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while, from the pics presented, it looks like the case boring was the problem, it is not conclusive.

the piston failed.

I made an educated guess as to why

a very real concern remains as to the bore and piston

was the piston defective ?

until all the relevant details are in, we are chasing Zombies

an early theory is just that, early

a lot of things ?

we disagree....

more information is needed for a conclusion.

I disagree with the conclusion of SE and moat above, no disrespect intended in any way.

I agree that the minor irregularities of the case opening is of no consequence and that a perfect fit is not needed.

What I think happened:

The opening was bored eccentric with respect to the studs. there is more metal between the stud holes and bore hole on the side opposite the cam chain passage.

the metal there is thicker than the corresponding space on the cylinder from stud hole to liner/skirt/sleeve.

because the liner has very thin metal in the skirt, the skirt deformed at assembly.

when you look at the bottom of the cylinder, ( in the first set of pics) you see that about half of the cylinder skirt is missing along with a big chunk of the piston side that goes up to the wrist pin hole in the piston.

It appears that the piston hit something and stopped (on a down stroke) and then was pulled apart, taking the missing cylinder skirt with it.

what I think the piston hit was a ridge in the cylinder wall caused by the eccentric opening in the case.

if the cylinder liner skirt had been thicker, it probably would have not seated/mated to the case without extreme force.

to be more certain of this proposed mechanism, more exam of the cylinder and piston is needed, along with measurements of the suspect area.

a retrieval and exam of the missing pieces of both the piston and cylinder skirt may give more light.

so I will leave my proposed theory up for critical discussion.

perhaps some one else sees something in the pictures that I do not and has a better explanation

I think that by hiding/deleting the thread, you do the forum and readers a dis service.

quite few of the posts were out of line.

at the same time, I still think the mechanism of failure is undecided.

we all learn by knowing how engines fail. when we see good info about failures perhaps mistakes, we can try to avoid having the same thing happen.

my guess based on GTI's few pics is that the bore and crank were malaligned, but that remains unclear.

the bore should index from the studs and not from case opening.

another point is: are these large cylinders with very thin base skirts prone to failure ?

I for 1 would like to see more pics of the failed engine.

was the crankcase opening bored eccentric ?

did the thin cylinder base skirt deform because the opening was eccentric ?

edit:
your stator makes no sense to me.... & it now makes sense why only 1 specific RR works

another option, the below ebay listing is in pounds or about $20 US for this stator

looks closer to the TT KTM and should convert with minimal problems

www.ebay.com/itm/8Poles-GY6-50-Moto-Engine-Sparking-Magneto-Stator-Coil-/250869207333?pt=UK_Motorcycle_Parts&hash=item3a68f85125



yes it makes sense.

edited: probably can get rid of the green (ground ) wire on this stator. after conversion... don't think that it is needed for the CDI trigger pulse

also you can see why winding are added to a pole in the TT guide...

a 12 V DC conversion has to end up with all things run from the battery

and the battery charged from the stator/RR, just like a car alternator/battery system

I think that the first diagram is probably wrong..... the center tap should be grounded rather than one end.

a center tap is used to provide a "neutral" point in a coil (or transformer) for comparing the 2 ends or to get different voltage outputs depending on the coil/winding count ratios

in that diagram were true, the yellow connection to the coil would be ground and you would only see voltage on white.

we know that is not the case.

scooter 6 and 8 pole stators have odd numbers of charging poles, 5 and 7, and you can not substitute white for yellow or visa versa. The "other" pole triggers the CDI.

so stator output is not "balanced" on white and yellow compared to ground. the output of white v yellow is 2v3 or 3v4 because of the odd number of poles

yellow powers the AC lights and auto enricher as well as the RR

full wave "floating" stator output is more or less comparable to 240 V AC residential power and is balanced

if you measure voltage from either wire, measured to ground, you get half the voltage compared to measuring wire to wire.

the CDI feed is from a circuit separate from the charging circuit, so a ground in that circuit would have no effect on the charging circuit.

yes, no more yellow and white, just 2 yellows

looks like good progress.

just wondering if you have an estimate on how much the radiator will drop the oil temp ?

glad the electric part is working out too.