99 YZ400 piston, what caused this?

[tntmo]

Here's the background on this bike, it's a friends machine and he got it from another friend of mine so I know it's history for at least the last 4 years. It was ridden mostly in the desert, a few times at the track and a few times in single track/technical stuff. It's always been a good running machine, one or two kicks usually got it going. The current owner let it sit for half a year while on a deployment and it wasn't running well. We cleaned the carb with minimal change, and after a second more thorough cleaning we got it running great again.

He brought it out to the desert two weeks ago and joined my family on a 40 mile loop. We weren't riding fast, lots of stopping due to wife and kids. The bike was starting and running great. As we got close to camp, he and I took off across a dry lake bed since we could open it up a bit. His bike has a bit of an edge on my WR400 so he got a good jump and then I noticed that he slowed down and I blew by him. He said the bike just felt like it ran out of gas, luckily rolled right up to camp.

We both looked it over and it had very little compression. When it cooled down, we pulled the valve cover and there were no tight valves. I knew there were problems with the piston or rings so we packed it up and brought it home. Later in the week when we took it apart this is what we found.

The valves look okay, some small sprinkles of aluminum on them but otherwise decent. What would cause the piston to do this? Was it just it's time to go? It may be the original, I'm not certain. Anyway, we just wanted to know if there was something to look into that may have caused it prior to rebuilding and having the same thing happen again.

[KJ790]

Most likely just due to the fact that it has been ridden for 9 years on the same piston.

[grayracer513]

That, my friend, is the result of detonation, which either caused, or was caused by, overheating the piston. When the edge of the crown melted, the aluminum went down the back side of the piston, ruining the rings and bore. It's kind of a classic example, actually.

This was either the result of lean jetting, inferior fuel, overheating, or a carbon hot spot heating up to the point that it caused detonation.

[Ga450owner]

Grey could this detonation also be a result of a original piston/rings on a bike the is 9-10yrs old with god only knows how many hrs

Whatever you do when rebuilding it....after new piston/rings and possibly cylinder buy a new camchain and CCT....

How do the valveseats look?

[tntmo]

Grey could this detonation also be a result of a original piston/rings on a bike the is 9-10yrs old with god only knows how many hrs

Whatever you do when rebuilding it....after new piston/rings and possibly cylinder buy a new camchain and CCT....

How do the valveseats look?

The valve seats look pretty good, I honestly don't think there are any problems with the head but will probably have it professionally inspected. The cam chain and tensioner are in surprisingly good condition also, but we will replace them anyway.

The engine was a little low on coolant, maybe about 12oz but it wasn't spitting steam or excessively hot when it quit. I don't know if that would have any effect on it. There were no changes in the jetting, same settings that were run for the last 4 years. Maybe an air leak somewhere? I didn't notice anything during the tear down.

Anyone want to chime in on the reliability of the 420cc Lukes racing big bore kit?

[grayracer513]

...could this detonation also be a result of a original piston/rings on a bike the is 9-10yrs old with god only knows how many hrs ?

No, other than that there may have been older, harder crbon deposited there. Age wouldn't really contribute to it.

[Britincali]

It ran lean got ultra hot and melted the piston, simple as.

[todds924]

That, my friend, is the result of detonation, which either caused, or was caused by, overheating the piston. When the edge of the crown melted, the aluminum went down the back side of the piston, ruining the rings and bore. It's kind of a classic example, actually.

This was either the result of lean jetting, inferior fuel, overheating, or a carbon hot spot heating up to the point that it caused detonation.

+2, lean lean lean and WOT running. I can't tell? is that the intake or Exhaust side that melted? OOps, never mind, i saw a better picture. Intake side! HMMMMMMMM?

[Markopolo400]

Most likely just due to the fact that it has been ridden for 9 years on the same piston.

My 9 year old piston didn't look like that...

[07thommi]

When you cleaned up the carb did you take it out of the bike?

If so you might not have put it back on perfectly right. I did this on my 250F, I put the carb back on but didnt have the clamps alligned right so I had an intake leak, which caused the bike to run really lean. Luckily I knew that it wasnt running right and caught the problem quickly so it was an easy fix!

