Putting the number in ebay advanced search comes up blank.
Is it just me or did someone buy them.
Doesn't matter & was only being curious to study them.

Here we go.

1973 Yamaha RT3 360 Left & Right Engine Cases | eBay
 

Ebay be careful. Number below says RT1 360.
I can see 291 cast into the case in one pic.
But has 275 in felt pen underneath.
Mouth of the case looks 360 size.
291 comes up RT2 RT3 so probably doesn't matter.
Price rules it out though unless you're want new only.
eBay item number:325821000009

1971 Yamaha Rt1 360 Left Right Engine Motor Crankcase Crank Cases Set NOS R5 | eBay

Looks like I should fix my “slow to return” kick starter before this happens!

I'm basing my claim on picture #9 of the ebay ad. I've tried to attach a close up photo of the picture clearly showing the part number and starting with 291. Not sure why, but it seems like it's not allowing me to upload it. Another photo does show the stamped vin as RT1, and that's why I'm asking, do these interchange? Because mine starts with 275. I'm not just dreaming this up, it's right there in the picture. Please go to the Ebay ad and zoom in to the triangle area and you will see it. Granted, it is the left case, but I'm assuming the right has the same starting numbers. They don't have a clear picture of that area on the right case. Sorry if I have offended you, but before I spend money on something like this, I like to see if anyone has more knowledge than me. These days, I don't like wasting money if I don't have to. 

250 and 360 cases are virtually identical except for the final machining.  Casting numbers are not necessarily part numbers.  I've seen dozens of castings with numbers from completely different models...  sometimes on parts that have been machined differently and don't fit that model. 

Seller represents those case to have come from an RT, and the factory VIN stamped into the case half with the "291" in the casting confirms an RT VIN.

Ebay has a money back guarantee and the cases are clearly described by seller to be RT3 so the argument about "wasting money" is a bit weak.

I am not offended.  Just frustrated.  Had you paid attention to the "model code" warning at all, you could have researched that 291 was a 250 code and asked about the discrepancy.  Or even a question like "What's the deal with the "291" on the casting in pic #9?" 

Nope, you didn't ask about the anomaly, you stated that these were absolutely 291 cases and asked if they would fit your bike.  Kinda blows the "just trying to learn" argument out of the water for me. 

And yes, I did jump to the conclusion that you assumed "291" from a parts list.   Yes, I could have asked why you thought these were 291 cases.  I guess I'm just tired of what seems like engaging in lengthy interrogations to answer questions when people seem to have already made up their minds.  (It especially got to me that PABDT, who was genuinely trying to help, seemed to feel bad for "misleading"... had it not been for that post, I would not have answered.)

Next time I'll do the smart thing, just not answer, and send a supportive PM to the "PABDT" who was trying to help.

I hope you find the cases you want or can get yours repaired. 

One last thing...  please do not take any of this personally!  Your post just happened to hit at a stressful time when my patience is short.  Fortunately, my desire to help and educate is still very strong.  I'll just have to not post when I can see and feel I'm headed for a train wreck! 

I just clicked along that parts fish to 1970 DT1CMX & shows 291 part number for crank cases. Not sure if i'm confusing the story without reading back on my posts but MarkT has it sussed thank goodness. 360 case mouth for the cylinder is obviously larger than 250.
www.partsfish.com/oemparts/a/yam/5004152...9bc78420e/crank-case
Don't let me confuse things, i'm just waking up.

A potential solution may be to weld the case? Hard to say without seeing it but i do know a guy in the community who welded the case on my 73 RT360 where the chain had flown off and broke one of the little "nubs" that stick out to hold the actuator cover on. He did a fantastic job.

I,m with you on this one Enduro Nut.

I have all my broken engine cases welded.I don't care how bad they are. I like to keep the original engine numbers with the bike with a original engine in it. Of course i have access to some expert welders at a few shops in my area. I never had one they couldn't fix and i have had some pretty bad damaged cases.  

Nersasian
Just google search for your local area for welders. Then go talk to them. You will still need to take the engine apart and have a clean bare engine case to hand to a welding shop to be repaired so it's a good time to rebuild your engine if needed. 

Good luck with what ever you decide.