Cracked piston

[roastdinajima]

What would be the cause of a cracked piston the bike is a '98 zx6r, as you can see the valves and piston have made contact ,piston appears to have touched the head and the bigend bearings appear to have spun in their seats.

What is likely to have failed first the bearing or the piston?

[porter_jamie]

my guess bearing first

[morepower]

Bearing...

[VeeTeeArr]

It was nice of you to put the little arrow on the piston to show where the crack is

[fran9r]

Was it standard? Miles? Not seen it that bad before...

[roastdinajima]

Was it standard? Miles? Not seen it that bad before...

God knows to be honest, the bike was brought as a non running track bike which was an easy fix. The engine was flushed out with a motul product then had new oil and filter. since then its just had a few short runs on our private test track but no real use, until friday which was a track day at eastern creek it lasted about 12 laps and felt real good until it went pop.

I'll get the motor out of the frame and have a good look at the crank and other pistons etc.

[roastdinajima]

Don't just look at the other pistons, crank etc....

Check the condtion of the cam chain, the cam chain tensioner, the cam chain guide and blades and the toothed wheels on the end of the crank and cams. If the timing has slipped and you've got valves touching the piston that could explain a lot. Edit - just noticed you said the bike was a non runner when you got it, did you check the timing before you started it up?

Also check the conditon of the cam lobes and buckets/shims plus the valves and spings, especially make sure the collets on the valve stems are in place - if the valves were floating becasue the valve spring tension acting on the valves had been lost that could also explain a lot.

Like I said before, I don't see a big end bearing movement or piston failure allowing valves to touch the piston as the mode of failure - I think it's more likely the other way round, valves touching piston casued the piston and big end to fail.

Thanks for the advice we didn't check the timing as it seemed to be running very well, the cams look in good condition, i half expected something like this to happen, if something seems to be too good to be true etc.

[will#224]

I've had the same thing happen on a car. Turns out that engine flush you've used loosened of some crap in the engine, partially blocked an oil feed and starved a big end of oil at high revs.

[potheadpixi]

It's likely that excessive big end clearence / failure could be the culprit - reducing the squish area to zero.

Being a track bike it's quite possible that the engine has been previously modified, eg head, block skimmed to increase compression etc...

Thus further reducing the safe running clearence and increasing the chance of piston to head contact at high rpm.

From the photo it doesn't look like the valve to piston contact was too servere... not enough to cause the piston to fail in such a catastrophic way anyway.

Also, I'd expect a tell tail misfire to become apparent before such a failure occured if it was purely due to valve / piston contact.

Just my two pence worth... PP

[Jay]

Your suggestion of skimmed head is a good one though - would explain the piston high points being clear of carbon and if too much was skimmed and the valve clearance before and after TDC weren't checked properly then that might explain the exhaust valve contact with the piston.

+1 I have seen similar with a R6 engine that had been home tuned, Matey had skimed too much from the head to increase compression, Bike overheated one day and Bang! valve clearances on the other valves was non existant also.

[porter_jamie]

is there any evidence of the other pistons touching the head, and or valves touching pistons? and what about the other big ends?

has the motor been apart before? did someone rebuild it with lashings of hylomar, and maybe some got into the oil/oil pump etc. cut the filter open and see what is lurking in there

just a thought

J

[roastdinajima]

Whilst we're dealing with the engine rebuild i decided to get busy and clean up a few things and powdercoat some stuff aswell while its in bits

while i had the shock apart to do the spring i noticed something wasn't quite right

This is truely the bike that keeps on giving its just a shame its giving me the ball ache :frown:

[fran9r]

while i had the shock apart to do the spring i noticed something wasn't quite right

Eeeek!

[roastdinajima]

Everyone who rode the bike before it shit itself (err 2 of us) all said how nice the rear end felt (suspension wise you filthy monkeys) just goes to show what we know ehh!

[roastdinajima]

I brought a wrecked zx6r at the weekend, what is the forums advice on the best way to go about firing up the motor, i've got no history on it but it appears to be well looked after (apart for being parked in the side of a car) would you just drain the oil refill and go from or what?

[lorenzo]

Is the bent up bike in a state that'd allow it to be run? If so, i'd do the basics; make sure there's enough oil and coolant in there, fire up the engine and let it get up to temp and make sure it sounds OK and doesn't spurt insides outwards. I'd then drop it out of the frame, and check everything you can think of. Valve clearances, timing, oil strainer, wear on the cams, cam chain tensioner, plus other things that I'm sure you'll remember even though I can't at the moment. I'd then stick it in and go have fun.

The thing is though, you were bloody unlucky to have the engine go pop in the first case. I don't know how many people on here have bought slightly shonky bikes as cheap runabouts for either road or track, and I don't think there's been many who've had catastrophic engine failures. Make sure you've done all the reasonable things with this engine and if it happens again, I'd be looking at the guy riding it and asking about over-revving.

[roastdinajima]

Is the bent up bike in a state that'd allow it to be run?

Err it was until a few hours ago but i've got itchy fingers so pulled a few things of exhaust loom carbs etc. you'd run it up with the old oil in it? then just change as normal when hot.

[Marb]

Just being curious, what did the big end look like?

I had a bigend failure on my 400 at about 14000rpm and did this to the engine:

The piston suffered very little damage. So I am stull curious what happend to your engine.

[Mr.Pigdog]

Couldn't someone have relocated one of the cams out when adjusting the shims?hence valves colliding with piston crown when overrevved.

[ronenige]

What cylinder was it on ? , most kwak's run no3 if run a bit low on oil , there are various reasons for it and all you can do is eliminate them all , don't forget to check the oil pump for wear and make sure the pick up pipe and filter is clear , it's not unusual to find these 50% blocked , it only takes a second for cavitation wreck a motor

[roastdinajima]

Just being curious, what did the big end look like?

This is the bottom of the rod it was no2 that let go

i'll probably do what mille says before i run it up, i might even drop the sump just to make sure there's no nasties hiding in there.