Just a thought

-mt-

[tntmo]

+2, lean lean lean and WOT running. I can't tell? is that the intake or Exhaust side that melted? OOps, never mind, i saw a better picture. Intake side! HMMMMMMMM?

Yeah, intake side by the center valve. It sure ran really good for being lean enough to fry a piston like that, but everything seems to point that way.

[grayracer513]

It's not conclusive that lean mixture is what caused it, IMO. It is certain that it was an out of control heat spiral, where something at the spot on the piston was adding heat faster than it could be got rid of, but lean mixture by itself will usually burn pistons in the center of the normal flame area, which is to say, near the plug. Detonation typically works at the edges of the combustion chamber, when the rising pressure from the flame front, possibly in combination with a carbon deposit that glows when heated, spontaneously ignites an unburned fuel mass before the flame ignites it in a normal manner. That creates a localized uncontrolled combustion condition, and is exactly the sort of thing that causes stuff like this.

[Britincali]

Most of the time detantion is caused by being lean.

[Birdy426]

Anyone want to chime in on the reliability of the 420cc Lukes racing big bore kit?

On the 400s, the small end of the rod was marginal if worked hard. 426 rod cures the problem. If you are going to go big, go for the 426 rod as well. The big bore kit itself is reliable as a rock, but does stress the rest of the reciprocating assembly. The rod fix is about all you need...

[grayracer513]

Most of the time detantion is caused by being lean.

I doubt that is a supportable contention.

[Britincali]

I doubt that is a supportable contention.

Notice I said MOST....

Most of the time if the bike is on the verge of being lean its fine, but add in high rev - high load of a dry lake bed and i'll bet money it went like this.

Bike is lean and combustion temps get silly hot, rider keeps it pinned

Bike is now running massively hot and starts puking coolant

Pinging rears its ugly head due to something melting or glowing

Rider keeps it pinned while wondering what that weird rattling noise is

Piston starts to melt and smears ally down the bore

Rider keeps it pinned

Ally builds up and creates even more heat leading to complete piston annihalation.

Rider keeps it pinned wondering why the power is gone.

MOST of the time lean causes the root issue.

[grayracer513]

Notice I said MOST....

I saw that, and I still say there is no support for that position in any data record.

The list of things that can and do cause detonation is much longer than that, and frankly, lean mixture becomes an even less likely candidate if you limit the discussion to 4-strokes. The common causes are:

> Poor fuel, either of an initially inadequate octane number, or one that has aged to a point where the octane number has degraded.

> Pre-ignition. This has a number of causes in itself. It can be from incorrect ignition timing, but this is very unlikely with modern ignitions. More often, it is from heat, carbon deposits, incorrect spark plugs, or bad gas.

> Uncontrolled heat, and any of the dozens of its causes, including poor radiator performance, poor circulation, low coolant levels, poor coolant performance, leaking head gaskets, and yes, lean jetting.

> Incorrect spark plug(s). Too high a heat range can lead to the plug becoming a hot point in the chamber and causing pre-ignition.

> Carbon deposits that glow under the pressure of compression or combustion and become a source of pre-ignition or secondary ignition.

> Oil consumption. Oil droplets in the combustion chamber can ignite spontaneously under pressure and set off pre-ignition.

It's important to understand that these causes are often, if not usually, tied together in some combination in which they become a series of mutual causes, eventually ending up the same way. It's also well to understand that pre-ignition and detonation are two entirely different things, even though pre-ignition almost always results in some detonation.

Lean jetting is, IMO, probably one of the least likely causes of this kind of failure, with fuel, carbon and heat problems being somewhat more common. This is especially true given the tendency of most tinkerers to end up with excessively rich jetting in the belief that it makes the bike run better. But, it may have even caused this particular problem, it just doesn't look like it to me.

[tntmo]

Poor fuel is a good possibility. The bike sat a lot, I was always trying to convince him to ride it and when he finally did this was the result. Good information guys, I'm a fairly competent mechanic but I like learning things like this.