Oil starvation or 4 corners

[Economy Cycle John]

Helping a customer in Australia diagnose a siezure. This looks like a 4 corner seize from a tight bore or oil starvation, and I'm leaning torwards 4 corner. I asked for some more pictures w/top and underside but assuming those areas are fine, what you do guys think? says his pump is working fine.

[m in sc]

not enough clearance.  got warm and -squeak-

[Striker1423]

Yea, my G3SS seized like 6 times in a row when I went with my buddy for a ride after a re-bore (OEM overbore piston at 2-thou). Seized in basically the same way. Not enough clearance. It would seize, shut off, kick again, go, seize, etc. Never locked the motor fully. Just kept seizing. Was scary, yet so strange. 

I referred to this when looking for answers back then:
https://mcbperformance.com/p-25418-piston-failures-causes.html

[teazer]

4 corners from lack of clearance or thrashing it while it was still cold (or both)

[Economy Cycle John]

Thanks Guys, much appreciated

[1976RD400C]

What brand is that piston?

[SUPERTUNE]

Too tight...4 corner seizure looks to me.
You cant't run stock piston clearances with today's pump gas unless it is 'CANNED Gas" (race)
Look like stock replacement pistons to me...Not stock Yamaha oem but copy's.
The copy's could be different base metal casting and stock pistons do require more piston to wall clearances as you go bigger in bore sizes as the pistons will increase thickness.
Chuck

[jmendoza]

He did not break it in enough.
I have used .0016" clearance on my air cooled RDs with OEM pistons and never had an issue, but I follow the break-in procedure and do not flog the engine.

Typically, seizures that occur where the piston clearances are tightest (below rings on the skirt) are due to lack of lube.
Seizures in the ring land areas are typically from overheating.

But in a new engine that has not been broken-in and has tight clearances, the piston wrist pin bosses are the thickest areas of aluminum and therefore expand the most, causing the typical 4-corner seizure mid-skirt as pictured above.

We used to coat our pistons with blue Dykem, assemble and fire up the engine, then take it apart and polish the piston skirts with 600 where the Dykem was scuffed off and then re-assemble go racing. This was a quick-break-in technique for racing only.