Wondering if somebody could shed some light - I need to replace a left crank arm due to damage. My chainset is 105 5800 170mm and looking through ebay there are a few ultegra 6800 170mm left hand cranks available.
Will these fit?
Thanks in advance
Perfect fit, no issues at all.
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Yes, they’ll work fine as both are (afaik) 11-speed setups from the same generation. Mixing different generations can be more complicated and Shimano doesn’t support mixing 10 and 11-speed setups although anecdotally even a 5700 transmission and an ultegra 6800 crankset work fine.
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Thanks for the advice, much appreciated!
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Every generation of HollowTech crank arms are cross-compatible as the splines haven’t changed, to the best of my knowledge. The left arm isn’t exactly what you’d describe as ‘speed specific’.
Mike
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My bad. I read this too quickly. I thought he was going to take this opportunity to upgrade the crankset. 
Detention for me!
I do that all the time.
Mike
No problem at all.
Re-cycling this thread:
My big Chain ring is worn and needs to be replaced. It’s a 105 5800 chainring and crank arm. Would an Ultegra chainring fit?
6800 would. Not R8000, as it is a 4 bolt pattern versus the 5 bolts of 5800/6800.
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5800/ 6800 are 4 bolt as well, but won’t fit bc the inner chainring on r8000 series moved inboard ever so slightly.
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But 105 5800 is compatible with Ultegra 6800, right?
Yes. And I’ve done exactly this, fitted an 6800 large chainring to a 5800 crankset.
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