I need a 56 mm piston and rings for an at3. I have some 248 pistons but no 316.

Standard bore?

Yes mark. I have a 318 mx piston without ring but I'd rather go back oem. I guess if the consensus says the mx is just as good I could just use a ring.i have 6 or 8 piston port 248 pistons I picked up at Barber. PO said the piston was stuck. He got it out and a little emory cloth or steel wool would polish it up. Its a really nice original bike so I'm ok with buying one. What type of hone do I need to get to hit the bore?

I might have one but most of my new pistons are earlier piston port type.  I will look tomorrow. 

Thanks 

Only standard 125 piston I found was for an AT1.

316 piston on ebay  # 193391373557
316-11610-00 rings not so many , but one set available ex ebay canada

I saw those rings. 50+ with shipping, 30+ us. I’ll wait it out some. Bought a piston out of AL reasonable. Thanks Mark for looking. 

I remember reading about using IT and maybe DT piston and rings on some earlier AT/CT bikes. Anyone ever make a chart on fitment or have any advice. I could also go 175 on the top end and could use 175 stuff. Just weighing options. Also wondering if anyone has used one of these and if OEM circlips work as should. I could do visual on a CT OEM piston vs these.

couldnt get link to post but mrssuperdeals has a kit for 18 bucks. Std Ct piston port. JuSt thinking on parts for a 71 ct as well. It’s G P cycles Japan made.

Be careful if you go piston port CT1-175 onto a reed AT3-125 bottom end as might be a conrod length difference between reed & non reed. Like for like would be ok--like a CT3 top end.
Ok cancel that incorrect info of mine as partzilla says everything's the same.
www.partzilla.com/product/yamaha/3R2-116...7a7f00da966aa3f42951