I'm needing to bore and would like to identify what piston I have based on the selection of markings. 74 dt250.

Photo(s) please!

You can also measure the bore, or a machine shop that does boring can, or a dealer should be able also.

Seems like a lot of info going on here.

Your piston is the Yamaha OEM standard size .
The next oversize produced was .25 mm larger . If things were really worn ,
that doesn't always clean up properly . Yamaha alone made the oversizes in .25 increments .
Both Suzuki and Kawasaki upsized in .50 jumps .
It might pay to take it to an engineering shop to measure up before you purchase parts .

I agree with Swoop... just for info... the "9 7 0" was used when the bike was being assembled to match the piston to a cylinder... sometimes you can see the inked size stamp on the lower skirt of the cylinder still...

If the standard bore size is say 70mm... your piston when new would have measured 69.970mm.

Anyway, Swoop gave good advice... a good experienced machinist can measure the wear, taper, and ovality and know what it will take to clean up... I've even had them take a few light cuts to clean it up... and then measure and tell me what size piston to order. Final bore and hone is done with new piston in hand.

Bore needs to be cleaned and ports properly chamfered... and beware of "experts" who add "a little extra clearance" instead of following the Yamaha clearance specs... unless you don't mind a rattly piston that will need replacing long before it would if bored correctly.

Mailing cylinder to someone who knows these old machines is money well-spent if you don't have a good shop near you that understands old two strokes... which can be hard to find these days.

Good info Swoop. Also refreshing to here it is standard. My mind was swimming with all the numbering. Definitely going to let the machine shop look at it ahead of time.
Any suggestions for where to purchase new piston and rings?

Thanks Mark. Also good info. I should be in good shape now.

Enduronut is the best source for pistons and rings that I know of... DEET might have them too.

Ebay, if used with care, can be a good source for pistons and rings.

Beware of any seller that says a piston fits bikes of different capacities.

Research carefully (ie triple check) the part numbers of the oversize items that you want and look for photographic evidence on the listing that that is what you're buying - eg part numbers on the boxes, 0.25 or 0.5 stamped on the piston and rings.

Be free to post again here if unsure.