This is a common thing to happen in the 400 engine. high rpm causes the piston skirt to chip away, and a chip of metal will fall into the crankcase. If a piece of the piston skirt gets caught between the crankshaft and the crankcase, you got big problems. This just happened to mine, and now its gonna cost 5 times as much to fix than it would just to buy a wiseco forged piston. In order to preveny this from happening, get a new piston immediately.

 

Yep, many have fallen prey to his happening. It's not due to high RPM's, rather just wear and tear causing too much clearance and it beats itself apart. The motors only turn 5700-5800, so the piston speed is kept fairly low. The rod angle causes some side loading of the skirt and that is where the wear comes from.Bubba

 

yes this does seem fairly common but im happy to say it never happened to me

 

yeah. You should see all the metal chips. It is a huge mess! theres chips in just about all the engine parts up to the carb. this sucks! by the way scrambler, did you mill the head for 14.5 compression or is there a piston for that? i asked a guy at wiseco and he says you have to mill the head, but a guy locally tells me that you can get hi comp pistons.

 

For a two stroke you either mill the head or buy an aftermarket unit with interchangeable domes for higher or lower compression. A four stroke uses a piston for higher compression.

 

as it's been stated earlier,those motor are not so tight designed and
does have clearances and wear on the skirt off and the the cast
iron cylinder doesn't help either,the main wear at the upper back
of the piston where the rod angle is toward the front,i'll always
¸tell the 400 owner to spray after run oil in their engine after some heat time up,it will highly keep the wear down as it come to piston.
keep on eye on fuel quality as well,as they depend greatly.

 

I had the exact same problem with mine. One day my bike didnt start so we toke it apart and found a hole in the crankcase. I even farther ina a dn found out that the pistone skirt shattered. I have been trying to get it back together but it is vey expensive. Make sure that if you have to buy new crankshaft bearings that you dont buy the cheap one becausethey wont go in right and will crack your crankcase.,

 

I talked to a local atv mechanic, or whatever you call them. I took my cylinder, head, and case to him to get new parts installed and he told me the cylinder was tapered, which means the bottom was like this / \ sorta, and not straight up and down: | | so the skirt was slopping around in there and thats what caused it to shatter. oh, and im getting the head milled to 14.1 compression [img]i/expressions/face-icon-small-happy.gif[/img] and the cylinder milled out to 84mm. hopefully he will have it done by thursday or friday and ill get it all back together for the weekend and blow it up again... not. and for u 500 ppl... you ask how it broke in the first place? i was dragging a 99' 500 and whooped him up and down on the platte river. i hate 4 strokes