What do you reckon caused this???

[Gumby190]

Hey all,

I have just rebuilt my bike from the ground up after going for a swim.

The old barrel & piston were stuffed so I replaced them, new Wiseco Piston, 2nd Hand barrel with new cast iron sleeve, I had the barrel honed professionally.

I have just hit the 50km mark since rebuild & taking it real easy, 40:1 fuel mix, haven't gone above 80km/h, keeping the revs low. I was going down the M5 motorway in Sydney when she started to hesitate, I thought the plug had gone so I was just about to hit the clutch & find neutral when the rear wheel locked up & I found myself sliding down the asphalt. I messed myself up pretty bad, alot skin missing, jacket stuffed & a really sore wrist & back. Quite lucky I didn't get snotted by a truck considering I was on a Motorway, my bike managed to slide a good 40mtrs before coming to a stop in a garden & I ended up in the breakdown lane.

My bike got away quite unscathed, Twisted Handlebars, Mirror gone, stand gone, clutchhandle gone.

I have pulled the head off & this is what I found.

Bear in mind this was a new piston, I am running a cold sparkplug.

As you can guess the inside of the barrel is a mess.

Any Ideas????

 

[heino]

Don't crucify me if I'm wrong, but someone told me that the cylinder has a hard coating of some sort. If you hone the sleeve, you take off the coating and you will seize the motor without even trying. That is apparently one of the reasons why you have to inspect the top end frequently. As I said, this is totally "hear-say".
Does anyone else have some input? :whoa:

 

[BSWIFT]

Don't crucify me if I'm wrong, but someone told me that the cylinder has a hard coating of some sort. If you hone the sleeve, you take off the coating and you will seize the motor without even trying. That is apparently one of the reasons why you have to inspect the top end frequently. As I said, this is totally "hear-say".
Does anyone else have some input? :whoa:

He used a sleeve that doesn't use the coating.
I think that you may have had an air leak and ran too lean. I siezed my KX that way.

 

[blackduc98]

It's very hard to tell from your photos, but it looks like a 4-point seizure. Assuming that you were not jetted overly lean, and you didn't have air leaks at the manifold and/or crank seal, then I'm thinking that whoever honed your sleeve didn't leave enough piston-to-sleeve clearance. Did your cylinder originally have a cast iron sleeve? If not, then did you account for the fact that aluminum piston expands more than the iron sleeve as the temperature rises?

 

[mhardee]

Hey all,

I was going down the M5 motorway in Sydney ...

I was there in January.. going down on the M5 could get you run over! The traffic is wild in Sydney (and Parrametta) especially for us "backwards" yanks that forget to look to the RIGHT first!

Hope you have better luck with the next piston.. that sucks after just doing a topend..

 

[hart125]

A kdx at highway speeds is a whole different deal. They arent designed to run at that speed with that throttle position. you are basically lubricating and running at idle position while the engine needs are the highest for lubrication and temp. you have to be sure that the pilot and needle are positioned so that you have plenty of gas at almost closed throttle. especially when the tolerences are real close like after rebuild.. You will notice a heat/lean problem when the exhaust side above the ports is scored.. yours is so bad that it has actually affected the intake side too but you notice the exhaust is much worse.. I vote the problem is a lean condition at low throttle.. you'll never notice this on the trail because your running different throttle positions all the time.the problem with the highway is you will be in a single throttle position for long times,,, and you have to be sure each area is jetted correctly.
sorry to hear of the problem,, I know how it feels from experience!!

 

[joebiodiesel]

My vote is with Blackduc. It looks like a 4 corners seizure to me too. I had some experience with those and a modded V-Max snowmobile. Sled ran great until I hit a sustained cruise condition. It would seize every time, and the piston looked very much like yours. I ended up putting in some serious clearance before it stopped doing it.
Joe

 

[Rotorranch]

FWIW...we used to have lots of problems with the Wiseco pistons back in the dark ages, when I was racing Bultaco flat trackers. They seemed to expand more than the OEM pistons, and we had to run bigger clearances.

Ironically, they didn't seem to have a problem in the Yamahas and Suzukis, only the Buls. ???

