I need to replace the right hand crankcase on my second CT1-C as the original has a crack in the kickstart area and leaks transmission oil.
Am I right in thinking the CT3 case is the same other than using the larger 23mm outside diameter needle bearing (22mm on the CT1).
I have a spare CT3 bottom end that I can take the right hand case off, but I wanted to confirm compatibility before I split that engine.
Also, any thoughts on what might have caused the crack in the first place?
The bike is new to me, kicks over, runs and rides fine, I haven't investigated under the clutch cover yet.
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Both of the CT1 cases I have contain different support structure there (compared to yours) and yet another CT1 I saw has 5 beefed up supports, so Yamaha must have been dialed in to this as a potential concern. If kick error is a thing, then maybe people tend to over kick and push down, so it looks like they added support for that. Also, thats a long lever arm on the back rear mount.

Pabdt, you're right there are three different versions of the CT1 right hand crank case to my knowledge (it's easier when you've had all three models).
First gen have no support webbing, second gen as on both of my CT1-C models have three support webbings, later CT1-C models have five webbings.
My AT1-C is later in the production and has five webbings and also has the later frame without the separate engine mounting plates.
What I really need is confirmation that a CT3 right hand crankcase will fit my CT1-C left hand crankcase, with just the bearing requiring replacement.
I'm 99% sure it'll be fine, just thought I'd ask in case anyone knows for sure.

Pete, we all know you are the CT1 guy! The only ribs I care about are from a cow! : ) Was just hoping to help out. I don't know about that case, but me I would fit it and test.

"Also, any thoughts on what might have caused the crack in the first place?"

* in your pic, the clutch cover case is not cracked but bottom right is a bolt connected to your engine case. What if the force from the kick were transmitted through that bolt. Could that have pushed on the case and split it?

* another option, casing a jump forcing engine in to frame, vice versa. not sure the case would even give or frame, just a thought.

* chain derailment/prying on kicker on disassembly??

Look forward to seeing this bike up and running.

 

The cases on the 69-73 AT/CT and 74-76 DT125 (12v e-start model only) are the same and "technically" interchangeable.  The right-side case was updated several times and the later DT125 electric start twinshock might be the "best" in terms of that.  (Kick starter parts are also stronger on the newer DT125 e-start models.)

Only problem I've ever had with cases was a small mismatch on the cylinder mount surface.  It was with a brand new LEFT side case (original was broken by a thrown chain) from ebay that was really cheap and may have been aftermarket?  

I tend towards keeping cases as a "matched set" but that isn't always possible if you want to keep the VIN number and the CT left case is the good one.  

Thanks gents, appreciate the replies.
I'll go for using the right side CT3 case as I want to keep the matching VIN