The 100mm bbshd is significantly longer than might be expected.
To give a better idea see the annotated pic below of a standard size BBSHD in a standard BB shell.
In this example it is having some trouble clearing the chainstay due to how chainstay flares out.
The annotations show how far the axle and spindle would be pushed out if this were a 100mm BBSHD and stayed spaced out to the same extent on the drive side.
100mm BBSHD has the axle (the cylindrical portion of the casing) basically flush with the motor core, rather than ~1" inbound as seen on standard BBSHD.
Similarly the spindle is also extended, and so the crank will also be extended by roughly this length.
This can be problematic because there is no good place to put that extra length.
If you space it all out on the drive side, it will mess up your chainline and the pedals will not be symmetrical. Realistically, symmetry is not really noticeable, but a bad chainline definitely is, it will drop the chain. And if you space it out evenly on both left and the right, the motor may slip due to the black triangle bracket not being able to bite into the bottom bracket shell and hold it in position.
The ridges on the black triangle adapter help hold the motor from twisting
Adding spacers on non-drive side may prevent this from holding on the frame itself if just pressing against a spacer
If you must put the non-drive side spacers right against the frame, this can somewhat be mitigated by getting the lockring very tight but you may run risk of stripping something.
Spacing it out evenly is also likely to make the chainline worse than it would be with just enough spacers to clear the chainstay and spacing out the rest on the non drive side. How critical chainline will be to proper shifting will be dependent upon the frame, but it is good to keep in mind. Generally speaking, the wider the axle on the rear, the less this will be an issue. So if the bike uses boost axle on the rear, or any other way that the dropouts end up wider than typical, this would be indicative that spacing out the drive side would be less of an issue (though still an important consideration)
What if you want to put all of the spacers on the right?
Putting all the spacers on the drive side definitely can work even with a 100mm motor on 73mm BB shell, although it might necessitate switching to an IGH on the rear, or using the low limit derailer screw on your derailer to keep it out of the lowest gear in addition to using the eclipse.
In any case, if going the route of 100mm BBSHD on a 73mm frame you are very likely to need extra spacers to take up that extra space.
Probably 2 or 3 of these spacer kits.
Alternately, in a pinch you could use these locknuts from a hardware store:
Additionally, to prevent motor rotation once mounted (if you spaced out on both sides) you may want to consider an alternate mount bracket. Here are the two options we provide.
Generally the BSB-1 is preferable, though it can vary depending on the frame.
Luna BSB-1 Stabilizer Bar
Luna BSBF-1 Stabilizer Bar (typically used with full suspension)
This is an example of a build that would benefit from use of a BSB stabilizer bar:
In conclusion, it is possible to use 100mm BBSHD on a 73 or even 68mm bottom bracket shell, but there are several things to keep in mind when doing so.
Alternative approach
In the first picture, the chainstay is fairly intrusive, but it can still work with a standard size motor without needing a larger motor and spacers. While both motors would work, generally speaking it may be preferable to use the smaller motor than switch to a larger motor, primarily because exchanging a motor can take time, as well as the other build considerations listed above. If using a standard size motor with an intrusive chainstay you would need to space it out, and you may run out of thread on the axle to use the black lockring if you space it that far. However you can use loctite in its place as the black lockring is only a jam nut.
To give a better idea see the annotated pic below of a standard size BBSHD in a standard BB shell.
In this example it is having some trouble clearing the chainstay due to how chainstay flares out.
The annotations show how far the axle and spindle would be pushed out if this were a 100mm BBSHD and stayed spaced out to the same extent on the drive side.
100mm BBSHD has the axle (the cylindrical portion of the casing) basically flush with the motor core, rather than ~1" inbound as seen on standard BBSHD.
Similarly the spindle is also extended, and so the crank will also be extended by roughly this length.
This can be problematic because there is no good place to put that extra length.
If you space it all out on the drive side, it will mess up your chainline and the pedals will not be symmetrical. Realistically, symmetry is not really noticeable, but a bad chainline definitely is, it will drop the chain. And if you space it out evenly on both left and the right, the motor may slip due to the black triangle bracket not being able to bite into the bottom bracket shell and hold it in position.
The ridges on the black triangle adapter help hold the motor from twisting
Adding spacers on non-drive side may prevent this from holding on the frame itself if just pressing against a spacer
If you must put the non-drive side spacers right against the frame, this can somewhat be mitigated by getting the lockring very tight but you may run risk of stripping something.
Spacing it out evenly is also likely to make the chainline worse than it would be with just enough spacers to clear the chainstay and spacing out the rest on the non drive side. How critical chainline will be to proper shifting will be dependent upon the frame, but it is good to keep in mind. Generally speaking, the wider the axle on the rear, the less this will be an issue. So if the bike uses boost axle on the rear, or any other way that the dropouts end up wider than typical, this would be indicative that spacing out the drive side would be less of an issue (though still an important consideration)
What if you want to put all of the spacers on the right?
Putting all the spacers on the drive side definitely can work even with a 100mm motor on 73mm BB shell, although it might necessitate switching to an IGH on the rear, or using the low limit derailer screw on your derailer to keep it out of the lowest gear in addition to using the eclipse.
In any case, if going the route of 100mm BBSHD on a 73mm frame you are very likely to need extra spacers to take up that extra space.
Probably 2 or 3 of these spacer kits.
Alternately, in a pinch you could use these locknuts from a hardware store:
Additionally, to prevent motor rotation once mounted (if you spaced out on both sides) you may want to consider an alternate mount bracket. Here are the two options we provide.
Generally the BSB-1 is preferable, though it can vary depending on the frame.
Luna BSB-1 Stabilizer Bar
Luna BSBF-1 Stabilizer Bar (typically used with full suspension)
This is an example of a build that would benefit from use of a BSB stabilizer bar:
In conclusion, it is possible to use 100mm BBSHD on a 73 or even 68mm bottom bracket shell, but there are several things to keep in mind when doing so.
Alternative approach
In the first picture, the chainstay is fairly intrusive, but it can still work with a standard size motor without needing a larger motor and spacers. While both motors would work, generally speaking it may be preferable to use the smaller motor than switch to a larger motor, primarily because exchanging a motor can take time, as well as the other build considerations listed above. If using a standard size motor with an intrusive chainstay you would need to space it out, and you may run out of thread on the axle to use the black lockring if you space it that far. However you can use loctite in its place as the black lockring is only a jam nut.