piston damage question

[Belgian1979]

This is about a case I'm working on right now.

Below is a picture of a hemi dragrace engine piston. Can someone tell me what the scratced in the top land mean ? What could have caused this ?

[Cubic_Cleveland]

Possibly contamination, but most probably detonation + got hot and grabbed. Is that the intake valve side?

Any more pictures? Any other pistons have any damage?

[FuelieNova]

I was curious about the other piston in the picture. It looks like theres a chunk out of it.
Tom

[Belgian1979]

Yes the other piston has a chunk out of the skirt. The engine fully disintegrated at the end of a dragrun. Alu rod broke and windowed the block.

This is a methanol engine. I'm wondering about wether or not it still needs cooling. It was setup as a top fuel car running on nitro (solid block, solid heads and such). Case was dismissed previously, but it turned up in court and we are discussing the cause. I concluded before that one of the contributing factors was detonation.

Engine was already disassemble and this was some time ago. Do not know anymore wether this was exhaust or intake side.

[Cubic_Cleveland]

If it was running too hot I would think more of the piston would have grabbed, and more than 1 piston. Is this the only piston damaged apart from the one where the rod let go?

[Belgian1979]

This is one of the others. They all show some signs in the top ring land.

As said, engine was all apart when I saw it, which makes root cause analysis a problem.

[twl]

I see this in old vintage hemi motorcycle piston failures.
It seems to be from piston rock.
We normally solve it with reduction in the diameter at the top ring land.

The top land gets a lot of heat and expands, and added to the rock, it gets scored. Sometimes it causes the top ring to get pinched.

[Dave Koehler]

X2 Just piston rock. IF it had not had an unscheduled dis-assembly you would still find that.

[robert1]

Piston rock would just be polished wear above the ring. That is detonation from too lean, too much timing, not enough fuel flow, too much compression. This goes back I believe to your first visits here talking about aluminum wrist pins. By the time that you get to this level of HP the operator should know very well what they are doing. Just having enough money to buy this much HP is not very helpful. That would be the intake side and that is always the side that shows the first signs of detonation. Too bad something like this ends up in court as the builder has zero control over what the user might do to the engine. Experience is very expensive. I had a guy many years ago the burned the number 3 piston in a SBC alcohol motor 3 weeks in a row. I kept telling them it was too lean. After sending the carb back several times they finally got it right. I had to stay late each week to make sure they were able to race each week.

[Dave Koehler]

I should clarify piston rock. It happens. However the one shown is the "good" hole so take it with a grain of salt. The real issue is heat. The upper land area is smaller than the skirt for that reason and in some engines way smaller to combat the rock issue along with heat. Fatten her up a hair next time out?

[Belgian1979]

Piston rock would just be polished wear above the ring. That is detonation from too lean, too much timing, not enough fuel flow, too much compression. This goes back I believe to your first visits here talking about aluminum wrist pins. By the time that you get to this level of HP the operator should know very well what they are doing. Just having enough money to buy this much HP is not very helpful. That would be the intake side and that is always the side that shows the first signs of detonation. Too bad something like this ends up in court as the builder has zero control over what the user might do to the engine. Experience is very expensive. I had a guy many years ago the burned the number 3 piston in a SBC alcohol motor 3 weeks in a row. I kept telling them it was too lean. After sending the carb back several times they finally got it right. I had to stay late each week to make sure they were able to race each week.

Your correct. This is the engine in one of my first threads which in the mean time turned up in a litigation.

I second your opinion. It's a bullshit case that should not end up in court. If you race, you will blow up an engine sooner or later, especially in a dragrace engine of this type.

I have seen other detonation issues as well among which a crack in the ring land, the hammered piston pin bore.

It also had oiling issues.

As stated this is an engine with an experimentational wrist pin (steel outer shell with an alum inner).

What still bothers me are the buttons on the skirts. I can only imagine the amount of clearance it needs to run to get them in. It will provide the opportunity for the piston to cock in the bore imo.

There is something else as well : when I saw the engine they presented me with a virgin piston and some pins. As I know now by experience, if well oiled up and with suitable clearances, the pin should slide through the pin bore without effort. The pins in question did not, meaning they were too tight as well.

It desintegrated at the end of the strip, meaning it stayed together pretty well, all else taking into consideration.

I think this team went up into a class with to little experience.

[Belgian1979]

I should clarify piston rock. It happens. However the one shown is the "good" hole so take it with a grain of salt. The real issue is heat. The upper land area is smaller than the skirt for that reason and in some engines way smaller to combat the rock issue along with heat. Fatten her up a hair next time out?

Dave this is not my engine.

It's a case I have to handle. A case that should not have made it into court.

[robert1]

I don't see any signs of aluminum wrist pins. The buttons have been used forever in all forms of racng, Top Fuel, Funny Car, boats, any high horse power application.

[keithdyer]

Looks like the guy who bumped the mag or put the main jet in should be the one on trial . . . . .

[Strange Magic]

Going to venture to say that this issue might have been caused by wrist pins sticking from either inproper clearance or expansion of that wrist pin for some reason. I would venture to say that it stuck in a certain position and tore at the top of the piston. The rod that exploded might have galled the pin and stuck it so bad that it locked/siezed it onto the small end shattering the rod.

As stated this is an engine with an experimentational wrist pin (steel outer shell with an alum inner)

This would seriously concern me. Why would someone do such a thing?