Hey guys I'm working on a full 1974 dt125 rebuild. I need a new crank and was wondering if a 175 crank will fit? Gonna ditch that stock ignition setup because its rotted and thinking about getting this. this www.dcvmx.com/xcart/product.php?productid=187861&cat=&page=1

From your picture of the side cover I'm guessing you currently have a 12v generator/starter DT125? 

If you had an earlier (69-73) 125, the answer on the crank would be a simple "yes" to a 69-73 175 (6v magneto) crank bolting right in..

1974 was a big revision year for Yamaha.  Not only did most models get the "DT" name but they had major redesigns of frames and engines.

One exception was the 12v generator/starter DT125...  the 74-76 DT125 12v generator engine stayed more like the 69-73 models.

Not exactly the same...  Do you know if your crank gear has a keyway?  Earlier models had a splined shaft and gear.  I'm pretty sure that you have the keyed type. 

Good news is I'm at least 95% sure you can use a crank from a 74-75 DT175. ***

Now the bad news.  Your DT125 will have the same stator diameter of the 69-73 125/175 (AT/CT) which is 130mm.  The ignition kit you linked will not fit your cases.  It is designed for the "redesigned" 1974 engine with a 120mm stator that the 6v magneto 125's and 175's had. 

For the "monoshock" models (First sold as 1978 models in the USA) the DT125 no longer had a 12v electric start option and that's when the US DT125 finally got the 120mm stator. 

Options are seeing if HPI can provide a 130mm stator plate with that kit and use the 175 crank.

Or rebuild your 125 crank and get the excellent kit from VAPE/Powerdynamo.  Mako has a nice write up on the conversion he did on his 12v electric start 125.  (VAPE makes a kit that fits the original 69-76 12v electric start crank)


***76-81 DT175 crank might also work.  I'm building a highly modified CT1 with a 1980 crank...  bearings and everything seem fine...  the only thing I'm not sure of is that in 1976 Yamaha changed the primary gear ratio (crank/clutch gears).  I'm using the later gears and not sure if the earlier gear you have will fit the later crank...  I think it will...  just not sure. 


 

Thanks for the great reply, Yeah my crank end was keyed, I have the one with this e start setup but I'm missing the governor assembly, rotor, and the stator/points have all wires cut and its destroyed.

Also my crank is garbage because the seal surface has deep pitting so need a new crank off ebay. Now that you know the exact cases and crank that I have, are you able to link me to any solid state ignition setup i can buy? If I have to I will get the same crank but if I have other options that would work and might be cheaper, I'd do it. Just tryna weigh out my options. The engine number is 444-108874

So have you ever seen anyone use a newer crank with the external threaded end on a 74 dt? Or that crank is only compatible with the 120mm stator starter won't fit in my bike? Either way I'm getting a solid state because the old governor piece is unobtainable. May just have to find the same exact crank I have. No big deal

One of the virtues of Yamaha, especially with the 125/175, is that there is often interchangeability over many years.

It's not the threaded crank end or the stator mount...  it's just your choice of ignition.  HPI ignition is very modular and they may have a 130mm two hole plate.  Or a kit for the Yamaha 69-73 CT

Basically if you install an externally threaded magneto crank you need an ignition that fits a CT1-CT3

 

Ahhh I see. Thanks so much for your expertise mark, I'll be back if I have any more questions.

I probably could have said it better...

69-73 AT (125) and CT (175) shared the same center cases with a 130mm diameter stator size. 

In 1974, the "kick start only" DT125 (not offered in USA) and DT175 got completely redesigned center cases with a 120mm stator size.

The 1974 to 1976 USA "electric and kick start"  DT125  kept the older 69-73 case design with the 130mm stator. (This is what you have)

For the 1978 USA model year we got the "kick start only" monoshock DT125 and DT175 with yet another redesign and 120mm stator.  (There was no 1977 model DT125 or DT175 in the USA)

Easiest way to tell if the cases have the early 130mm stator is the clutch actuator is hidden inside the ignition side cover.  The 125's and 175's with the 120mm stator all have the clutch actuator in the left side center case with the actuator arm exposed on top of the engine.

Crank bearings and seals are the same for the 125/175 from 69-81.  Best ignition side seal to use is the 77-79 IT175 seal. 

Cranks are also very similar through the years.  Pre-74 models had a splined crank gear.  74-on had a keyed crank gear.  Around 1976 the primary gear ratio was made taller.

On the ignition side, the electric start 125 models had a shorter stub with internal threads.  Armature is held to crank with a bolt.  The kick start only models had a longer stub with external threads.  The magneto flywheel is held to crank with a nut. 

Electric and kick models were 12v.  Kick start only models were 6v.

Wow that's all the best info right there. Exactly what I was looking for. So no matter what I need a kit with a 130mm stator plate? I assume a 120mm plate doesn't line up correctly with the threaded holes in the case?

Correct. Not just the threaded holes. As or more important is the raised boss that precisely centers the stator plate to the crank. 

I did alot of research and I decided I'm gonna go with a vape ct1-3 ignition with the 130mm stator and get a 1974-75 dt175 crank.

I found a nice crank on ebay with the same keyed primary gear end on the wet side as the 74-76 stubby crank so I wouldn't have to change the gear, it has the external threads on the dry flywheel side that looked identical to the ct cranks that should fit the flywheel that comes with the Ct kit from vape.

It's very hard to find a nice stubby crank so that's why I'm going this route, plus the kit is a bit cheaper at 320$ shipped from the manufacturer