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How did this happen!?

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Hey all, first time posting on Thumper and I had a question in regards to my 4 stroke and it's major malfunction. A little history, This 2005 RMZ 250f has been a track bike and has quite a few upgrades in regards to suspension and engine modifications. When I bought the RMZ it was broken down and the owner "Wasn't sure exactly what happened". So i did the work and pulled the engine and opened up the top end only to see a smashed piston, an intake valve completely snapped off and missing. Side note: all the bolts were loosened previous to me opening it up. Someone had definitely inspected the bike and knew exactly what the damage was. 

So my #1 question is, what could have caused this disastrous malfunction? 

*Apologies if I posted this in the wrong section, I did not see one for maint or plain old repairs. Move if needed please!

 

 

20170728_181911.jpg

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Usually over-revving if everything else is intact. Are the keepers still in place? And is the spring unbroken?

Is there marks on thhhe piston where the other exhaust valve has been contacting? kinda hard to see in the pic.

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Yeah, springs are intact and yes it does look like the other exhaust valve made some contact or it could possibly be fragments of the broken exhaust valve. Also, i'm not sure if this makes a difference but they had stage 1 hot cams upgraded but did not do a larger bore, just a standard cylinder works jug.

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@ S.O.A.N.Z - I have not gotten that far yet in the disassembly. Will let you know when I pull them out for inspection. Also, I forgot to mention. This motor was fully rebuilt, top and bottom. Was ridden for 2 hours and then stuck in a garage for 6 months. Maybe it didn't get enough heat cycling before storing? 

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Could be assembly error as well, say the valve keepers.

 

Perhaps didn't show up during the first few hours after the rebuild because it was ridden moderately during engine break-in

but later grenaded once the RPMs were pushed higher.

Edited by mlatour
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On 7/29/2017 at 6:44 AM, mlatour said:

Could be assembly error as well, say the valve keepers.

 

Perhaps didn't show up during the first few hours after the rebuild because it was ridden moderately during engine break-in

but later grenaded once the RPMs were pushed higher.

i was kinda thinking maybe cam timing too, but as the heads off now he can't tell.

Or they simply didn't account for the higher lift/longer duration of the cams.

I wonder why it was rebuilt last time?

I have an old GPZ turbo...which suffered much the same thing, it was due to a high lift cam and the guy who put the head together for me (i was being lazy...) forgot to account for the lift when setting the spring heights....coil bound them and ripped off the bumps in the keepers that go into the valve groove after an hour of so running.

Maybe this should go in the 'engineering and tech' section.

Edited by DEATH_INC.

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How do you know it was rebuilt?  To me it just looks like lack of maintenance.  12 year old MX bike dropping valves.

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