Anyone Running Power THRU the Bottom Bracket?

Sounds like you are almost on the same train of thought as me, just a boxcar away. That should work too, I just hate bronze bushings.
If you make external cups for the outer shaft you can use a much larger outer shaft. Then you can use sealed bearings through it for the inner shaft to ride on. Use as much space as you can in the BB and have the bearings outside of it.

Dan, I dislike bushings too!

It took a tremendous amount of time and research to realize that no off the shelf parts exist to spin a larger shaft thru my BS/BSA-type bike frame.

My frame's shell is only 41mm ID.

A standard external BB can only accommodate a custom-made($$)37mm hollow spindle.

There is an external 54mm bearing cup.

However, it accepts a 30mm, not 37mm spindle.

The lack of 52mm bearing availability would prompt you to fabricate custom($$)bearing cups to accept ($$)bearings from factory assembly lines.

The maximum size custom-made($$)spindle to pass thru a 41mm frame is still 37mm.

So the hollow($$) custom-made 37mm spindle won't accept the standard 35mm sealed 1 3/8" bearing INSIDE its core.

Not enough "meat".

Your 37mm ($$) spindle can be designed to accept smaller ID needle bearings to support the 5/8"(16mm) inner shaft,

And THEN! You'll need to find special sprocket adaptors and lock collars to attach to your ($$)37mm hollow main shaft.

And that is why a larger custom($$) shaft is impractical for this project.

Cost of my 1" hollow shaft..10.00

13" long shaft, 3/4" diameter.....$47.00.

Bronze bushings.....$5.00

Used external BB.....$8.00

New 25mm bearings.....$20/pair.
 
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Have you ever thought about using a roll pin to attach your sprocket? I've done that before. My jackshaft I was putting the sprocket on was too long for the application and I didn't have room for the locking collar. So I cut the shaft and used a roll pin, works get and hasn't come apart yet in 5 years.
 
Well, i take some stuff back.

My Ebay vendor has a 1.5" (38.1mm)hollow shaft, which can be cut to 37mm.

However, there's no bearings that fits 37mm BB shafts.

The next available bearing size is 30mm.
But there's still no room for the inner shaft's sealed bearing.
 
I was EXTREMELY relieved when the one-piece bottom bracket (BB) cartridge AND threaded cup in my bike unscrewed easily.....

and my used Shimano GXP BB screwed right in!!

Now all I need to do is install correct bearings for my 1” hollow spindle.

Thankfully, the BB bearings are cheap and easy to find.👌
 
As a side note, the bike’s original bottom bracket threaded cup can be resourced to be used in a shift kit.

The cup has the same 1.37x24 tpi threads as the standard right sided freewheel:

https://www.ebay.com/sch/i.html?_fr...ACKET+CUP+UN54+ENGLISH+LEFT+HAND+NEW&_sacat=0

It can be easily welded onto the head of a steel offset crank arm and used in the standard shift kit for more engine clearance:

https://mbrebel.com/product/offset-crank-arms-square-taper-chrome-steel/

That’d be a $20 welding job, instead of trying to find a fabricator willing to “build up” the crank arm’s head and CAREFULLY threading this $200 job
 
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I WAS gonna use 25mm bottom bracket(BB)bearings and machine the 25.4mm spindle to fit,

However, maybe I can find a BB that’ll accept 30mm external bearings.

I have aluminum bearing adaptors that’ll allow a 25mm spindle to be used with 30mm bearings.
If everything fits, it’s easier to clearance bearing adaptors than a 6” steel shaft.
Besides, a turned-down spindle throws off sprocket and adaptor fitting.

Hopefully, 30mm bearings fit the BB that I have.
 
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