Hi - do any of you knowledgeable people out there know what the markings on the top of my piston mean?
At the front, 8-4
At the rear, 8525
Is it standard bore and is it a standard Yamaha OEM piston?
Many thanks…. 

Hard to say if it's OEM or quality aftermarket...  what are the markings cast inside underneath?  Might say YAMAHA or AZUMI or something like that. 

Piston is first over in Yamaha-speak.  85.25 mm from the markings.

Thanks for the info. It says ART on the side. Does that mean anything to anyone?  I’m not planning on taking the piston off, so can’t check for markings on the underside of the crown as there is carbon build up and I don’t really want to disturb that. 

It's probably original Yamaha.  Enduronut would know for sure.  Casting marks underneath would be inside the skirt, not under the crown. 

 

Hmmmm.... except for the reed valve windows, it reminds me of a 4-stroke piston.
I only know certain pistons....  seeing windows in the sides around the wrist pin doesn't seem right for a DT400B.
If it has been running good, then it must not make a difference.

Art is a piston manufacturer and did make Yamaha OEM pistons as well as aftermarket ones.  OEM pistons often have a model code or other identifier cast inside the skirt

On ebay there are several OEM pistons for sale with the windows by the wrist pin...  I've seen a 400 piston come apart where the crown broke off through the top of those window right below the "ledge".  Allegedly the rider downshifted into a low gear at high speed and let out the clutch...  tach pegged and piston let loose. 

Anyway, here's one of the Yamaha pistons on ebay. 

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Yes . . . you have a stock Yamaha 1st oversize piston
There is no word "Yamaha" cast into the underside of piston on this particular model
. . . but you will find the numbers 50002 on the inner exhaust skirt.

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It’s been a while since I’ve seen a DT400 piston. 
I didn’t recognize the holes in the side. 

 
50002 Correct !!!