Had the devil bike DT175 on a nice ride out this bank holiday afternoon.  I was naughty and put about 30 miles on her, enjoying the new found zip and the motor doing two stroke type things rather nicely.  I did not want to go home.
On the way back, she went onto reserve about six or seven miles from home, so headed back the shortest route.
Came out of a neighbouring village onto the lane back to my village with about four or five miles to go and the lane being unrestricted, opened her up through the gears and into top.
My word, she was flying.  60mph, lovely smooth power. 
She has never achieved 60 and 6000rpm in top on my overgeared setup before.  I was revelling in it.  

Running absolutely lovely.  Then she wasn't.  A single backfire and shut down.  No nipping up, just stopped firing.

I thought I had completely run out of fuel, but no, the filter was still full...

A prod on the kickstart was.... A little disappointing.  Like standing on a rotten tomato. 
Oh God.  I know what that is.

I pulled the plug, observed the dark gray colour insulator and ground electrode with alloy flecks in it, threw it into the field as far as I could in disgust and started the long push home.

I have no answers....
Ignition spot on.  Set by dial gauge.  Double checked by strobe.
Rotor key is intact.
I did a wide open throttle plug chop previously, showing slightly rich at the top end.
Genuine Yamaha piston.
B8ES plug.
New crank seals including an IT175 item on the left.

I did consider leaving it at the side of the road overnight..... Fecking feckity fecker of a bike.  I feel like I did ten rounds with mike tyson after pushing the fecker five miles.  It did the tendonitis in my ankle no good at all.

It is going to have to wait in line now.  I will finish the little one before I get around to the devil bike again.
 

Man, that sucks. I don't think I'd walk my CT-1 home 5 miles. Probably lock her up and call Uber, then come back with my truck and a trailer (I don't have a ramp) Or, I wonder if AAA would bring it home? Yesterday I opened her up and got to 65 MPH at 6500 RPM, with the 17 tooth front sprocket (one up), but having previously tested the speedo with cell phone GPS, it was really closer to 62 MPH.

BTW, I hope the repair goes well, and isn't too major. I also called AAA and added RV/motorcycle coverage. $30 extra per year, and I do have a motorhome as well.

Oh no!!!!

When you get around to looking at it, I've found it's very helpful to pressure test before teardown to rule out an air leak someplace as the cause. I had one where somehow the intake manifold suddenly started leaking like a sieve at the sealing surface to the reed valve. (But usually I want to assess the damage right away and tear it apart before testing.)

May have just starved for fuel for a few seconds at the wrong time though. I've starved for fuel when tank got low and holed a piston during a desert race.

After that I've run completely out of fuel at full throttle during a desert race.... no issues. There was a difference... we modified the fuel tank to have a small "sump" to trap fuel near the outlet and started running a snowmobile Mikuni fuel pump that ran off crankcase pressure/vacuum to provide consistent fuel pressure no matter the tank level. This was done at the advice of a two stroke racing legend/guru.. Never holed a piston when low on fuel again. When it ran out of fuel, it was OUT of fuel and just shut off without going lean a few seconds first.

Oh splinters! Maybe the bike should be painted red? Add some horns by the lead lamp…. I’ll bet that bike was cussed out all the way home! I hope it didn’t hobble you after the walk.. down satans lane!
Oh dear..
Sneezles61

Actually I am not too downhearted.  When it was running so well, I had an inkling that something was going to blow up in my face. Nevertheless, if it was going to do it, lets get it over with.  I confess I wrung its bloody neck down that lane.

I never came across an engine that did not run the best ever shortly before it was going to die.

350 Bultaco. Always a lovely motor.  Suddenly became hyper smooth and seemingly more power and torque.
Then the crank pin tore itself loose from the right flywheel 30 minutes later.

CB900FA.  Same thing.  Always a nice engine. Suddenly seemed to lose 20,000 miles and 15 years. Hyper smooth and refined as one would expect of a bike fresh from the dealers showroom.  Running in perfection.  It died under the wheels of a mitsubishi shogun 10 miles later.

Did I ever mention how lucky I am?

OMG Tinkicker - such a bummer to hear, I experienced the same a couple of years ago (DT360) not due to running out of gas but a series of blunders.  It hurt badly, I was down and depressed...I ended up going thru a redo with the support of this crowd and knock on wood its still running nice.  A couple of positive thoughts for you.  1. you have a good heart, which is more important than a piston, and 2. perhaps you got lucky and you can get away with a new piston and wrist pin/bearing which is probably metal sprayed.

I wish you luck as you delve it this motor

When you said it was running the smoothest and fastest it ever has at that moment I felt I knew what the next sentence would be, I experienced the same circumstances on a 1977 it400 about 40 years ago and I holed my piston, at first I thought I was a bad ass mechanic/tuner by how good the bike was running and a few minutes later after it died and I coasted to a stop I realized I could  push the kick start lever down with my pinky finger, so much for me being a bad ass mechanic/tuner. I like MarkT’s idea of doing a pressure test, 40 years ago I never heard of doing a pressure test, we just turned the bike upside down and got the garden hose and flushed the crankcase RT325 style, got another piston and rings and was back out riding within two days, never did find out why it holed the piston, we blamed it on some new fangled spark plug we had tried, rode the bike for another two years without issues and sold it.

Disappointing to say the least.
I've no answer but i'd probably have used a 9 plug or higher i have 10's too, the thin wire type..
Maybe a notch or two lean on the needle if you weren't on absolute full throttle at the time or in particular just rolled it back a bit from wide open.
Is it poss reserve was almost at an end & fuel that's left was just sloshing & not constant flowing down the fuel cock.
Easy for me to hypothesize & not being there, but gotta start somewhere.
So hole in the piston then?. Be good if only half through & rings stuck.
Look forward to what you find.

I seem to remember that you had the cylinder relined on one of your bikes?

If that's the case, then the liner might have moved? On cylinders which have cast in skull type liners, if at all possible, it's a good idea to avoid putting liners into them.

The cylinder casting itself, in most cases, isn't substantial enough to support the liner, and after a few heat cycles, the liner comes loose.

There are various ways to make a loose liner less likely, but none of them are all that satisfactory.

Hopefully, your liner hasn't moved, but it has then the cylinder is effectively scrap, and it might be an idea to look at one of the Chinese DT cylinders, which are advertised on ebay?

I brought home one of my inspection mirrors and had a peep down the plug hole...

Confirmed.  Hole in piston maybe 12mm in diameter.

I finally got my phone to talk to flickr and send the plug pic I took before launching it on a ballistic trajectory into the shrubbery and starting my walk of shame.

Here we are:  
Comments appreciated.  I am out of my experience zone.  I never saw a holed piston before in the flesh.  Everything I know about it is anecdotal.
I think the stock plug is too hot and as RT says could do with a 9 or 10. I also read elsewhere that the stock plug in the 175 was very borderline hot.
Piston crown is same colour though so it might have gone lean as the fuel got low.  I do not know what I am looking at though, it does not look lean to me.
This is my first ever holed piston.  Does it make everything turn grey?  I was expecting burned white.
I was expecting mounds of alloy on the plug, not a tiny dusting.

Snapshot of conditions at exact point of failure.