Hi,

I'm currently building a Yamaha AT3 Frame I bought at a junk yard. Basically I want to get some reassurance on what I have found on Google and some additional insight / guidance going forward. From what I understand AT1 2 & 3's are nearly identical to the CT series & DT 125's.

The frame I found has a 1973 AT3 engine with a 171c header and piston, I'm assuming it came from a CT series. I took the original engine apart and the innards look pretty nice, considering it was in a yard for some time (pic 2). The dynamo assembly itself is a pain in the ass, and there aren't many videos / articles on the web that cover it. Plus, I would rather switch to a static stator & rotating flywheel for many reasons.

My big question is, if I have a AT3 engine, can I buy a CT3 175 crank case and migrate everything (innards) over? Buy a stator / flywheel, find the correct crankshaft, ect.

Pic 1: Frame



Pic 2: Clutch Side



Pic 3: Dynamo I want to change (the whole reason for this)




Appreciate anything y'all can give me!

This is what I'm working with:

You just need a 1969-73 CT crankshaft and of course the ignition (stator and flywheel).

Crankcase is the same on the AT and CT.

I should know--but think--the left case has a different way of mounting the generator stator whatever ya call it, compared to the flywheel stator. Just check it out. I'll go look on partzilla if drawings are clear enough. --Had a look at AT3 & shows the generator & magneto in the same pic & only one crankcase in other pic so i must be wrong--again lol. Not in same pic but says Magneto & also says Generator then in crankcase link only shows one option. Top end swaps over ok. AT1 [piston port motor] has a different conrod length but AT2 & 3 just swap CT2 & 3 175 top end. I've got a TY175 top end on my piston port AT1 magneto model but think i added a base plate--must go look, too long ago. 175's go very well. Link below if it works. Rough old think we've had as a farm bike since the mid "70's as a trade-in at work. Clutch cover has never been off it. Sorry to woffle on, on your page. Have super high road gearing on it these days.
www.partzilla.com/catalog/yamaha/motorcycle/1973/at3
www.partzilla.com/catalog/yamaha/motorcy...3/crankcase-cylinder

It was the 250 that changed the rod length in 1972 when they went to reed valve... 125 and 175's have had the same rod length from 69-81 both reed and piston port.

Ok--umm, maybe i was trying to run an RX125 top end which has the super tall piston & is 5mm taller from pin to top. Never mind, so long as OP's works.

This is super helpful, thank you!

Okay, so just to verify?

These styles will work:



This style will not:

1969-1973 CT will work.

The 175 had a major redesign in 1974... the primary gear end of the crank went from "splined" to "keyed" so even though a 1974 DT175 crank might work, you'd at least need the crank gear too... and maybe other parts. Best to stick with a 69-73 CT1 crank.

A 250 crank will not work in a 125/175.

Be careful of cranks that show any rust... if the big end bearing got rusted at all it will fail quickly. Unless you are rebuilding the crank anyway.

Powerdynamo/Vape offers a modern CDI for the AT series motors.  Mine works great and have never missed the electric start option.  One kick and were ready after brief warm-up.  CT barrel and pistons is all you need for the conversion.  Thats all I needed on mine.  You may need to rejet, depending on elevation. 
 

Need head too. Mind you machine a 125 head out to 66mm at the edge & she'll be a high comp head--got me thinking now with my conversion haha.