First Post! So many resources here but I can't seem to find a definitive answer to the following:

Currently have two cylinders (123CC and 171CC) at Millennium Tech plating to get overbored to the next size, but I am having trouble determining what size they currently are and what the next size is.

Measurements taken by the shop before doing any work:
123cc is 2.279" or 58mm
171cc is 2.589" or 65.75mm

Measuring my old pistons that fit pretty tightly into the pistons as they are now:
123cc Piston: Is stamped with 080 CD on top
Top: 57.19mm
Mid: 57.89mm
Bottom: 57.83mm

For the 171cc Piston: Is stamped with 96 on top
Top: 65.5mm
Mid: 65.8mm
Bottom: 65.7mm

With those numbers in mind it seems the CT1 may be at the stock size, but I am not sure what I am looking at with the AT1 cylinder. seems it should be 56 nominally but both my cylinder and piston are way over.

Appreciate the help!

Here's the info. on the bore for both cylinders. It was here on the forum. You just needed to get into the tech library and look through the service manual for the AT1/CT1.
Oversize pistons were made in .010"/.25mm oversize increments.

Pete

Thanks for those details, def helps get me going in the right direction, RT1.

Given the factory spec for the 66mm CT1 bore, I think it is safe to say that I am running the standard piston for the 171cc, especially since its stamped with the 96 on top.

With the factory 56mm AT1 bore, it seems I am way outside the bounds of the last standard oversize max of 57mm. both piston and cylinder are measuring close to 58mm. This supported by the fact that the steel piston jacket looks rather knife-y so the at1 cylinder has probably already been bored a few time.

Not great news. dang.

Wonder what this piston that came with the at1 is....

I believe your 125 piston is a Wiseco piston, they made pistons up to .080, and that would be 58mm. I am pretty sure that is the limit on a 125 cylinder. As to the 175 piston, it looks like a stock piston that somebody modified on the exhaust side, why I am not sure.

Great, i will be asking them to bore out the CT1 for a 0.25mm oversize piston and call it a day there.

looking at the bottom of the at125 piston I think you are exactly right. It has the wiseco W and the number "236"

I cant seem to find a 1969 at125 specific #236 .080" oversize kit with two compression rings, seems that somebody may have isntalled the piston for a DT125 from 1972? Are they compatible? ebay.us/ZEo5rd

There are some other wiseco .080" oversize kits meant for the at1 1969 out there that all have only one ring. ebay.us/TcwQoC

I will be getting new pistons for both cylinders which is pretty straight forward for the 175 but what would you recommend for the 58mm piston that I have to use in the at125? I will have to ask millennium to plate and recondition the bore for the 58mm piston since I cant go any bigger.

Umm... you can't have your cylinder bored correctly unless they have the new piston in hand. If the shop is willing to bore without the piston, run, don't walk, someplace else.

Nobody swapped anything... Wiseco simply doesn't make that .080 over 125 piston anymore... remember, it's over 50 years old. If you wanted a batch made Wiseco would do it... I think you have to order about a dozen pistons though.

Also, unless there is something very special about that AT cylinder, there are lots of good used cylinders out there and lots of Yamaha pistons to fit them. Or convert to a 175.

Yeah I will most likely be running the bike with the 175 installed. Just would like the 125 on hand just because. My plan is to order the pistons and have them delivered to the shop to have them be able to finish the work correctly.

Mark, makes sense that those simply aren't available anymore. sounds like I will order one of the proper at125 pistons with a single compression ring. It appears that the ct1 cylinder may be brand new if its coming in at 65.75mm so I will just ask them to fit and hone it for an OEM factory piston i send them.

That link to the Wiseco DT125 piston kit won't work. "possibly wrong gudgeon height" -- but has the cut away so is later model reed orientated Can see the cut away in one pick. Did try something like that in my AT1 piston port cylinder & went like hell. But was a Kawasaki KX125 56mm piston with a dish top so super super low compression or i might've persevered with it. Think it was a case reed model piston so the cut away in the skirt wasn't too radical.

Also, i see there's been a bit of trickery going on modifying the 175 piston to advance the exhaust duration. From 'that' i wonder if the rest of the ports have been modified in some way. Maybe just tried the easiest option to increase the top end--'or not'.