I was cruising at 75 on the 74 360 on the way across town when I felt the motor getting tight. I pulled the clutch in and it promptly seized. Traffic allowed me to get over to the shoulder and I pushed it for a short while until I was is a safer spot. It had freed up by that point and restarted easily, just with more piston to cylinder clearance than usual. I got to my destination and back again without issue.

Upon disassembly I found nothing other than four streaks of aluminum on the bore in the vicinity of the transfer ports:

 

With the piston out I find zero scuffing on the intake and exhaust sides and only light scoring corresponding to the streaks:

 
 
 
 

I'm thinking insufficient lube. Hard to say though. Lot of changes recently in this motor, one of which is a 39mm carb. I suspect the oil pump needs to be advanced to account for the additional airflow at low throttle openings.

Damn nice carbon pattern though, in my opinion. Porting is working pretty well:

 

It is running great other than this little hiccup.

My guess is not enough clearance for the type of material the piston is made out of. Thinking old type Wiseco that need about 4 thou or do exactly 'that'. Maybe 4 though is exaggerating a bit but not my much. What brand/type of piston is it. Not seen one with holes high up like that unless drilled by someone [or the owner]. Piston is probably good to go now its changed shape to fit how it wants--maybe. Lucky you felt it going tight & avoided damage or getting dumped in the road which wouldn't be good. When you say cruising at 75 i dare say kph not mph. Hope she's good now but don't think its a lube thing but could premix a bit as well as the pump if running the pump. Was looking through a DT1MX manual recently & wow they say 30/1 mix plus the pump, or 15/1 mix with no pump. Not relevant to your problem really but thought hell she'd smoke the track out & catch up with it on the next lap at that ratio. That'd be what's 'really called' smoking the opposition lol. Sorry, wandering off course.

What does the underside of the piston look like?

Same thing happened on one of my '73 360s on a hot August day at 50 mph. After it cooled down I started it back up and rode it home. I put another few hundred miles on it before I tore it down and had it rebored to the next size up. The shop owner advised against sustained hiway speed for the 360s. Vary the throttle position and go up on the main jet size. I have #280. He also suggested keeping 2 fingers on the clutch lever while riding.

 

Have the cylinder ports been chamfered?

I have kept a 360 to try to figure out the jetting compared to the 400's.  My 360 does this same problem while trying to ride at 5k rpm and up for a sustained period.  Not really sure why the 360 is different than the 400. I have come to think it is just a retained heat problem of the big bore 2 stroke. Thru all the years of racing moto I could never get a big bore Yamaha to run as clean as a 250.  Then HONDA and Kawasaki built the 500 liquids and they both seemed to run a lot cleaner than the 465 and especially the 490 Yamahas.  Regarding the 360 though that engine is just not as easy to jet as a 400.  One thing I have done on occasion is to reach down and open the choke and it seems to help at the beginning and then it starts to load up and you can feel the engine pull harder as it is cleaning out. I know I do not have this issue on my 250 DT's and that is why I believe it is the retained heat problem. This is also kinda the same as the deceleration popping and bucking you get on these big bores and you don't get on a 250
doug

Underside of the piston shows no sign of overheating: 

 

This is a forged wiseco.  The FL350 piston I was using has vanished from the market.  This piston is actually pretty promising, much lighter than the FL350 and it only needs a 2mm spacer under the jug.

I was cruising at 75 American miles per hour.  High speed cruising and top end pulls are the true test of any high performance machine.  Most of the trip was made at 70 but I punched it up to 75 just to see how it would do.  A minute or two of that was enough to do it in.

Clearance was specified at .003" and that's what I asked the machinist to do.  In hindsight though I remember that this piston was originally intended for use in a water cooled application.  Perhaps .004" or .005" would have been a better choice.

 

The ports were chamfered but I did go back over them last night to make sure he didn't miss a spot.  A little more chamfer couldn't hurt.