So,what caused this then??

[dave411]

Was the piston seized in the bore?Spigot casting probably smashed by flailing con rod.

[Avantone]

Blow by on piston indicates worn piston/rings and so probably sloppy piston in bore.
Sloppy piston likely to stress piston skirt as it rattles up and down the bore
Skirt broke and lodged in port and locked top end solid.
Conrod broke, scooter travelling at speed, clutch not pulled in and motor self destructs.
Probably just luck that the rod didn't punch a hole in the crankcase.

[kevspeed]

Was the piston seized in the bore?Spigot casting probably smashed by flailing con rod.

Piston not seized,but probably as suggested,piston skirt cracked,lodged in a port then BOOM!!!!

[goldeneye]

shimmed crank AND piston shims used? that's my guess.

[bazza3004]

SIL rod failure due to excessive piston to bore clearance (evidence the blow-by), Ring failure (where is the top one). Small-end bg failure: check as suggested by roscalla. Pics are poor: check big-end and main bearings. Coupled with poor quality components?

Top ring is present,small end is still free,mains are good,mostly suprised at the chunk of barrel spigot thats smashed off,and the fact that the engine casing isnt damaged!
Original 1979 Indian 150cc motor,no modifications except for a 12v electronic,as the lights on the original points set up were dire!,pity as was running faultlessly,still,its Lambretta isnt it!

they always seem to run at there best just before they go BANG.

[Knowledge]

Steve "Klondike" Cook, scooter legend of Derby, used to say that Lambrettas only run well twice: once when they are re-built, and again just before they blow-up.

This thread is in the latter category.