Today I pulled out the new OEM head gasket I received from Yamaha to install on my CT2 project bike.
Oddly, the inside diameter was smaller than the head dome. I put it on the bike and found that it leaked during the leak down test (the compression ridge was floating out over the dome instead of on the head deck).
I pulled out a head gasket I found in an old Sudco kit and found that the inside diameter matched the head. The same was true for an old used head gasket I had lying around the garage (see photos attached).

I looked at the Yamaha parts diagram and found 2 part numbers for the head gasket. I OEM one I have is the 248-11181-00-00. Could this be the part number for the AT2 head gasket and the second part number (prefix 251) is for the CT2? Did I order the wrong one?

Thank you in advance for your advice!

Here is the parts diagram

Tons of posts on here about this. When Yamaha digitized the parts lists, some of the information was lost. A general rule is the prefix... the 3 digit Yamaha model code... determines the correct model when there's a choice. This is almost always true.

Take a look at the 69-71 AT-CT parts list in the Tech Library to see how the charts were laid out originally and how the model info is now missing.

You can use those earlier charts as a reference for many parts on your CT2. If the AT parts were different from the CT parts on the earlier models, they are likely to be different on yours. Most obvious are cylinder and piston parts and gaskets will be different due to the different bore size. Wrist pin and bearing too. Some electrical will also be different... those are more obvious.

250/360 and later 400 also shared lists. It can be a challenge to find the right part number sometimes.

Several versions of model code lists are in the Tech Library and are invaluable in decoding the 3 digit model code prefix.

Think you've answered your own question but it's annoying isn't it. Amazon pic shows a 175 gasket which is obviously out close to the stud holes. They show what they fit below but they also say AT2 etc which is wrong!!. Ok the stud pattern is the same but 125 is 56mm against 66mm for 175. Anyway, you've found that out. I always thought of the 125 as a 248 part number. Even Partzilla have it messed up.

www.amazon.com/Yamaha-251111810000-Cylin...Gasket/dp/B005C599W8
www.partzilla.com/catalog/yamaha/motorcy...2/crankcase-cylinder
www.partzilla.com/catalog/yamaha/motorcy...2/crankcase-cylinder

What confuses things more is the early bikes... and the head gasket didn't change for the later reed valve engines... use the lower number (248) for the 125 and the higher number (251) for the 175.

When you get to the the reed valve "2 and 3" series of the AT and CT, the 125 has the higher number (316) and the 175 the lower number (314).

So if you need a head gasket for the CT2 you use the 251 prefix part and for the CT2 head you use the 314 prefix part. Very confusing.

When installing the head gasket, I assume that the compression ridge (raised ring) faces down to make contact with the steel cylinder liner?

I always put ridge up.

Not sure it really matters though.

Thank you MarkT and RT325 for all your help!

Yeah i put mine ridge up too but never thought why, just sits flat underneath i guess. Also i grease mine but i'm probably the only one so don't read too much into it. Probably had a theory it lets it squash better when torquing it down. Actually i don't even torque then so i'll leave it there before i get deeper in trouble.