Mark,  wish i could figure this picture link out.  

definitely a 2 no question 

I was questioning the 3 vs 8 until i decided the 8 would have equal circles, this one does not, lower circle larger. 

unfortunately, at the 10 oclock position of this 8 / 3 is gone. 

I may have taken it off cleaning the carbon off.  

gotta get this mic set, check the diameter around the skirt, get a bore gauge in the cylinder and hope i dont need to get it bored.   Would be nice if it was in tolerance and a honing is all it needed.  

Thanks for your input.. its valued 

Drac

Have a read of this previous thread.

A few ideas on how to post IMGUR Pictures - Vintage Enduro Discussions

Piston doesn't really tell you much except the general size.

The important thing is the bore size, taper, and out of round... 

The factory bored and honed the cylinders to "spec", say 80mm on a production line. (It would be cost-prohibitive to have a machinist hand size each cylinder to clearance the piston as is done with a rebore.)  The "9" number on a new Yamaha standard piston for these bikes is the size.  A "96" on a piston made for a 80mm nominal bore meant the piston was 79.96mm.  Then it was a simple task to measure the new cylinder and select the correctly sized new piston to get the clearance correct.  (The factory often had more standard sizes than were sold as spares).  So if the new bore size measured at 80.02mm, and the piston is 79.98mm, the clearance is 0.04mm.  If the new bore was exactly 80.00mm, then a 79.96mm piston would be selected if the target clearance is 0.04mm.

Factory new cylinder sizes the piston to the fit bore size.  Oversize bore repair sizes the bore to fit the piston.

If your bore is still in spec, you can often do the same thing by sizing the new piston to get the correct clearance.

On some Yamaha models , not all , standard pistons were offered in a couple of sizes .
Usually .96 and .98 . The variations can sometimes be found , but NOS stock is likely
rare for the larger option after all these years . The size variation was added to the end of the part number ,
a Oceania market RD 400 F piston ( standard oversize ) is 3J7-11631-00-98 .

Hey Mark, 

lotta great info here on this subject..  the keystone piston / ring set..  I came across a service bulletin explaining the update Yamaha made in 69..? It states everything after that was fitted with a keystone piston / ring set.. so is it safe to assume, while shopping for replacement rings and or piston, they will be a keystone type setup with the taper… or will i have to visually confirm these? 

Next thought, oversized sets.. (.25, .50,.75 etc)  1969 ——> , were they mfg (Yamaha) in the keystone fashion? 

Finally, looking at 360’s 71—> ..  would the rings and piston sets be a option..  i see a lot of part numbers starting in the 308 prefix, perhaps a super-ceded  number ?  

Again, i wanna let you know that your knowledge is greatly appreciated on this restoration. 

KR

Drac
 

after market,

Thanks for sounding off here, I appreciate the information.. 

Im probably getting my cart before the horse here but as you mentioned.. NOS PARTS, scarce… e
aftermarket stuff seems to be around, caveat emptor.. i have learned.. lol.  

Im out of country on a work trip.. this existing K piston in my project, cylinder, will have to be the first thing to settle when i get back..  

Thanks again for your time..  I appreciate every bit of it you use to comment here.

Kind Regards,
Drac

Pistons and rings are marked if keystone, and you do have to be careful. I'm sending you a PM with contact info for an expert that might have the piston/ring parts you need or will certainly know what part numbers you need to search for.  

The 308 prefix is RT2/3 fitment . the pistons have windows in the rear skirt for reed valve ,
and the rings are Dykes rings which are L shaped for the top ring .
RT1 need 275 prefix parts .

Mark,  message received..  thanks so much for that info in the PM.  

This was a great bit of tutoring for this old guy.. I think ill name it Service Bulletin 215 amended page 4/4 print and add it to my service slash / part number manual.  

seriously..  Thanks for your time, a valued commodity that guys like you, Swoop.. offer for free.

Until next time..

Drac

 

Swoop,  my goodness, that bit of info confirms another search i did shortly after i posted it..  

The additional bit about the name “Dykes” and L shape is noted. 275 it is.. 

I wanna thank you again for your time.. your knowledge is greatly appreciated.. 

Drac