Rotor

 

[adam728]

They seemed to expand more than the OEM pistons, and we had to run bigger clearances.
Rotor

This is true for any forged piston compared to cast. You simply get more thermal expansion with forged, and therefor greater clearance is needed. You can get away with a tight engine a long time under the right conditions, but get that thing hot and suddenly the piston becomes a "press fit" in the cylinder, and bad things happen, like the above.

 

[hart125]

since we probably all agree that the piston and cyl are likely machined and forged very close to perfectly round,,you will know if these guys are right because you will see the expansion happen on the left and right sides too, not just on the intake and ex sides. IMHO If you see scoring only on the exhaust and intake sides I'll still argue it is a lean/heat not expansion problem.

 

[ridejunky]

Maybe the exhaust port relief on the re sleeve was insufficient and you got her heated
up by backing off the throttle at higher speed and letting her coast too long. When you do that sometimes there is insufficient lubrication because the throttle is closed cutting off fuel/oil but she is still cranking pretty good. soon the bridge expands from heat and Whamo... seizure.... Just a theory.

 

[whenfoxforks-ruled]

ridejunky,I am with you.Even though he did not post any crown pics,lack of lube would have toasted the bearings also.Air leak would have damaged the crown to some degree.How much that piston scored relates to how long it ran and how hard it swelled shut!Case splitting party at Gumby's garage!How is the road rash doing?

 

[76GMC1500]

This is interesting. There is another post in the CR/KX/... forum about a bike who also suffered a piston failure. I notice one major difference between the two. In yours, the scuffing is at the top of the piston near the crown. In the other, it is low on the skirt. I am going to say that this failure is very likely heat related while the other may have been lube related. I guess we should start at the basics. Check the jets, check for air leaks (crank seals, carb boot, throttle slide), what spark plug are you running, is your coolant circulating, does the cooling system hold pressure, etc...

 

[blackduc98]

How about the very simple basics first, like the answer to my question? To reiterate: what was the clearance between cyl. sleeve and piston that your pro honing shop set? What authoritative source did that clearance figure come from, and did it account for the specific materials such as forged piston in a cast iron sleeve?

Lean jetting could certainly be contributing to the problem, but enriching it will not be the solution if clearance is too tight to begin with.

 

[whenfoxforks-ruled]

How about the very simple basics first, like the answer to my question? To reiterate: what was the clearance between cyl. sleeve and piston that your pro honing shop set? What authoritative source did that clearance figure come from, and did it account for the specific materials such as forged piston in a cast iron sleeve?

Lean jetting could certainly be contributing to the problem, but enriching it will not be the solution if clearance is too tight to begin with.

When they changed the cylinder material I was hoping they were slick enough to select the proper piston,oem will not work anymore!Blackduc,any shop that does their own machining I would double check for sure also!For under a grand anyone could get the chinese equipment to perform the work,probably the work will match the equipment!

 

[ridejunky]

Well, whatever the cause, the one thing this reinforces for me is to stay in the dirt. Too many friends and acquaintances of mine have had life altering or ending
experiences on the road. I hope you all stay safe and major injury free.

 

[Gumby190]

Thanks for all the input, I have been avoiding my bike for the last week since meeting the road, she is still in bits in the garage.
I was running a BR9 plug, the crown is undamaged just the usual 2 stroke gunge.
I am cracking the case this weekend, & will have her up & running by Sunday arvo.

And yes, my ass still hurts but is looking less & less like a Hamburger Patty everyday.

 

[blackduc98]

I've been thinking about your situation some more, particularly about the fact that you were riding on a motorway. Were you going at a steady cruise at the time of seizure? Can you try to remember what speed and thrhrottle position this was at? Were you by any chance engine braking, i.e. trying to slow down by closing the throttle?

Here is why I'm asking all these questions: 2-strokes which use premix for lubrication really don't like to engine brake because when you close the throttle, you drastically reduce amount of mixture delivered to the cylinder. The mixture carries lubricating oil, as well as cools the piston. Engine braking forces the engine to spin at high rpm. If both cooling and lubrication are drastically reduced while revving high, seizure is likely to occur. Apparently this was a common problem with old Bultacos.