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When reading through this thread firsts thing it occurred to me was what @FrankS29 mentioned about the different spindle lengths for the M8100/M8120/M8130 and the spindle spacers used to take up the slack and allow to properly set the preload, the same thing also applies to M9100, M7100, M6100 series.
But @Gman7 said that he's not using any spindle spacer, then I noticed something on the BB picture you posted, the plastic insert/seal should normally be flush with the aluminium cups on the M800 BB, but on your picture it seems to protrude a couple of millimeters, is this an optical illusion, or the plastic insert is really not flush with the aluminium cups of the BB?
 

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The plastic piece acts as a bearing shield and is part of the bottom bracket and non-removable as far as I can tell. The 24mm spindle slides into it. It appears to be just about perfectly flush with the face of the housing. @FrankS29 made an important observation that the right arm assembly was supposed to be FC6120 for boost (has longer spindle) but FC6100 was provided instead. I am fairly certain the wrong crank was installed.

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damned rocks...
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You can remove the plastic insert if you want to do some tlc to the BB, I only mentioned the plastic insert because it looked like it was not flush with the aluminium cups, but clearly it was just an illusion.
The longer spindles on the M6120 is just to allow better clearance on boost frames because they tend to have larger chainstays around the BB area on more recent models, the BB shell width is supposed to be the same, 73mm on a mountain bike, meaning that the M6100/M7100/M8100/M9100 should be able to be mounted on any frame with a 73mm BB shell, regardless if it's a boost frames or not as long as the chainring don't hit the chainstay.
The M6120 longer spindle on a 73mm shell has to take the one spindle spacer on each side because otherwise you can't properly preload the BB bearings, it was not supposed to be longer because boost frames have larger BB shells.
Did you measured the BB shell just to make sure it has 73mm?
 

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The plastic piece acts as a bearing shield and is part of the bottom bracket and non-removable as far as I can tell. ...
Thats the 2.5mm spacer shimano specifies for a 73mm BB shell. You can remove it by taking off the BB; if I did that on mine the chainring would rub the stay. The same crankset + BB can be used on a 68mm frame by adding couple more 2.5mm spacers (one each on DS and NDS). 3 of those spacers were supplied in the box my 7100 cranks came with. 6100, 7100, 8100 cranks can be used for boost and non-boost frames which work with a 52mm chainline. 6120 cranks have 55mm chainline and a wider q factor for frames which need more clearance (and they take an extra 3mm spacer for the cranks). Looks like you have the right cranks and it was installed as shimano specified. Maybe check the BB shell like the suggestion above; if its slightly wider than 73mm. If not hopefully shimano or your shop come back with a good explanation. Comparing your pics to the one i posted its hard to tell but i don't think i have 1/3rd of the splines exposed like yours.
 

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I measured the bottom bracket shell and it’s exactly 73mm as it is supposed to be. The spacer behind the right side cup is 2.5mm so everything measures out for boost spacing. The only thing left that I cannot check is the chainring spider due to the fact that Shimano now has the crankset. I will post the response from Shimano as soon as they reply.
 

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Quick update. I received the crankset today under warranty but no determination as to the cause of the failure. At this point I will reinstall and run it.
 

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Shimano's 12 speed crankset product line is quite confusing.

The FC-M6100, FC-M7100, and the FC-M8100 works with (142 and 148mm rear spacing) BOOST rear ends designed with a 52-53mm chainline
The FC-M6120, FC-M7120, and the FC-M8120 works with (148mm rear spacing) BOOST rear ends designed with a 55mm chainline. It's spindle is 6mm longer than the (FC-MX100) which is why you need to use two 3mm spacers (one on each side) that it comes with.

^ It's meant to be used on newer BOOST frames designed around a 55mm chainline. Some bike manufacturers are now designing frames (2020-2021+) like Santa Cruz around the 55mm chainline for their boost frames. People want wider tires (2.6+ inch), shorter chainstays on their 29ers hence the need for this, requiring a larger Q-factor as well. Pretty sure this is Shimano's new design template.

Th FC-M6130, FC-M7130, and the FC-M8130 are meant for Super BOOST and isn't really applicable in this discussion. It's spindle is 9mm longer than the FC-MX100 and 3mm longer than the FC-MX120.

With that said, if you have a 73mm width bottom bracket, and your bike's boost rear spacing is designed around a 52-53mm chainline (e.g. 2020 Specialized Stumperjumper), then you should be using the FC-M6100. The engagement on the spindles will be exactly the same as long as you aren't using any rubber spacers on the spindle - and those plastic spindle guides on the Shimano bottom bracket are flush with the bearings. The 6mm spindle length difference between the FC-M6100 and the FC-M6120 is taken up by the two rubber spacers (3mm on each side) on the spindle (provided on the FC-M6120) and isn't really relevant to spline engagement.

