Piston Skirt Explosion


New Member
Apr 22, 2024

I am trying to help a friend rebuild his yamaha blaster he purchased. To make a long story longer he had a clutch issue and got drunk and tore the whole dang motor apart….down to the crank and actually tried to press the crankshaft out🤦🏻‍♂️. I am rebuilding this tub of parts he brings over to me and have gotten to the point of putting the jug on the case. I am finding that There are multiple paper style gaskets and 2 metal spacer like plates. (I think they are aluminum.) I started with one gasket and tried to install jug and head. Well the piston hits the head and the crank will not rotate.

So experimenting, i found If i stack these metal spacer plates along with 2 paper gaskets, (like Paper Gasket, metal, paper gasket, metal.) i eventuallY get enough clearance to be able to rotate the crank without the piston hitting anything.

How do I tell if someone has changed the stock crank in this thing?? I know they previously deleted the oil pump, and the carb does not seem to be stock either. Its a KeiHin PJ.
I work on mostly 4-strokes so this is a lil new for me. I have a manual and have followed everything on the rebuild to a T. But I dont understand why they have all these spacers. Other than an aftermarket piston rod, like a stroker kit or something.

Seeing that I now have clearance enough that the motor rotates freely I finish the build pour some trans oil in the case and put the motor in the frame, kick it over a couple times to verify everything rotates smoothly, throw the carb on, put the spark plug on and kick it over. After about 3 kicks she fired up. It was running pretty nice, very responsive throttle. I started trying to figure out how to adjust the idle. Its a screw in style on the choke lever, (thats broken off🙄). As I blip the throttle I hear it bog up and then a rattle sound and it just died. I went to kick it over and could feel it was completely locked up. I pulled the head and jug assembly off and found the intake side of the piston skirt was destroyed and a million metal pieces are now in the case……. Please help me on what steps I should take to understand what I have going on and what could have caused this.

Thanks for your time and advice!


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Sounds to me like that engine has some mods done to it like a stroker crank and/or re-chambered head. The stack of spacers needed to allow the engine to turn over leads me to lean toward the stroked crank.

As to piston skirt destruction, there aren't many things that will cause that. Excessive wear of the piston and/or cylinder can cause it for one. Also, is the piston oriented correctly on the rod? There should be an arrow on the crown, the arrow should point toward the exhaust. A piston installed backward I suppose could lead to premature failure of the skirts.