Posts
1588
Joined
2/20/2015
Location
Flowood, MS
US
Edited Date/Time
6/21/2015 7:13pm
Bought a yz250, 05 model a month or so ago. Bike came with tons of receipts and service history. So i felt good about adding it to my stable with my other yz250's. The last service was a top end. Receipts showed early April, all oem. It was done, new nuts for the cylinder, new cap nuts for the head, you could see a new base gasket. But, it wasnt oem parts....
Ive put maybe, 12 hours on the bike strictly mx, race gas, jetting is spot on. Fmf fatty, carbon reeds, 178 main, 48 pilot, stock needle 2nd clip down. No toil filter changed every ride.
Wtf caused this to happen?!! No lean condition happened. I was coming around a fairly fast left hand sweeper in 3rd gear and laid it on coming out and POP! Pulled the clutch and shut it down asap. Pulled the plug and balls of the piston were in the plug. Pics above were from the tear down.
So pissed....
I see a lot of piston slap signs.
It was either older than stated ( more hours) or a poor fit.
Check the cylinder size.
Paw Paw
Not sure about everyone else, but that one needs to come apart totally, IMO. Crank rebuild ,as well, as there is surely tramp /stray metal in the big end rod bearing / pin area.
The Shop
That said, I agree with all the above comments.
Pro-X is a decent piston, they don't do that in 12 hours if everything else is right.
That looks like tons of hours or a bad cylinder or air-filter issues judging by the pics.
Just to rule out detonation:
On the YZ250, pump gas can cause detonation on a stock engine and stock jetting under the wrong conditions.
Check the outside edges of the head for roughness and pitting. If there is pitting (even a tiny bit) in the head, you have detonation. On the YZ250 that pitting always shows up around the perimeter of the squish band.
Get that bore checked by a machinist if it's not obviously in need of a re-plate.
Thing with this bike and why i didnt question it, i know they guys nephew and know him somewhat through mutual riding friends. Everyone has always said he is meticulous and anal about his bikes. So with the paperwork and visual cues, i didnt question it at all. Until she grenaded.
I split the cases and checked runout on the crank and rod/ bearing. Down there, new seals where in, which leads me to think he knew she was on her way out and wanted to unload it before she blew.
In 30+ years of riding, this is the first piston that has let go like this on me. Ive only had 1 other actual piston failure and that was on a 125 being murdered on a daily basis! So seeing this is new to me in a way.
Thanks guys, sounds like i got duped by paperwork and bought a lemon. But she will be lemonade after today.
I had the same problem with a clapped out YZ bought to a guy who never did maintenance. Inspect your cylinder, mine was cracked.
Now the bad news, I would also look at doing the bottom end since you just ran a bunch of Al through it. Plus, that thing looks to have a lot of hours on it.
The good news is that it is a 2 stroke and the cost of repair on something like this is 1/3 of that if you had grenaded a 450.
Like i stated earlier, i run a machine shop and have every tool at my disposal to do anything needed. Ive inspected everything on it, i find myself veey fortunate that the piston/ rings and reed cage were the only things damaged.
Ive already assembled it with oem pieces, D size piston was perfect match. Compression is now 240 and she fires up first kick and sounds nice and crisp.
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