Two stroke piston ring terminology ordering help

[EDGECRUSHER]

I’m in need of help figuring out my situation rebuilding an old Yamaha two stroke dirt bike.
When I had the cylinder checked they said it only needed a hone and was still within standard specs for rings.
The rings I ordered a couple months ago are what Yamaha refers to as 2nd size (.50). When I put them in to check end gap I’m getting 0.020” without any filing. Spec needs to be 0.008”-0.016”.

I have no idea what I was thinking when I ordered the 2nds in March, but knowing how they don’t fit now and factoring in that end gap I’m getting, is there a way to figure out what ring size I actually need?
I’m wondering if a 3rd at 0.75 will get the gap where I need it or if I’m looking at this wrong?

Im really terrible at figuring this out. I can measure cylinder bore and calculate clearance, but translating that into how Yamaha designates their rings confuses me. I’ve done a ton of reading on the subject and understand how the rings and piston need to be sized. However, when it comes to ordering rings I don’t understand how you read the description of a ring set and know it’s what you need.


If someone could enlighten me I’d appreciate it

[Nessism]

Sounds like you have the right rings only either the bore is a little worn or the rings are shipped with a larger gap than ideal. Regarding the later, it's common since a larger gap rarely causes problems but tight rings, when guys don't set the gap, will cause catastrophic failure.

[Agemax]

most 2 stroke pistons have any oversize stamped on top of the piston crown. give it a good clean and see if you can see any markings relating to which oversize it is

[EDGECRUSHER]

So 0.004-0.006” bigger in gap the factory spec and you’d still run them?

I was told after it was honed and I confirmed that the bore is very close to stock. I believe I’m the only person who’s rebuilt it. I did it once ten years ago. I wish I had taken notes so I know where it stood. Seems to me like a second size bigger ought to be tighter. These were some OEM ring I got off ebay. I’m still hoping someone can explain what the sizes mean exactly. I imagine if my bore is over 0.05” I would go with what I have. Is that right?

thanks for taking time to help me out

[EDGECRUSHER]

I’m feeling stupid for not grasping something that seems simple to everyone else. Maybe I’m over thinking it?

[Nessism]

Oversize rings are sold to match oversize pistons, and those come in .25mm OS increments for Yamaha 2 strokes like the IT175. If you have .50mm OS rings and there is a large gap like you mention then the bore can't be standard, it's gotta be .5mm oversize.

And yes, it's perfectly normal for the ring end gap to exceed the new gap limit.

[EDGECRUSHER]

So if I have an oversize ring I need the matching piston?

[Steve]

You may only recall doing one rebuild, but you admit you don't remember all the details. Maybe it's already oversize. You say you have had the cylinder honed. Was the actual dimension checked? Top, middle and bottom? In-line and crossways?

Did you measure the piston? Both axes?

Your piston needs to match your cylinder bore.

Your cylinder bore needs to match the piston.

Adjust whichever one is necessary, THEN order the rings needed for that combination.

.

[EDGECRUSHER]

I think where my confusion started was from conflicting data I received from others along the way.

I had my cylinder checked by my local stealership. When I went in there I thought it was going to need a bore, however they said it looked pretty good and just needed to be honed. I had left them with the piston as well so they could tell me what rings I needed. I practically had to beg them to give me this info which was kinda silly. At that point they said I could hust put the standard rings back in but that didn’t help. I want to say Ten years ago I put a new piston in it. Whether it was oversize I’m not sure. I’m gonna measure again when I get home from work so it makes more sense. I just happened to try fitting the rings last night before I came in for the night and got frustrated hence the questions without measurements.

[Steve]

Go to posts #2 and #6, look in the signature for Ed's oh, so valuable advice:

"To measure is to know."

.

[EDGECRUSHER]

I was thinking maybe there were oversized rings for each oversized piston. But really there’s just 4 chances to win including the original piston and rings.