It’s been a while since I’ve posted. I’m doing a top end on my 70 DT1 and using partzilla for parts number confirmation before going on eBay. Found a first over piston in Canada using what I thought was a standard two ring piston number but I received a keystone type. Rings bought separately were regular first overs without expansion rings. I’ve, since, ordered a set of keystone rings. My question is about reading part numbers. The partzilla number description didn’t note as a keystone piston nor did the eBay post. Maybe what I received had the wrong piston in the box. Is there a better way to get accurate part numbers or read the part number format? 

Usually all the OEM Yamaha DT1 pistons that take keystone rings have a faint K marking to the right of the arrow marking on top of the piston. Sometimes you have to look carefully because the K is very light or 1/2 gone and it won't even look like a K.

1st over Yamaha piston (std style) is 214-11631-10
1st over Yamaha piston (keystone style ring)  is 214-11631-11 
1st over Yamaha piston ( keystone style ring) 214-11635-01-00

When i buy on e-bay i always ask the seller to send me good close up pictures of the tops of the piston and even ask them to check the ring grooves to see if there tapered or not. Some will check other don't know what i am talking about.

I also will use  MC or Rocky brand complete piston kits as they come as a set and work well. 

 

Yes, there is a way to tell by the part number suffix for the early 214 prefix DT1 piston/rings.

"Square" ring Piston part number = 214-11631-xx
Suffix -00-96 = standard 
Suffix -10 = first over
Suffix -20 = second over

"Keystone" ring Piston part number = 214-11631-xx (same base number!)
Suffix -01 = standard
Suffix -11 = first over
Suffix -21 = second over

"Square" Rings part number 214-11601-xx
Suffix -02 = standard
Suffix -12 = first over
Suffix -22 = second over

"Keystone" Rings part number 214-11601-xx (again same base number!)
Suffix -03 = standard
Suffix -13 = first over
Suffix -23 = second over

The "K" is marked on pistons and rings as Skydancer mentioned.  If you have a "square ring" piston, you can easily install "keystone" rings accidentally (=bad!). "Square" rings won't fit into a "keystone" piston.

P.S.  Yamaha superseded the "square" ring pistons to "keystone" type so the "keystone" ring pistons were shipped when they ran out of "square" ring pistons.