Anyone know how I can get this to work? It's a 1974 dt125 engine with a crank swap. I got the crank off ebay from what he said was a 1974 dt175 and got it rebuilt. It has the external threaded flywheel side and the keyed primary on the other

You need the parts to match your crank on the clutch side...  74-75 DT175.

On the ignition side you need either a complete 1969-73 CT ignition system...  but if you want to remain street legal it would be easier/cheaper to use a VAPE/Powerdynamo 12v CDI kit for the 69-73 CT.

I looked at my crank halves and looks like it's a 1978 crank because my counter weights looks like the 1978 compared to a 1974 seen in these pics. Will this work if I get a 1978 basket and primary gear? Even though they're both keyed on the clutch side the 1978 looks shorter too

Maybe i'm not thinking right but is yours electric start & now you have a mag flywheel crank. Save the pain & build your good rod into the old crank. Ok i'm half asleep & on the wrong track.

My e start system was screwed up and missing parts so I needed to start from scratch with a 74 175 crank with the external threads and planned on getting vape system. I gotta sell this crank now because I thought it was a 74 because I only took note of the ends of the crank and the ebay listing didnt show the sides so I couldn't tell that it was different.

Got it rebuilt anyway but now I see they changed alot in 78 including the main shaft, primary gear, clutch, etc.

Would cost more trying to swap parts than to just sell it and use the funds to get a 74 crank. The keyed wet side is shorter on the 78 which is why it's not fitting my 1974 parts

Hmmm...  I'm not sure you need to quit just yet.  I have a later crank in a CT1. 

It will take me a few days to dig out the parts and check but I'm pretty sure it can be done without a bunch of parts. 

I found some notes...  I think all you need is the 90387-20488 spacer from the later model.  Might be worth the $10 or so to see?  (Still available new from Yamaha, dealer might even stock it....  don't overpay for a used one.)

Does a magneto stator plate [or vape] fit in & locate in the case that is an electric armature case. I can see a locating pin for the field coil stator that will need to be removed. & are the two screw holes the correct thread 5mm or 6mm. I dunno, just throwing it out there before ya get in too deep. MarkT will know i hope.

Just got one for only 11$ off ebay from a yz80, it's the same part #. it does look shorter. If it works that would be amazing

I see the CT and AT series use the same crank cases so I found a kit on VAPE website for under 300$