Here's my bike:

Here is the only picture I have showing the sprocket offset of the 175 engine when in the 100 frame.

I can take more pictures at some point should anyone want to know/see more, just ask.

Here's my bike:

Here is the only picture I have showing the sprocket offset of the 175 engine when in the 100 frame.

I can take more pictures at some point should anyone want to know/see more, just ask.

It appears the engine case and frame changed between 1975 and 1976 on the DT100...  1G1 is, at earliest, a 1976 engine.  I suspect that has something to do with it.

I'm curious if a 1976-on 100 would have the same issue. 

It looks like you have an aftermarket expansion chamber on it. Very hard to find for the crossover style pipes. If you decide you would rather have an original exhaust, maybe we can work something out. My bike has the original stock pipe and mine is nowhere near original anymore. And I have been looking for an expansion chamber.

imgur.com/a/8g2eldX

EDIT
Or maybe that’s just a newer year pipe on your bike. It’s not stock for that year.

Yes, it's an aftermarket pipe for a monoshock dt175 that I have cut up to make fit my project.
I have the original Dt100 pipe like yours aswell.