(Chris, I've yet to look at the other side, although it should give a clue as to whether this is a set-up issue or local fault.)

So far I've got two thoughts about what caused this, they're both just possible theories.

Firstly there may have been some corrosion on the bore as a result of being in storage for a while, this caused a broken ring which scored the bore and allowed gas to blow past and burn the piston. Problems with this theory are, does Nicasil corrode? (I would assume not). And that there is also evidence of overheating and scuffing on the opposite side of the piston.

Theory two is overheating/detonation. Evidence against this;
I have a temperature gauge though the sender is in the block and not the head, it wasn't reading any higher than usual before the problem started, iirc it was getting quite warm while limping on the last few miles, still not hotter than normal, but high considering how little throttle I was using. I also have a fuel ratio/lambda gauge, it wasn't running lean. I didn't hear any unusual noise(knocking), but then you don't here much over the ambient wind/rattles/exhaust noise, and I wouldn't trust myself to identify engine noises anyway. There's no damage to the head which I thought would be expected from knocking/detonation.

The main area of damage is directly below the lower exhaust valve, I can't see any damage to the head at all.



Scuffing to the top side of the bore.



Corresponding scuffing on the skirt, and a small burnt away section near the crown (top land). The rings are undamaged here, just a bit nipped.

Just a thought some Nicasil barrels did have a failures of the plating with bits coming adrift,which would cause damage like this. There were quite a few problem with these early oil head boxers, like this and the through studs head/barrel to crankcase were just pulling the threads out of the castings.
I got to know about such through my good friends in the police workshop.

Just an idea or two to consider...

An update, the other cylinder appears fine, although I've not taken the barrel off the bore is unmarked and the piston crown is complete.

Looking at your bore - the honing marks are still there so do not think it was corrosion etc. Does your fuel / air ratio gauge take in both cylinders? What I am trying to say because you have 2 invidual barrels and injection/ carbs then 1 could be lean and 1 rich and the sensor would say engine ok...

The lambda sensor is in the exhaust after the Y-piece so might not show up a problem that affected only one cylinder, but what might that problem be? A blocked injector, they're the same throttle bodies that were in the car before and are back in the car now. Or an air leak between the throttle body and head, but that would show up as a bad idle, and have no effect at full throttle.

That licks balls!
I feel your frustration...
And have no mechanical expertise to offer.
Perhaps that is why the engine was taken out of the bike in the first place, do you know it's history?

Harley

The more I look at the area of damage the more I think that the bore coating has failed,I also wonder if that hole/chunk does look more like a structural failure rather than a burn type erosion to me.Mind it's 40 years since I worked in a metallurgy lab.I also have a history from my karting days of running things a bit to lean resulting in the odd seize here and there,but even with a 2 stroke revving at full tilt I've never seen that much damage

Joolz,
pages 24, 25 & 26 should confirm the cause...
http://www.boosttown.com/engine/piston_damage.pdf

Note that once the piston rings have been nipped or broken by a momentary or partial seizure, compression on that cylinder will have been drastically reduced, hence no more detonation would occur.

Also, the Nicasil coating was in good enough condition to tear molten aluminium from the piston, so I still reckon that the gouged area in the bore happened as you continued your journey, with the dead cylinder being 'motored' by the other side.

ken

p.s. I don't know much about the cooling arrangement on these engines, but is the oil cooling confined to the cylinder head?

Do an injector check and see if they deliver the same amount of fuel, however by what I have seen of lean/knocking engine burns always a hole in the middle of the piston.... not at the edge......

Could a piece of metal got inside the cilinder???? doing this damage, or broken and loose pistonring???

p.s. I don't know much about the cooling arrangement on these engines, but is the oil cooling confined to the cylinder head?[/quote]

The head is oil cooled and the cylinder air cooled although there is of course the cooling from the oil cooler in some respect for the rest of the engine