I'd say once you've inserted the spindle on the bottom bracket, tap the drive side with a rubber mallet to make sure it's flush. Put some grease on the splines, connect the left crank arm, tighten the spindle star nut and tigthen the two bolts to lock it in place. Loosen up the two bolts slightly, back out the star nut and make sure you are getting 80+ percent engagement on the spline (gap wise). If it doesn't look good, I'd say tap it slightly with a rubber mallet and see if slides in more. If it looks good though, tighten up the star nut, tighten up up both bolts in alternating fashion to 13 nm and you should be good to go.

The LEFT crank arm is exactly the same on all three variations (e.q. FC-M6100, FC-M6120, FC-M6130), additionally the right crank arm is the same as well (albeit it's just connected to a spindle that has different lengths based on desired chainline / Q-Factor). The FC-M6100 has no spindle spacers, the FC-M6120 comes with two spindle spacers (3mm each side), and the FC-M6130 comes with two spindle spacers (4.5mm each side). Basically, longer the spindle, larger chainline, and larger the Q-FACTOR.

I'm using an FC-M8120 (XT crankset, 55mm chainline) on my Commencal Meta TR 2021. Will take a pic of the spline engagement gap for reference.
 

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Shimano's 12 speed crankset product line is quite confusing.

The FC-M6100, FC-M7100, and the FC-M8100 works with BOOST rear ends designed with a 52-53mm chainline.
The FC-M6120, FC-M7120, and the FC-M8120 works with BOOST rear ends designed with a 55mm chainline. It's spindle is 6mm longer than the (FC-X100) which is why you need to use two 3mm spacers (one on each side).

^ It's meant to be used on newer BOOST frames designed around a 55mm chainline. Some bike manufacturers are now designing frames (2020-2021+) like Santa Cruz around the 55mm chainline for their boost frames. People want wider tires (2.6+ inch), shorter chainstays on their 29ers hence the need for this, requiring a larger Q-factor as well. Pretty sure this is Shimano's new design template.

Th FC-M6130, FC-M7130, and the FC-M8130 are meant for Super BOOST and isn't really applicable in this discussion. It's spindle is 9mm longer than the FC-X100 and 3mm longer than the FC-X120.

With that said, if you have a 73mm bottom bracket, and your bike's boost rear spacing is designed around a 52-53mm chainline (e.g. 2020 Specialized Stumperjumper), then you should be using the FC-M6100. The engagement on the spindles will be exactly the same as long as you aren't using any rubber spacers on the spindle - and those plastic spindle guides on the Shimano bottom bracket are flush with the bearings. The 6mm spindle length difference between the FC-M6100 and the FC-M6120 is taken up by the two rubber spacers on the spindle (provided on the FC-M6120) and isn't really relevant to spline engagement.

I'd say once you've inserted the spindle on the bottom bracket, tap the drive side with a rubber mallet to make sure it's flush. Put some grease on the splines, connect the left crank arm, tighten the spindle star nut and tigthen the two bolts to lock it in place. Loosen up the two bolts slightly, back out the star nut and make sure you are getting 80+ percent engagement on the spline (gap wise). If it doesn't look good, I'd say tap it slightly with a rubber mallet and see if slides in more. If it looks good though, tighten up the star nut, tighten up up both bolts in alternating fashion to 13 nm and you should be good to go.

The LEFT crank arms are exactly the same on all three variations (e.q. FC-M6100, FC-M7120, FC-M7130), additionally the right crank arm is the same as well (albeit it's connected to the spindle). Only the spindle length that's different, and where or not you got no spacers (FC-M6100), a pair of 3mm spacers (for the FC-M6120), or a pair of 4.5mm spacers (for the FC-M6130). Basically, longer the spindle, larger chainline, and larger the Q-FACTOR.

I'm using an FC-M8120 (XT crankset, 55mm chainline) on my Commencal Meta TR 2021. Will take a pic of the spline engagement gap for reference.
Thanks for the advice on installation. I have a 2021 Ibis Ripley with 73mm bottom and it has a 52mm chain line. I will run through the steps above and will measure the end clearance. 80% engagement should be something like 2mm overhang.
 

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BB is 73mm BSA with one 2.5mm spacer on drive side. My understanding is the 6100, 6120 use the same spindle length - 115mm. Boost has different spacing on the chainring to get the proper chain line 52mm. Here is a pic of the interface.
View attachment 1914273
The bike shop and Shimano are working on it and will take care of the issue. I have other Shimano cranks (11 speed XT) with the same design (non-boost) and have never had a problem but the difference is that they engage a bit more of the spline. The failure could also be a difference in material or annealing process. Hard to tell without subjecting the arm to lab testing. Nonetheless I never had a crank arm come off on me before. As stated earlier, the minimal engagement makes me nervous.
I also had this happen . . . also on a V4 Ripley Deore. Happened within the first 50 miles. Noticed it was missing the plastic spacer between the bottom bracket and frame on the drive side (like yours has). Reinstalled everything torqued to spec and now using an aftermarket aluminum preload cap with loctite, and seems to be holding fine. Figured that missing spacer was part of the issue, but maybe not, since you had the same problem with it in place.
 

