I thought I would put a quick bit of information in here for anyone who has wondered about doing the same thing. After much googling there was no solid information available about putting a bigger engine into a DT100 frame, so when a cheap dt175 engine came up on FB marketplace local to me, I went for it.

I have a 1g1 175 engine, it bolts straight into my 1975 DT100
However I did have to widen the frame where the swingarm mounts by around 8mm as the chain was touching the frame. The sprocket on the 175 sticks out a little further than the 100, just enough to cause bother.

I modified a cheap second hand gianelli exhaust for a later model 175 to fit. I didn't have a standard DT175 exhaust to try, that may or may not be an easier option, I don't actually know how different the 175 frame is from the 100.

So in summary, if you can't weld or don't dare cut your classic Yamaha then don't bother. If you can then it makes your cute but slow 100 much more fun!

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Interesting. Doesn't that mean your front and rear sprockets are out of alignment? 

Yes, but not by enough to concern me.

Complete guess on my part...  but an "educated" one from a similar situation with a Yamaha engine swap I did years ago.

The DT100 and DT175 share the same case design and output shaft as well as a lot of other power transmission parts during the same and similar years.  There is a spacer under the engine sprocket and Yamaha made different heights for similar engines put into different frames.  Using the 100 sprocket spacer on the 175 engine would likely prevent having to modify the frame. 

 

Spot on Mark.
I noticed that the sprocket nuts on my 100 and 175 are different.  The 100 is more hollowed out. 

You have now made the reason for that obvious.  To accomodate different width spacers whilst still snugging down tightly against the sprocket and not fouling the outer of the splines.

100 nut.



175 nut.

Haha! Brilliant. Will have a look when. I'm back at my workshop.

Tinkicker made an excellent point.  As Tinkicker mentioned, you may need to also use a more "hollowed out" nut if going to a shorter spacer. (In my case, I needed a taller spacer) 

I repaired a bike once that friend bought with a loose front sprocket.  Someone had installed the nut upside down and it was bottoming on the splines instead of clamping the sprocket tight.  (Owner had tried to fix it and even replaced the sprocket...  but didn't notice the relief in the nut and put it back on the way it came off.  Flipping the nut over was a 5 minute fix...  owner was worried he needed a new output shaft or other expensive repair.)

It's also possible you might need to find a spacer from a different model to get the alignment right.  For example if I remember correctly the 175MX (monoshock) spacer is tall, the 77-79 IT175 spacer is slightly shorter, and the earlier 175 spacer is shorter still at around 10mm tall.  Certainly checking parts lists is helpful.  We did not get the 1G1 engine in the US and I haven't seen a parts diagram so I'm not sure what spacer(s) it might have come with over the several years it was used in models sold outside the USA.  But I believe it was around 1976 some of the US (YZ100/125/175, TY175 and IT175) models started coming with the taller spacers...  and I think the 1G1 first came out in 1976. (DT and YZ engines share the same DNA)


 

I am slightly relieved to report, that the engine cases are different (though very similar, all the covers are identical) and no amount of spacer swapping would have given sufficient clearance for the chain.
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My Dt100 is a 1975, and I believe my 175 engine to be a very similar age. From what you guys are saying, I think it must be later models than mine that would have been as simple as swapping spacers.

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