When replacing main bearing on  RT 360 crank do you replace the r type circlip with new or is it ok to use the old ones.Always been told to use new, if so where do you get them. Are they a Yamaha part ??? Thanks in advance.

Does it look worn? any dings or bad scrapes? Look cracked?

If they are truly worn, yes, replace. Did you remove to change the bearing, or leave in place? (actually, I haven't done an RT, only a DT1, so maybe different?)
I looked at a parts diagram for a '73 RT2, which shows the clip. It is a Yamaha #, but pretty small $.
 

Alan, are you talking about your 73' motor in a 71' frame bike or another bike?
Cause the 73' DT3 has 3 each circlips.

#14

 

Piston pin clips - ALWAYS replace.

The large bearing/seal retaining clips can be reused if not damaged.  Always place the sharp edge of the clip facing "out" on the side the clip is trying to keep the seal/bearing from moving.

For a laugh--or just showing my stupidity, but back in the late "60's & when saving weight meant everything in motocross i even left out the big outer circlip for the right side crank seal the first time i had mine apart--when the gearbox broke 3rd gear & it exited out the bottom. Wasn't a good day. Anyway i knew it'd never blow a seal out as are big & tight fitting. I'm sure i could feel the weight saving lol.
But, what i was going to say 'was' the circlip behind the seal is meant to go in so it ends up 'Not' overlapping the oil feed hole to the bearing. Wouldn't totally block it if you got it wrong & there's that much mixture firing around that i'm sure it'd survive. Parts pic i think shows the circlip ends pointing down but should be 'up'. I'll check one of my cases later to be sure, but that's my thoughts.

The engine is the 73 in the 70 frame. The reason Im taking it apart is the big end bearing has about 8 millimeters of side play, (its pooped). I have a crank to put in, so seeing that it is 50 years old engine, my thinking is renew all of the bearings, seals etc while its in pieces. I dont think its been apart in the past,so everything looks good. If I can get new clips I will use them. Also can anyone tell me if the early RT1 piston (solid skirt)can be modified to fitt, ie have the transfer windows cut into the back of the piston.The reason I ask is the bore is good but the piston is +50, and there is a lot of damage form detonation or plug damage.Thanks for your replies so far. Alan.