can't seem to find an oversize piston for my 72 DT2 250, which has standard piston.  hoping for 1st over, but not picky.  yamaha shows part number 438-11635-01-00, but can't find anything on ebay or on wiseco website for the bike.  I see it might be same as suzuki RM 250 or perhaps YZ 250, but hate to make that mistake.  any advice?  thanks!

I'd be fairly certain that the original part number was 311-11635 and there are at least three of these on fleabay (one with a set of rings). 311 is the model code of the DT2. 

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Ask . . . and you shall receive  

I have some of the genuine Yamaha NOS 1st oversize sets
Contact me for further details.

Full credit for you, Al, but no extra credit, as you left the -01 off the end of the correct part number.

Now, now!   

Never let the facts get in the way of a good mickey take! 

For there to be a "-01" (first revision), it's almost certain that there would have to been a "-00" (see link below). Therefore, by leaving off the suffix, I felt I was giving the original poster a wider field to search. 

Of course, my first thought was to suggest Enduronut, but I'm conscious of not wanting to sell all of his stock for him! 

311-11635-00 NOS Yamaha Piston 1st OS Piston DT2 DT3 W3976 | eBay