Im in the process of putting the top end on my 73 RT360 and i noticed that i have 2 expander rings? Im re-using the standard Yamaha 80mm piston because its in great shape. Whats funny is the rings that originally came off it are the same rings. What i mean is there was no keystone ring in the top grove? I have NOS rings ready to go i just need to know if im supposed to use the expanders or not?

If it's OEM Yamaha, will almost always be keystone rings which have a very slight taper on the top surface. 

What you probably mean is you don't have a Dykes top ring?  ...which is an L shaped top ring.

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I think we need a picture and part number of your new rings, as well as a picture of your std piston please . . . so we can see precisely what we're dealing with.

The OEM Yamaha RT2/3 piston rings did not come with expanders, so you must have purchased aftermarket, or possibly even RT1 rings
Hard to give you sound advise, until we can see whats on the table!

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Ok so here is a picture of the piston and the rings that were together. The numbers on the head reads 74 K and 960 down at the bottom there. That could be an 8 and not a 6 but it looks like a  6 to me. They dont seem to want to fit in in the grove but they are rusty and i know they came off that piston. As i recall they were tough to get off. They measure 2.0mm thick. The 2nd ring in my NOS kit measures 1.4mm thick.

Here are the pictures for the NOS rings as well as the 2 expander rings that came off that piston you see in the previous post.

  

This is a 72-73 cylinder that takes reeds. I do not have the serial number for the engine. somewhere along the lines the cover got replaced without a number stamped on it or someone sanded it off to paint. I spoke to DEET back in the spring on the phone about this bike. its a 68 frame with a 72-73 RT360 motor. I have 2 extra motors that are 72's (currently parts motors) and thats where this jug came from. To my knowledge there is no difference between the top ends on the 2 years but im no expert here so maybe im missing something. Not sure what that XO stamp means?




 

Might be my eyes but the top ring of the new set looks like a Dykes ring (L-shaped cross section).  I get confused easy because initially it sounded like the new and old rings were the same but the new ones came with expanders.  Now I see I was wrong about all of that.    Pictures really help.

Luckily you have a real piston and ring expert* (Enduronut) helping you sort this out! 

*Probably more correct to say THE expert when it comes to piston and rings and a lot of other stuff about these old bikes!

Thanks for the kudo's Mark . . . 

The cylinder you're showing is definitely "reed induction" . . . from a 72 or 73 model.
However, the piston being used is not . . . it's from a 70 or 71 "piston port" model.

You can go in two different directions to correct the problem and get the motor running again.

1.  Buy the "correct" set-up . . . . a reed valve piston (308-11631-01-96), and install the new rings that you already have.
or . . . 
2.  Buy Yamaha rings (275-11601-00) . . . to fit the "incorrect" RT1 piston that you currently have

Option  #1 is probably a slightly more expensive fix, but the bike will run MUCH better, and be correct.
Option #2 may cost you less, but will not be the correct set-up, nor will it run anywhere near as good as option #1.

.Case closed!

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I really appreciate your help on this! i was just about to ask if i can use a 70-71 RT360 piston in the 72 and 73 years and vise versa as long as the rings are correct? Im always afraid to try something like that in case the ring gaps line up different in the cylinder, id hate to catch a port and destroy the top end.
Heres where things get more fun, i have an old wiseco piston for one of these in better shape then the Yamaha one lol. I have new rings for it but i plan to use that in the 360 im restoring. Choices, choices.
On a side note I may need an 80 over (1.75mm) piston from you too for one of these 360's. Thats another story though and i havent quite decided what i wanna do with that yet.

On a side note I may need an 80 over (1.75mm) piston from you too for one of these 360's. Thats another story though and i havent quite decided what i wanna do with that yet.

80 over is 2mm over so i use 2x5 kits with slight mods. That's if i understood your meaning right above. Or does it read you need 80mm plus 1.75?, might just be my interpretation in error.