Bottom end of motor blown??

[Rockjaw]

I built a 2 stroke 50cc bike and the bottom engine end was used...i tried everything to get this bike started but it just wouldnt turn over. Im puzzled..the magneto was giving spark to the plug, i replaced every part on the kit...when i swapped the bottom part of the engine...the engine started..in other words...the bottom end was blown..( i used same cylinder and piston) i dont understand how the lower part of the engine could be bad? what exactly was went wrong with the lower end that would cause the bike not to start? it was getting spark!
Please inform so i know the next time this happens.
Thanks!
RJ

 

[Slogger]

Probably leaking seals.
Just guessing here.

 

[Rockjaw]

and also i just put another bike together ..new topends tried 2 so its the bottom end (crank case) not starting the bike..making a crackling sound

 

[crassius]

magneto wrong way around would do that - crackling could be rings backwards & now broken

 

[Deleted member 12676]

crackling could be bad bearings.
but that didn't prevent the engine from starting.
it was the seals which let fuel mixture blow past them. that mixture was intended for the combustion chamber.

 

[Rockjaw]

ok thanks ..whhich bearing cause crackling?? the bearing that is in the piston arm? also which seal ..should i replace the lower gasket and use sealant on that? would a bad lower gasket stop the engine from starting?

 

[crassius]

I suppose it could, but usually it just makes them run really badly.

 

[Deleted member 12676]

just move it to feel if the bearing action is not smooth.
crassius is right- a bad gasket usually is not enough to keep it from starting. but the seals are