours. Couldn't have noticed that if you didn't point that out. Pretty hard to take a picture of it too, but I'll try.

My second tube though, I can't spot it with that, at least not in the same location, probably not anywhere I think. The third is to be examined later maybe. Can't shut it down just now.

About the sun, yes sun could easily do that kind of thing. But I've just removed a sticker from this same tube today, and I can't see much of a color variation after IPA clean up. It's not the same shape or location though, it was a user attached sticker.

 

[KeithCody]

It looks as if there was a length of tape that has been removed.
The whole unit has slightly changed colour due to UV light, but the bit under the tape has not.

Thats what it looks like to me too.

 

[Pakaku]

I have no idea how the thermals work in that machine, but it looks like that strip is around the area inside where the top grills start. If nothing has ever been placed on the outside, maybe something inside is causing that, and my guess would be that maybe the heat has been taking a toll on that spot inside for some reason and is discolouring it. It might explain why it looks "bubbly" in some areas, too.

 

[arw]

I am pretty certain this looks like the residue of a sticker. Although not sensible, the glue/chemical has reacted with the metal/coating. Given the temperature of the housing, it accellerates any process.
I had very similar stains on a cMP after removing some sticky dots.
Can‘t you simply ask the previous owner?

 

[KeithCody]

I have no idea how the thermals work in that machine, but it looks like that strip is around the area inside where the top grills start. If nothing has ever been placed on the outside, maybe something inside is causing that, and my guess would be that maybe the heat has been taking a toll on that spot inside for some reason and is discolouring it. It might explain why it looks "bubbly" in some areas, too.

Aluminum doesn't change color thermally. If you weld it and it cools back down it returns to aluminum color, unlike say titanium. Maybe the aluminum oxide anodizng could change color but I think it would have to be really hot, since annodizing is a dye in the pores of the aluminium oxide.

 

[timidhermit]

One of my tubes does have it, but it's almost invisible to me. It's to the right of the Apple logo, shape is the same as yours. Couldn't have noticed that if you didn't point that out. Pretty hard to take a picture of it too, but I'll try.
View attachment 1932151
View attachment 1932150

My second tube though, I can't spot it with that, at least not in the same location, probably not anywhere I think. The third is to be examined later maybe. Can't shut it down just now.

About the sun, yes sun could easily do that kind of thing. But I've just removed a sticker from this same tube today, and I can't see much of a color variation after IPA clean up. It's not the same shape or location though, it was a user attached sticker.
View attachment 1932154
View attachment 1932155

I was about to accept the UV discoloration or sticker residue reaction theory until I saw mikas' photos of his/her MacPro. The shape of the marking in mikas' Mac Pro looked identical (tape shaped like) to the marking on my Mac Pro. This, by itself, meant that the marking was not from sticker residue.

Mikas, is the strip of marking located near the top or bottom of the cylinder? I can't tell from your photos. Mine is located near the top of the cylinder on the front side, directly opposite to the Apple logo which is on the back side.

This is very bizarre...

 

[mikas]

Nope, not on the opposite side. It's more on the back side of the cylinder, you can locate it by the apple logo and by the IO hole of the case in my picture. In my cylinder it is very subtle, like I said I wouldn't have noticed it without your post about it.

Strange it is, and I wonder why it's in there, but I can live with it.

 

[timidhermit]

Nope, not on the opposite side. It's more on the back side of the cylinder, you can locate it by the apple logo and by the IO hole of the case in my picture. In my cylinder it is very subtle, like I said I wouldn't have noticed it without your post about it.

Strange it is, and I wonder why it's in there, but I can live with it.

mikas, can you check on your third Mac Pro and post your finding? Is the shape of the marking on your Mac Pro also rectangular and strip like (measuring about 10cm x 1cm)? Are there any peculiar etching-like marking within the strip (like bubble shaped)?

 

[mikas]

Sorry, I can't take it down now. I tried to look at it a little bit, but couldn't find anything with a flash light. Later this month I'll take a proper look.

 

[timidhermit]

Sorry, I can't take it down now. I tried to look at it a little bit, but couldn't find anything with a flash light. Later this month I'll take a proper look.

No problem. I am tracking this thread and look forward to seeing your finding later this month.

For your Mac Pro that had this shadow or etching, did you recall putting a sticker on the cylinder shell that matched the shape and location of the shadow or etching?

I am increasingly suspicious that this is actually a manufacturing issue by Apple. Perhaps during the manufacturing process when a coating is being supplied on the cylinder shell, some shells had a mask label applied on them for parts identification in the assembly line. This mask somehow interfered with the coating process so that it left behind a shadow that was more evident in some batches of the shells than others.

This would be a major find if it turns out to be a newly discovered issue.

 

[mikas]

Ok, I did a proper cleanup to all of my 6,1 cases. I have a pretty hard time to find that mark anymore, I believe it's gone. I am now doubting myself if it ever was there. But if it was, it must have been residue from a sticker or alike.