Hello All,
I decided to do the big bore swap on my 125 and have been looking for OEM top end parts. Today, I found what looks to be a near complete 1979 MX175 motor (minus carb and magneto assembly) at a swap meet.  The "completeness" of course needs to be verified, but is there any benefit to just swapping out the whole motor instead of just the top end?  I think I can reuse my points system in the MX, right?  From what I can tell the only thing I would be missing is the neutral safety switch.  The one thing I like about a motor swap is I don"t  risk totally killing the original motor.  Maybe I'm just over thinking it.  Anyway, I'd appreciate all insights.
Thanks,
Craig

If it is a 1979 MX175 engine, yes, it is virtually identical to the 78-81 DT175 engine and will bolt right in...  ignition will swap over, etc. 

It may have a "better" clutch than your engine too...  I'm fairly certain the 78-79 DT125 monoshocks had a smaller (less discs) clutch than the 78-81 monoshock DT175....  starting in 1980 Yamaha started using the same clutch (and CDI) in the DT125 as they did in the DT175. 

MarkT, Thanks for the explanation.
I guess my next step is to validate the "completeness" and decide from there what makes most sense.  Should be a fun project.
 

You can also swap the clutch assembly onto your DT125 if you need to...  best to swap crank gear with it to keep gears as a set.

Yeah, the clutch is pretty much brand new and works great thanks to your help.  Good tip on keeping the gear sets together, I would have over looked that for sure, thanks!
Now, as far as the 125 carburetor goes, I'm guessing I need to rejet it to be similar to the DT175 or MX175 specs.  If so, how close does that need to be?  The DT and MX 175's jetting is slightly different from each other (Main jets and pilots).  Both have the same main nozzle but that part is different from the main nozzle in the carb.(at least per CMSNL pats list).

Hey 'nextgenrider' - fwiw I did a similar swap on a '78 DT 125 a few months ago, and took advice from a different thread with a reply from 'bigdog302' here and went with 22.5 pilot jet and 145 main jet - limited saddle time so far, but seems to work.
www.yamaha-enduros.com/index.php/forum/p...t175-jetting?start=0

OTBMF,
Thanks for the input!  I understand I may have to play around to get a perfect combination, but it's nice to start with a know set up.  Thanks for the link too, very interesting.  Engine temp is yet another variable I have never even considered.