Hi,
What do the numbers stamped on the top of a RT360 piston mean? are they anything to do with the size of the piston? Mine has 7 2 on the arrow side and 9 6 5 on the opposite side.
Is there any info on this site about the dimensions of the standard piston and cylinder please?
Thank you.

I think you have a standard piston. Otherwise youd find the oversize 0.25- 0.50- 0.75- 1.00
Pic would be good if pos but need an external link to post it like 'imgur' i believe.
All 360's are 80mm std bore size.

RT325 is correct.  Standard bore would have been very close to 80mm.

965 means the piston was originally 79.965.  Sometimes skirt of cylinder still has ink markings with the deviation from 80mm when new.  That number was then used to select the correct standard piston (which I believe came in 0.005 increments at the factory level) to get the cylinder to piston clearance correct. 

With a 965 piston (if still original), the cylinder skirt was likely marked "010" or "015" in ink.

That was over 50 years ago.  Means almost nothing now.  No telling how much bore has worn without measurement by a qualified machinist with the correct tools and experience. 

Thank you. Yes I did try to post a picture but the site won't support it at the moment it seems. I will look into the external link you recommend but for now your comment is helpful, thank you.

Thanks, that's really helpful. I do have some capacity to measure the piston and bore myself but "qualified and experienced machinist" I am not. So perhaps I will hand it to a pro. 
Thanks again.

Just thinking, if it 'is' quite worn it'd pay to let your machinist know that Yamaha only go in 10 thou steps or 0.25mm.
It doesn't take a lot of wear to exceed that & if you think of it in terms of 5 thou each way so might not clean up around the ports where often the most wear takes place.
Good you have one on std bore though after 50 years so best of luck for maybe just a new std piston kit & many more mile of riding left.

It seems pretty good in terms of wear. If I do decide to go up a piston size do you know of anywhere I can get a new piston kit? I see a few things on eBay but not with parts numbers that match what I think I need.
By the way I did post to a Facebook forum on the subject too and I'm going to try to put a link to the post I made which does have a picture of the top of the piston.
www.facebook.com/groups/633386550560589/...nk/1460418877857348/

Private group.  You can upload pics to a site like Imgur and link them here.

My apologies if you know this, but to "go up a size" you really need to have the cylinder bored and honed to fit the new piston size.  As RT325 said, the amount of wear determines how much material needs to be removed to get the bore straight and true again. You may need to go up a more than one oversize.

Oh, forgot.  For a piston you might contact Enduronut.  I'll pm you his email, not sure if he still visits here.