Is there any reason I couldn’t use an R8000 (4iii) power meter LH Crank on an older R6800 chainset?
Cheers
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Nope. Go right ahead. I even used a R7000 4iiii power meter LH crank on a GRX 810 crankset. It changed the q-factor on that side, but not enough that I was sensitive to it.
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Thanks, there is a 0.6mm difference in Q-factor between the two cranksets (146.6 v 146mm)… so I guess it will only be 0.3mm.
The human body is not perfectly symmetrical, you likely won’t notice. If you do you can always use a washer between RH crank and pedal to even up.
I have a 4iii LH crank on a non matching model version and it’s perfectly fine, no issues. As long as crank type matches, Hollowtech 2 in my case.
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Yeah i’ll not notice. If i do I also have some play with the cleats.
same here, didn’t notice a thing
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I use a mix of a 6800 4iiii with a GRX400 crankset. Works fine and my old knees and ankles see no difference.
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Slightly different models IMC but I had just bought a LH arm power meter for my Ultegra R8000 cranks when I was offered a Dura Ace R9100 groupset. I used the Ulegra LH arm on the dura ace crank no problem.
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Anything that is HT2 is going to fit, be it deore to dura ace or 105 to XTR. Only thing you really need to consider is Q factor and whether it will clear your chainstay
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As a side note, the Deore MT510 Crankset is not HT2. While it might use the same BB and it may appear like it will work (same splines, same installation method), the axle is a few mm longer and the upgraded crank arm will not preload, at least not without a spacer of sorts.
On the plus side, good excuse to match that XT LH arm I bought with a proper XT crankset 
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What width BB do you have it mounted on? The MTB Deore range is designed for 73mm width. If you have road 68mm you just fit the supplied spacers as surmised to bring the HT2 BB width up to 73mm.
BB92 mtb with Shimano cups.
The M511 and the XT cranksets both fit without any spacers. The crank arms are a different width at the splines.
