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I have never had the bottom end of one of these apart. Now, I may get the chance.

My college son called and said that the motor had stopped running while on a trail ride. It would still kick, according to him, but wouldn't re-start.
I am going over the list of possibilities in my head, and I am thinking fuel related, like a fuel pump.

I get the bike home, three months later, and it won't kick. Locked up solidly.

I go to drain the oil, so that I can look for metal. No oil came out. Heart attack and stroke simultaneously.

I take out the oil filter, spread it out, and no metal particles. I fill engine oil and drain it, just using it as a rinsing agent. No metal particles. Refilled engine and trans oil, install new filter, lay the bike down on each side to slosh around the oil. Poured oil in the top and coated the cam and all of the parts there.
Now, I will try to rock it back and forth in gear and see if I can unlock it.

So, the question is-with a lubrication deficit, what would seize first? Piston in the cylinder? That I can fix. Crank bearings? Above my pay grade so far, maybe I will get a great winter project on my hands.
Any chance that a piston hot seized and locked up the motor before the rest of it grenaded? Help me feel better about the prospect.

Man that sucks .... I feel for ya ...... I am guessing Piston to Cylinder , or rather Rings to Cylinder , or the rings broke and hard seized , that would be my first guess , I am assuming there was no antifreeze in it either ?? , so who did the last service on it ?? , or was it done then never checked after countless hours ?? , A new engine will burn thru about a 1/2 qt in about 2 tank fulls , then it should have dramatically decreased in consumption of oil , maybe 1/2 qt in 15-20 tank fulls indefinitely , but these things always burn oil from my observations , so either the antifreeze boiled out from trail riding with no overflow tank , which overheated the oil and burnt the oil down in which it would varnish everything and seized , which would mean everything inside would smell like really burnt oil I would think .... Or it just overheated and lost the rings , the oil then went by , then went into the trans side via the crank seals and overfilled the transmission side (did you check to see if the tranny was overfilled ??) and dry seized .... or , the oil was never refilled on oil change and only the tranny was and it dry seized , but I would think the cam would have been the first to go , so that is interesting , but dry seizing would not explain no contaminants in the filter , as it would have been running then just ground itself to a halt ..... , hopefully its just someone brain farted and did not add oil and/or check the coolant

If the top end looks ok you would have to think the main and big ends still had oil. I tend to think rings on the bore and maybe the little end. The other possibility is the engine ingested some water, whether during the ride or after assuming it was washed down and the rings rusted on the bore. It doesn't need much water to do that. Either that or coolant made it to the bore.

Feel for you Leardriver, seized pistons are not fun to get out of bores.
Do you still fund your sons riding?

Yes, I am grateful to be able to help my young people enjoy bikes. We just seem to have different standards about maintenance.
The bike is on it's first top end. The motor has never been apart. 70 hours on the motor since new, 18 since I last saw it.
The anti freeze is full.
The bike was over an hour into a trail ride when it seized, and had been at the track during the last week.

I will try to free it up today, while the mild weather makes that bearable. If I can get it started, I can listen for noises, and maybe get some metal to show itself and help with the diagnosis. Then, engine out of the frame in anticipation of splitting the cases, and start by pulling the head and looking for carnage.

My son has moved to road bikes but I am sure he will come back to the dirt side.
Being a dirt rider has saved him a few very close calls with cars.

Be interesting to see what you find or don’t as the case may be.

I rocked it until it unseized yesterday. Didn't take much.
It kicked with normal feeling compression, but wouldn't start. I ordered one of those $20 borescopes that work with an Android phone, and I will take a peek into the cylinder. It is cold outside now, and I am in no rush.

The teardown is complete.

The failure chain was as follows-

No oil=Right intake valve bucket getting dry, moving slower, building up heat, and breaking off a chunk of metal from the casting that holds the bucket. That camshaft lobe is gouged, and the top of the shim bucket is gouged. Powdered metal around just that valve. The bucket partially blocked the valve open, leading to the no start/sudden stop. Maybe a blessing is disguise.
The very good news? It happened quickly and seems to be isolated to just that part.The crank is clean, tight, and smooth.
The cylinder and piston look great.

So, new piston, rings, cylinder hone/light scuff, and all will be well with the rotating assembly.

I need a new loaded head. Valves, shim buckets, and a cam. It may have to wait a couple of months.

Anybody got a stock cam for a 2015 laying around after they put in a Hotcam?

I'll post the pictures if someone will tell me how. I enjoy seeing failures so that I can recognize them.

This bike is built so well! The fit, finish, engineering, machining, etc, is just great.

PS-Those Android borescope are several different kinds of awesome. You can see clear as day into cylinders, plumbing, dog's ears.

Hi Leardriver,
Good to hear the bottom end is ok.

It seems the Honda does actually stop quickly when something goes wrong.
The yamahas I have looked at post destruction, kept going until massive failure.

I guess more by luck then design but easier on the wallet.