Which is the correct plug for a 1973 RT3 360? I have seen B8ES and B9ES listed in what should be good sources. Thanks.

My owner's manual lists the B9ES for the RT3.

My owners manual does too. Other sources say B8ES. The B9ES I believe is a hotter plug. I don't want to cause any problems.

With NGK spark plugs, the lower the number , the "hotter" the plug. Opposite of most American plug manufacturers. Best to keep an eye on plug insulator color after a short ride. A ''10" will probably be too " cold" and will foul easily. A "9" is a good starting point.
 

I just checked the NGK website to be sure. You are right. Lower number is a hotter plug.

Just wondering if the "other sources" was one of the Yamaha online parts lists? 

The problem is that many of those lists were originally shared among similar models...  for example the DT3 and RT3 shared lists.  The original lists often had columns for each model covered...  a check in the DT3 column meant that part fit the DT3.  Parts that were the same for the two models would have a check in the DT3 and RT3 columns. 

On things like pistons and sometimes spark plugs, the parts were not the same so you'd have two part numbers listed and one would have a check in the DT3 column and the other part would have a check in the RT3 column. 

The problem is at some point the "column information" got lost.  So it's easy to accidentally get the wrong part.  Sometimes the "orphan" part is listed as one of the "0" parts in the beginning of the new lists.  Other times there is a note or something to help.  Mostly, nothing.  Obviously you wouldn't be able to install a 250 piston into a 360...  but you could easily install the wrong spark plug!

Which is all a very long way of saying "Good job on checking to make sure you're using the correct plug". 
 

9 is pretty cold. That's the heat range you see in a 125 running 13000 rpm. I run an 8, a 7 would be fine as well.

I don't remember what the other sources were. I've been all through this site and all over the internet in the last couple of weeks. It wasn't a matter of 2 columns though. It also wasn't a plug manufacture. I'm leaning towards some reference on this site. I may be wrong but pretty much everything that I've seen has been on here.

I figured that the owners manual was correct. It better be! But I figured that a fellow owner could confirm. You guys are the knowledge base and I'm just a newbie. Thanks for the help.

I stumbled on one reference on this site. It's under Tech Library - Spark Plug Table. It shows a B8ES for the RT3. Wonder if any more are wrong.

Info from Yamaha itself has a lot of errors. 

P.S. I was trying to explain there is no longer two columns so you have to be very careful with the parts lists sometimes if you want to get the right part.