bad finding

[gohan]

There was fouled spark plug and blue smoke on load. After I took out the heads, I found the condition of the cylinder and the piston of #6 as the following pics.

What would cause this?

What need to be done, other than replacing the cylinder and the piston?







Regards,

[TimT]

That looks like damage from broken rings..The bottom pic shows that you have taken the cylinder all the way off. Are one or any of the rings broken? If not a broken ring, perhaps you got something lodged between the piston and the cylinder walls. the wear appears to be concentrated in one locale.

[gohan]

I did not find any of broken rings. The damage is concentrated in the location pictured. But what could be sit between ?

[ruf-porsche]

By foul plug, did you mean that they were foul from oil or too much gas? If it was to much gas the plug would be black and sooty. The additional gas that cause the plug to foul could have also wash the oil that lubicated the cylinder off causing the cylinder wall to be scoured. Check the injector for either too much flow or leaking.

[TimT]

This type of failure is purely because something is caught between the piston and cylinder. The fouled plug is a symptom of the lost compression, not the cause of it..

Perhaps a ring land broke because of detonation. A ring may have broken because of ring land wear...Something was wearing between the pistoms and cylinder.

[Early_S_Man]

In the motorcycle/kart racing world that would be called a 'stuck' piston ...

Galling of that type is almost ALWAYS caused by too lean mixture or overheating of the cylinder & piston by restricted air flow! In simple terms ... piston is overheated and expands beyond ID of cylinder.

[Doug E]

Warren - would you be able to notice a change in the engine sound when something like that was happening?

[Early_S_Man]

Doug,

Yes ... sometimes the engine will sieze up and lock the rear wheel ... other times there will just be a violent 'shudder' and momentary loss of power. When it happened to me around 1970 on a single-cylinder bike ... it just shuddered, and I killed it instantly to find out what happened! After cooling down, it had noticeably lower compression, but it did start and ran with low power and much smoke!

[Doug E]

Thanks for the explanation Warren.

[RoninLB]

Hey Gohan.. I don't see what model this is under your signiture

[Wayne 962]

Shoot, Warren beat me to the punch. Yes, (I think that this is explained in the new book too) when the engine overheats, the piston skirts begin to scrape against the walls of the cylinders = very bad. As they scrape against the walls, they create more heat and expand more (like a chain reaction) making the whole situation worse. Leaves stuck in the engine cooling tin may cause inadequate cooling that would result in this. Also, placing the cylinders upside down, or not installing the engine cooling sheet metal as well.

If you shut the engine down (like Warren suggested), then you can possibly stop the chain reaction, but you have to move very quickly!

-Wayne

[gohan]

That is 2.7L on 7R case. Had an overheated run on a track day last year. Had the similar symptom said by Warren and Wayne. That's part of reason, I bought Wayne's book and parted the engine to check.

[JohnC]

are those head studs corroded?

[gohan]

no, they are not.

[john walker's workshop]

of course, there's particle matter imbedded in the bearings from that, so do a full teardown.