The bike is the '68 DT1 I am rebuilding (still...  ). on disassembly the nice surprise was that there was very little wear on the bore, and it was running a first oversize piston. OEM as far as I can tell, marked 70 25 (cannot see a decimal point between the #s).
I took cylinder and piston to a good machine shop and had them measure and verify that things were still 'in spec'.  I am trying to re-use this 1st O/S piston as it really looks ok. I got a set of Yamaha NOS rings, part 214-11601-12 (came in a OEM box, rings have that light green coating on the outside edge, with a very faint "25" on the topside of each ring.

The buggers will NOT fit in the ring lands. 

I have cleaned and cleaned the piston ring lands, and I really don't think they are hanging up on a burr. Both rings and both lands have the same issue. The ring lands I am measuring at 1.8+/- .01mm. The new rings measure at 1.96/1.97mm, but so do the old ones (!?!). However, the old rings fit in very nicely. THAT is my puzzle. The new and old rings measure the same, yet only the old ones will go in the groove.

The 'style' of the rings appear to be the same to my eye. They are both a plain rectangle cross section, I see no tapering to make it a keystone type, let alone a dykes (yikes!! can I even say that anymore??).

Any experts out there who can show me the error of my ways? Right now I have reinstalled the old rings, but it is still on the bench.

Is there a "K" stamped in top of piston? 

Early bikes had regular rings...  later models AND replacement pistons for early models used keystone rings. 

Try this...  try to insert outside edge of old rings into ring groove.  If ring grooves are square, it will go right in and bottom out. If not, keystone groove. 

Visual representation is OC where O is piston and C is ring being inserted into ring groove outside edge first. 

I got regular rings sent by mistake from ebay when stuff was cheap years ago. They sent the taper set at no cost which was nice as i'd asked for the right rings but were so cheap i was happy to pay. Now costs have gone crazy [worlds gone crazy].
I thought keystone rings [incorrectly it seems?] didn't have an expander under the top ring. But all pics i can find show them on both.
Maybe yours had regular rings & piston from new but a keystone oversize kit when she was rebored.

Just remembered I took a picture of the "keystone test".  The taper is so small that it's hard to see.  Easy way to test other than the "K" stamped in the top of piston is to take rings that fit and try to put in groove outside edge first... if ring groove is keystone, they will barely go in backwards.

 




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My test is to put the verniers across the rings & can flex them if keystone but not if parallel.

RT325 said: "My test is to put the verniers across the rings & can flex them if keystone but not if parallel."

Yes, great test for the rings! ....and both ring and piston grooves should always be tested if there is any question (open box, etc.)..  keystone rings will fit in parallel grooves but not a good idea. 

P.S. Morley, when your typeface changes on you, you might have accidentally hit "control" and "i" at the same time instead of "shift" and "i" or something.  Cntrl i is the shortcut for italics

Thanks Mark, i'll take note. I can't make quote work either. Did you quote me or copy & paste. I'll try a practice shot incase its my computer.

Nup, just come up with

Error

Cannot save message: Message body is empty!

I used copy and paste to quote you.  Quote function is broken… not you!!

Comment on cntrl I was for those times the text changes in middle of your post