I think I posted the 69-73 CT VAPE kit would work...  but it could have been a different thread. 

Don't get the AT kit!  It will have the flywheel for your original crank that uses a bolt. 

Keep up posted if the spacer works for you...  I know it can be done...  when I did mine I had a ton of parts from various 175's from 69 to 81 and actually turned the CT into a 6 speed...  so I changed more than the crank.  But pretty sure the spacer is the magic part you need to make it work.

Wow MarkT sounds interesting a 6 speed in a CT you must be an engineer. How does it run? 

I haven't run it yet.  It's one of my land speed record project bikes....  one of the challenges some other members here had seemed to be if they geared it up it couldn't pull 5th... and if they geared it down to make the jump from 4th to 5th it would run out of rpm.  That was with the close ratio MX 5 speed gears.  They still hit impressive speeds. 

My idea is to use a close ratio 6 speed YZ gearbox...  on paper it should help.  Was a lot of mixing and matching parts but no case mods were needed.  Engine looks stock from outside which it needs to as I hope to set a "stock" class record.  Life has gotten in the way but maybe this year?

 

Sounds wicked I was wondering about the gear ratios in 6 th gear please let me know how it works and I hope you set a record 

Yeah, i'm sitting up & taking notice now Mark.

Unfortunately it's not gonna work :( technically it would work but it's only gonna be grabbing half the teeth on the primary. Could this cause it to break apart If ran this way?

Are you sure clutch is on all the way?  Doesn't look right.   

Also it's possible that they changed the thrust washer arrangement....  Yamaha was trying to narrow down the engines.

I have the clutch on right going off schematic www.cmsnl.com/yamaha-dt125-1974-usa_mode.../partslist/A-08.html , the way these teeth are angled arent they always engaged the same amount even though it's like 70% on? Its 2.5mm away from being all the way lined up.

I took a video and its turns really smooth with no play. Wouldn't running it this way be the same as the gear being a little more thin if my guess with the teeth being engaged all the time is correct? If that makes sense. Btw I'm cleaning that dirt out of the gears haha i just wanna make sure I can get it to work first.

There should be only one thin washer under the kick gear under the clutch basket.  Yours looks spaced pretty far from the bearing in the case for some reason.  Are you sure you didn't put the thrust bearing there?

Later I will compare diagrams...  my engine is buried in a shed right now and I can't get to it to compare.