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[papasmurf]

JTL in the second picture down on the unmachined part of the piston there are a number of what look like tiny "woodworm" holes. Basically underneath those holes the metal could look like the inside of an Aero chocolate bar.

[Alan R]

----- I'd put my money on the chocolate bar !!... A few years ago I fitted an Aluminium replacement barrel on my 500....There was an inclusion halfway down inside the pushrod tunnel which was so bad black oil seeped to the surface as if a leak had occurred !!

[Presto]

In light of the ‘mileage’ this piston has covered it seems unlikely that there is any fundamental fault with the item. If there had been it might have been expected to have failed much before now. The damage to the piston and con-rod seems to indicate that failure occurred on a ‘compression’ stroke. It is significant that there is no evidence of seizure on the piston skirt. I’m puzzled by the difference in colour either side of the crack shown in 4th photo – with the duller surface showing on what would be the ‘main thrust face’ of the piston – the intake side of the piston. It may also be significant that there is no evidence of oil on any surface – not even in the crankcase. But I’ve no idea what may have caused this failure to occur at this precise point in the life of the piston – nor it seems have any of us!

[Gwilly]

Burt Munro had a shelf for all his broken pistons and rods..

Just went right back and made another.

[JTL]

Hi all... once again thank you for all thoughts and suggestions on the cause of the broken piston. After reading about porosity and a close study of the piston I'm pretty sure porosity is not an issue in this case, but a weakness in the casting/material may somehow be the cause of faillure. Presto... the bike has high capacity oil pumps from our host, so there is plenty of oil around in the system. Maybe your reference to no evidence of oil on the outside can be explained in the fact, that I cleaned the engine outside with kerosine and viped the piston clean before taking the pictures. Or maybe you had something different in mind? Gwilly... I have taken a picture of the gudgeon pin (see below)... After all, maybe this little piston incident can not be explained in a rational way one can conclude on in such a way a "next incident" can be prevented. Hopefully I can rebuild the engine to a greater strength, that will suit my way of riding the bike... regards Jacob