Hey,
I’m looking to replace my chainring because it’s starting to wear out, will an R8000 chainring work on a 105 5800 crank? Both are 110mm 4 bolt patterns
Thanks!
I would have thought so. If I ever need to down gear, I’ll swap my (correction) R9100 4 bolt pattern 36/52rings for the same sized, speed and patterned R8000 34/50 rings off my older bike. I swapped 34/36 R8000 rings on it for 34/50 R8000 rings a few times.
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I think I’ve seen that it will mount but the aesthetic lines on the crankset might not match up. And the glossy/matte areas might not line up either.
6800 chainrings will not mount on 8000 crank. So I think the answer is no.
ETA - I’m deleting my previous answer. I was looking at the wrong version of 105 crank when I made my comment.
Have you actually tried it?
The R9000 is same series as 6800:
And I can tell you 100% that 6800 outer chainrings will not mount on 8000 series crank. I tried it with 6800 50 tooth chainring, failed, took it to the bike shop, stumped the junior guy, and then the shop manager walked over and said “not compatible, you need to buy an 8000 series 50 tooth chainring.” So I did, at it mounted instantly.
And that is implied in the Shimano compatibility chart: 2023-2024 SHIMANO Product Information Web
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Only indoors to check they fit. I’ve never actually tightened them or ran them but I can’t see an issue if it the same speed/size/pattern. I’ve just googled it and its 9100 though and 8000
Edit I have an older 6800 some where and that definitely wouldn’t fit it’s a 5 bolt pattern.
9100 or 9000?
The 9000 chainrings are compatible with 6800 and 5800.
The 9100 chainrings are compatible with 8000 and 7000.
They are both 4 bolt, and the newer 9100/8000/7000 outer chainrings are not compatible with 9000/6800/5800 outer chainrings. I didn’t try with inner chainring.
Sorry I’ve edited my OP and it probably caused confusion
The exact same speed/size/pattern 9100 36/52 has been swapped for the identical set up bar name R8000 (indoors only, not tightened or ridden).
I wouldn’t even try swapping the 4 bolt 9100 chain rings for an older 5bolt 6800 crank set chain rings. I have as it simply wouldn’t fit👍
my 6800 crank looks like this:
4 bolts.
my 8000 crank looks like this:
4 bolts.
I’m not sure how your 6800 got 5 bolts???
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Apologies, my older crank set must be 6700 then, its 5 bolt and 10 speed which is definitely not compatible with my 9100 or 8000 which both have the hollow tech II crank pattern shown in your 2nd pic (11sp/110mm/same crank pattern) 
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@Nasar_Maatougui no because the bolt locations are slightly different. For the 105 FC-R5800 you would need 5800 or 6800 or 9000 chainrings.
For example this would work: https://www.excelsports.com/main.asp?page=8&description=Ultegra+FC-6800+Outer+Chainring&vendorCode=SHIM&major=1&minor=12
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6800 or 5800 will fit nicely. I think r8000 will work but not fit totally flush.
I have the same issue and have been looking at round absolute black versions but not sure about shifting.
No that will absolutely not fit on 105 / 5800 crank. Shimano changed the bolt locations.
Good thing I posted! Absolute black versions it is then!