Different cranks use different axle sizes and different BB attach to the frame differently depending on the standard which also specifies the BB outside diameter. So a normal PF92 BB has a 24mm axle and will work with any Shimano crank or that has a 24mm axle. There are other standards that are bigger and take a bigger axle which is usually stiffer. There are some aftermarket companies that make a BB for a certain standard but that accepts a bigger axle. The only problem is there is less room for the bearings so they have to be thinner and are normally less durable.
The ZTTO thread together BB for PF92 is wider than a normal one which would probably be an issue with a standard crank with a 52mm chainline. But a Boost crank with a 55mm chainline has an axle that is slightly longer and is what I used. With a Shimano crank, they specify to use a spacer on each side of the frame and this made the axle not long enough so I removed one spacer on the drive side which placed the crank as close to the frame as I would want and about perfect. The other side fit on the splines as intended and worked well for the AM831 frame. The crank uses one less spacer which with the extra bb width, moved the crank in roughly 1.5-2 mm VS the non drive side but I don't think anyone would be able to feel this. It also moved the crank as close to the frame as I would be comfortable with which gives it the best chainline possible which is also good. For reference, a 32T chainring has 4 or 5 mm of clearance with the frame at the tip of the teeth which is normally the closest point. However, the Shimano rings have a screw or rivet head that attaches the ring to a permanent spider which only has about 1.5mm of clearance but is closer to the BB and I think will be fine with any normal flexing. An aftermarket ring that's all one piece would give more clearance where the tips of the teeth would be the closest point.
Anyways, sorry for rambling but I dealt with this exact setup. Also, there are obviously other issues with the frame and I would advise anyone to wait until it's updated. I've only been able to ride mine around the yard and has sat in a work stand for over a month. In addition to the bad upper link design, the derailleur hanger is outside of Sram specs for 12 speed. Mine won't shift anywhere near acceptable but it sounds like most are OK with the shifting but I have to think it would be better with a correctly designed hanger. A few others have mentioned shifting problems but the other problems are getting all the attention and I don't think anything is going to be done about the hanger. I emailed Eddy over 2 months ago with pictures and the Sram technical documents showing how it's out of spec and he said it was sent to the "engineers" and every time I ask he hasn't heard of anything. I really want to like the bike but even with an updated rear end, it's just art if it won't shift correctly. I'm probably going to order a Carbonda FM1002 and maybe one day all the issues will get worked out of this frame, but now I'm really rambling...
