‘72 CT2 175

All original 6,800 miles. I decided to do top end because I had a couple small air bubbles at base gasket with air pressure test. Otherwise ran great.

New rings, base gasket, and upper copper gasket. Honed cylinder. Some visible verticals After first 20 seconds, so I gave it another 20 seconds. Slow rpm and slow hump for cross hatch. I thought hone went fine. See pic.



After assembly, fired right up. Rings seated and felt happy. About 80 miles after assembly, on a long ride and was at full speed for awhile. 6000-6500 rpm, 3/4 throttle, 50-55 mph.

Just shut down.... see carnage...



Ugh.

Couple things of interest...

The wiseco rings that I put in felt thin compared to what came out. ~.038” vs ~.052”. Ring gap was ~.016” (I think target should be ~.012”?)

I pulled piston pin from one side thinking it would be easy to install pin after starting jug insert, but that wasn’t necessary. Went back same way it came out I’m sure. But worth mentioning.

Taking look at combustion and plug, that burn looks a bit whacky. Plug is prob 400 miles used.

Cylinder and piston both scored pretty bad like it was interfering. Were these rings too thin? I’ll pull up part number from receipt, but feels fishy.

I guess first I’d like to know options and second how not to repeat the mistake whatever it was.

Thx.

Wiseco rings are only for Wiseco pistons. That didn't help. Not sure if that caused the whole problem as using incorrect rings is not one of the many things I've done wrong over the years.

It looks to me like it ran out of oil. Autolube tank ran dry?

I bit my tongue on my first post... but just have to say it. I would not call the first repair a "new top end".

Your "after honing" picture looks like the bore was already fairly well worn and scored. No reason it might not have run like that for a while longer but certainly wasn't "new".

I'd suggest sending cylinder to a reputable shop that knows two strokes for a re-bore and oversize piston fitted to Yamaha clearance specs. Ports will also need proper chamfering and parts cleaned well before assembly.

At least my dunce cap seems to be compatible :/

I’m guessing that’s the answer.

Oil I checked, it’s there. I’ll go spin the wheel and make sure it feeds. It was calibrated few hundred miles ago.

In your picture of the underside of the head, it looks like there is a step in the copper head gasket in the upper left corner. I wonder does it just look that way in the picture or did something actually cause that. Could the piston have been hitting the head gasket?

I'm not the expert that some of these ex-racers are on the forum, but, I'm careful with break-in of a new top end. On my 76 DT400 the owner's manual calls out the break-in procedure. I use this on all my rebuilds now. First 100 miles keep maximum revs no higher than 4000, between 100 and 250 miles keep rpms 5000 or less, 250 to 500 miles keep them 6000 or less. Beyond 500 miles they just say no prolonged full throttle in the red zone. Red line is 7000 rpms. I've treated all my rebuilds this way and just rolled over 100 miles on my RT3 so I'll free to run her up to 5000 rpm once in a while. Yamaha also says to vary the rpms while riding during break in. By reading your issue you broke about every recommendation that Yamaha says for break-in. I also always stick with Yamaha parts. But like I said I'm no expert...I just treat stuff right so it will last. Hope you get her going again.

After honing the barrel , what cleaning steps did you take ?

Autolube feeding okay. I think the wiseco rings compatibility was the problem.

Thx for suggestion. I just looked at it. Lighting from pic makes it look different there, but that was just the detent circle that crushes. It looks fine and the gasket registers with the unthreaded portion of the head bolts so it’s hard to misalign.

Thx for comment. I’m the guy that needs to learn more, but I did seat the rings by going through 5 heat cycles at gentle RPMs before riding. After that I didn’t flog it. But now I suspect I will be much more pedantic with break in routine.

Looks like a major seizure. I would guess no oil was being delivered to the air mixture.
Lack of a proper start-up procedure may have been a contributing factor, but the seizure is pretty extreme to just be caused by that.
Could also be incompatible rings BUT if they fit into the cylinder during installation, it might affect performance... but they did fit.
Maybe lack of clearance.
What piston-to-cylinder clearance did you end up with? Did you measure and record it?

My opinion.... There was a lot of heat involved here. So unless the mixture was extremely lean and you ran the crap out of it... I am guessing oil pump.

I would run the primer wheel on the pump and see if any oil comes out into the cylinder connection. Very important that you see it go into the cylinder to prove there are no obstructions.

^^^ this... Has been on my mind. I sprayed with brake cleaner through the ports and inside but I don’t have parts washer for full dunk. Yes, I am concerned there was residual honing grit in the intake and I will be more careful in the future, but hard to imagine that would be catastrophic. But again... I have more to learn and will err to caution. Thx.