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I would suggest taking a close look at the spline in the crank arm, inspect it for damage. Mine ended up having to to be replaced by Shimano as a result of distortion to the teeth. The failure for was also similar as it happened on the 6th ride, around 60 miles. I checked all of the fasteners for proper torque on the bike before the first ride but did not pull the preload cap to check overhang. It’s possible something wasn’t fully seated, at this point I don’t know. What I do know that the arm was properly torqued to 14nm, that I am certain of. I normally ramp up the torque gradually alternating the tightening sequence on the two SHCS. I just don’t like the fact that there is only ~10mm of spline interface. Doesn’t seem robust enough when compared to my other bike which has a much larger positive engagement area on the spindle ( RF cinch). As for the design, Shimano has employed it for quite some time and I don’t see a lot of complaints on message board, so I will chalk it up to a rare occurrence. Nonetheless I plan to watch it closely.
 

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Might not be worth much, but on an Ibis facebook group there was a shop employee who reported they have issues with new bikes running Deore 61XX cranksets loosening while still on the floor. I think he said they suspected a metalurgy problem where the crankarm material was just taking a set when torqued down so the pinch bolts didn't hold tension.
 

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I also had this happen . .
This fastening design just plain sucks - lol. I know Shimano's been using this for a while but they really should trickle down the way they set preload on the XTRs down to these cranks. And replace that stupid plastic star nut used strictly for setting preload with a real bolt as a backup to prevent backouts similar to the way old Octalink V2's did it
 

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Might not be worth much, but on an Ibis facebook group there was a shop employee who reported they have issues with new bikes running Deore 61XX cranksets loosening while still on the floor. I think he said they suspected a metalurgy problem where the crankarm material was just taking a set when torqued down so the pinch bolts didn't hold tension.
Some people also don't use torque wrenches and believe in "GUD-N-TITE". The crank pinch bolts only needs 13 NM (and needs to be tightened in an alternate fashion), and once you over torque it and stretch the aluminum threads - it's game over as you won't be able to get the same clamping force ever again. Once that happens, best thing to do (if you don't want to replace the crank arm) is to drill out the threaded portion and see if you can find female metal inserts (Time-Sert). These aluminum threads are guaranteed to strip over time, given galvanic corrosion (metal bolt -> aluminum thread) and the threads being a softer material that easily deforms when over torqued.
 

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I'm using an FC-M8120 (XT crankset, 55mm chainline) on my Commencal Meta TR 2021. Will take a pic of the spline engagement gap for reference.
@funks I'd be interested to see the thread engagement in yours as I am using the same crankset on a Meta AM with a Hope PF41 bb. Cheers!
 

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@funks I'd be interested to see the thread engagement in yours as I am using the same crankset on a Meta AM with a Hope PF41 bb. Cheers!
Here it is on my Commencal Meta TR (XT FC-M8120) - bottom bracket is Wheels Manufacturing Thread Together BB86/92 (BB86-OUT-BB)

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And here is one of my other bikes - Shimano FC-MT510-1 & SM-BB52 Bottom Bracket

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Thanks! My XTM8120 has pretty much exactly the same length of exposed splines.
Here's the wear on the splines (not that bad, but still was not expecting to see any after 500kms) :
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FWIW here's my SLX 7100-2 crank; that's a 2 for the 2x chainring version. It's my understanding that the arm and spindle is the same as the 1x.

Exposed teeth measures just shy of 3mm.

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Might not be worth much, but on an Ibis facebook group there was a shop employee who reported they have issues with new bikes running Deore 61XX cranksets loosening while still on the floor. I think he said they suspected a metalurgy problem where the crankarm material was just taking a set when torqued down so the pinch bolts didn't hold tension.
Thanks for the input. It could be a metallurgical problem such as improper hardening / aging of the material to what’s known as T6 condition. As I read on another post, Shimano makes the crank arms by placing a billet of 6061 aluminum in a cold die forge, which requires a post heating step in an oven to get the right combination of ductility and through hardness. The hollow tech arms use 7000 series aluminum (aircraft grade, can’t be welded), and are bonded together. If the heating and quenching isn’t done right the crank arm could be too soft. When I inspected the spline, the teeth were 50-70% distorted, the damage was totally unexpected given short amount of use. Anyway it’s all speculation on my part and impossible to know without lab testing.
 

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The hollow tech arms use 7000 series aluminum (aircraft grade, can’t be welded), and are bonded together. If the heating and quenching isn’t done right the crank arm could be too soft. When I inspected the spline, the teeth were 50-70% distorted, the damage was totally unexpected given short amount of use. Anyway it’s all speculation on my part and impossible to know without lab testing.
The original post stated it's a 12-speed Deore Crank which I'm assuming is an FC-M6100. It's not a HOLLOWTECH (bonded) arms but just plain forged aluminum with sections machined out to save on some weight.
 
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