Problem engine clang- crack noise found :)

[Bobbyoutback]

I have a few Mbikes this story is about an old 4 year engine thats been ridden & ridden but never abused , just recently it developed a loud crack ~ clanging sound from the motor .

Well I went to the trouble of a engine tear-down looking for the fault & found nothing that would cause this noise
Now at this point I was considering just mothballing it " But then tried the long screw driver to to ear trick to try & isolate the sound , guess what I found it !!!

The problem was that over time the con rod has stretched allowing the piston to go that bit higher each stroke to slap into the head after distorting the thin metal head gasket because the Chinese made head is that tiny bit smaller then the jug diameter .

Hope this helps diagnose earlier model motor problems .

 

[GearNut]

Are you sure it is not the big end or small end bearings wearing creating excess slop and letting the rod travel higher than designed?

 

[srdavo]

I'm betting on the bearings/bushing, too.

 

[AussieSteve]

I'll have $1 on that too.

 

[motorpsycho]

you have ceated a stroker!!!
grind out the combustion chamber so the piston clears it, and run that sucker as long as the bearings are still good.
lol...you know i'm just kidding right?

 

[GearNut]

Conrods do not stretch unless they are waaaaaaaaay overheated. The piston would seize in the cylinder before engine internal temperatures ever reached that amount of heat, especially with the way 2-smokers breath the intake charge into the crankcase first, assisting in cooling the bottom end bearings as well as lubricating them.
The only other exception could be **** poor design (Chinese?) or horribly inappropriate alloy (Chinese again?).
I still bet on worn out bearings.

 

[Bobbyoutback]

Hi guys,
Thanks for the input , I did check both big & small bearings and found them still OK with no slop , when I back off the head tension the clang-clang stops so I'm going to make a thicker copper gasket & see if that works

 

[GearNut]

I bet that will help indeed.