sad day :(

7 replies to this topic
  • rufusz

Posted May 24, 2008 - 05:29 AM


Today is a sad day !

Just got everything to SM, did a couple of miles together with some friends, and after couple of hours, going 5th gear, full throttle, enjoying the weather and the curby road, something broke in the engine. I would guess a ring, or part of the piston, because I can easily turn over the engine, but I hear metal shrapnel moving around in the bottom of the engine. Most annoying thing that I don't have time to work on it, I have to take it to a shop.

02 YZ426F SM

:p :mad: :mad: :thumbsup: :ride: :bonk: :lol: :mad: :lol:

  • rufusz

Posted May 24, 2008 - 11:33 PM


Update: Today morning I will take the engine apart.
A question popped into my mind : is there any possibility for the rev limiter not to work? Maybe one of the wires to the CDI unit broke? Is this possible? I know there is a different ignition mapping for neutral and a different when the bike is in gear...maybe there's no revlimit for in gear???!!! I've wanted to see max speed in 5th gear full throttle and sometimes I've hit a limit, but it didn't sound like the "classical" rev-limiter sound...any ideas on this? Or basically I shouldn't ride supermoto in 5th gear full throttle for longer time?

  • Kasjok

Posted May 25, 2008 - 03:21 AM


Or basically I shouldn't ride supermoto in 5th gear full throttle for longer time?

I think you shouldn`t ride anything full throttle for longer time.

Rev limiter isn`t a wire, if you meant that. The wire you are talking about is probably TPS. If this is disconnected your ignition map is on full throttle mode all the time. But that doesn`t matter a thing at the moment.

Your engine blew probably because of some mechanical problem. Riding WOT rises the odds of engine blow-up significantly.

When was the last time you rebuilt it? How was the maintenance?

Good luck with tearing it down!

  • rufusz

Posted May 25, 2008 - 12:12 PM


I've bought in October, changed ex-cam with HotCams, repaired transmission, changed oil frequently, bike ran OK.

Update: Pulled the engine apart today !! One intake valve broke, fall into cylinder, piston has 2 holes in it, the head of the broken valve has been pushed back into the cylinder head (don't know how it's called in English, the hole where the valves come out from :thumbsup:), and one of the valve lifters on the exhaust side has been broken to small-pieces, cylinder suffered minor scratches on the inside.
I think cylinder can be rescued, don't know about the cylinder head....
I don't know if it can be welded and then machined.... Maybe it would be a better idea to buy a used cylinder head...
So if someone hears my rescue call and has such an cylinder head...don't hesitate :ride: Will the cylinder head from other model/year fit (450/400) ?

Will post pics soon...

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  • grayracer513

Posted May 25, 2008 - 05:04 PM


All 400/426 heads will fit, although the early 400 head has a port on the intake that would need to be blocked off. No 450 head will fit.

  • rufusz

Posted May 27, 2008 - 09:31 AM


Took couple of pictures of the head, piston and cylinder. What do you think? Can the head be repaired? And about the cylinder?

Posted Image
Posted Image
Posted Image

  • rufusz

Posted May 29, 2008 - 05:44 AM


should I replace the connecting rod also? I know that the piston absorbed most of the force, it possibly went through it like knife in hot butter, but the connecting rod's bearing could also have suffered...
what do you think?
and another question, could bad valve clearance produce this? and why? I'm not completely sure why valve clearance could lead to breaking a valve (Ok if the shims are too big maybe the piston could hit the valve, but if clearance is bigger then normal???). Through normal wear, when you need to readjust the clearance which part of the valve/camshaft assembly is worn?

  • grayracer513

Posted May 29, 2008 - 06:42 AM


The rod will almost certainly have been bent at the small end, and the big end bearing damaged, yes.

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