What caused my melted piston?

**Tomas Wallin** · 10-26-2009 05:44 PM

As some of you know I melted a piston in the beginning of summer, I never found any answer to why but today something struck me...

Would a loose spark plug might been the cause? Or was it an effect of what happened. Both from lack of heat transfer and also maybe from causing a lean mixture due to the bad seal.

When I removed the spark plugs after the incident I found that #6 was loose but I always thought that was caused by the detonation or the heat, but is that really that common?

One thing that always bugged me was that all the other cylinders looked so good and this melted, why would #6 get so lean and no other???

I have plenty more pictures of the pistons, plugs, valves etc...

It's a carbed (2x Holley 800cfm) 509ci with Superchiller and 8-71 with 9.5PSI boost@6000rpm. Iron heads and 8.3:1 in compression, very little total run time on motor since rebuilt and 10minutes run time since dyno (warmup)... This happened at wot.

**Chris** · 10-26-2009 07:09 PM

What engine?

Detonation will definitely loosen a plug. Loose plugs don't cause ultra-lean conditions- and that's what melts plugs and pistons.

**DAREDEVIL** · 10-26-2009 07:37 PM

I would say ,,that looks like a timing problem,,,,since if it would be a lean problem ,,it would show on more then on 2 !!!!!!!!

TRUSST ME,,,I KNOW,,BUT PLEASE DON'T ASK WHY ....

**Ratickle** · 10-26-2009 07:39 PM

Is it fuel injected? Or carb?

**mj2562** · 10-26-2009 08:16 PM

Mine went lean and got 2 # 2 & 8. How much run time on motor?

**DAREDEVIL** · 10-26-2009 08:20 PM

This looks like a little 4 and #6 thats why i think its timing !!

If they run lean on a carb its usually the outside ones that go first !!!!

**Ted** · 10-26-2009 10:51 PM

Recently a buddy of mine took out a cylinder on his boat with very similar circumstances. Loose plug for a bit (see exhaust on body) and lean in one cylinder. As best we could figure the plug allowed too much air past and/or detonated from being loose (couldn't shuck heat off because it didn't have good head contact). It was a very similar failure and he only did one run with the loose plug.

**Ted** · 10-26-2009 10:52 PM

BTW, this was a carb HP500, if your engine is injected look at the injector for poor flow, possibly while pulsing (lost one myself that way).

**MOBILEMERCMAN** · 10-26-2009 10:54 PM

I doubt a loose plug caused the damage..One could argue the loose plug wouldn't fire as well and would miss.

**DAREDEVIL** · 10-26-2009 11:45 PM

Originally Posted by MOBILEMERCMAN

I doubt a loose plug caused the damage..One could argue the loose plug wouldn't fire as well and would miss.

AIR WILL NOT DO THIS AND A NON FIRING PLUG WILL NOT BURN ( LEAN OUT ) ANYTHING !!!!!!!!

No fuel ,,or way to much timing will !!!!!

**WeaponX** · 10-26-2009 11:46 PM

It's 5 and 8 that are the worst on big blower motors on detonation.

I've seen some just like that and alot worse.

**WETTE VETTE** · 10-26-2009 11:57 PM

I think a loose plug could cause this. Spark plugs transfer heat into the head at a particular rate, which is why there are different heat ranges. Hot plugs transfer heat more slowly than cold plugs. If the plug was not installed tight enough it could overheat because it doesn't trnasfer heat into the head the way it should. If it overheats it could glow causing pre-ignition which will eat a piston or worse. My opinion.

**Ted** · 10-27-2009 12:05 AM

The engine will not run more than a few seconds with pre-ignition. The only way to control pre-ignition is just keep any pre-ignition sources at bay. Spark plugs should be carefully matched to the recommended heat range. Racers use cold spark plugs and relatively rich mixtures. Spark plug heat range is also affected by coolant temperatures. A marginal heat range plug can induce pre-ignition because of an overheated head (high coolant temperature or inadequate flow). Also, a loose plug can't reject sufficient heat through its seat. A marginal heat range plug running lean (suddenly?) can cause pre-ignition.

http://www.enginelogics.com/detonation.html

**MOBILEMERCMAN** · 10-27-2009 12:09 AM

I can follow the loose plug overheats theory. I have trouble believing loose lets in air though.

If the plug was glowing wouldn't it have blued?

Thanks Ted, Great read.

**Ted** · 10-27-2009 12:13 AM

Looks kinda dark up near the insulator, it is a silver plug normally.

**SFOcean** · 10-27-2009 01:24 AM

I had the same problem last year on #7 BBC normally asperated iron head. Loose plug (yes, I know it was installed loose) caused extended pre-ignition. My plug looked similar to yours, no ground strap. Forged piston was good, just tuliped both valves and lost compression before piston failed.

**Tomas Wallin** · 10-27-2009 03:12 AM

Sorry guys,

It's a 509ci with Superchiller and 8-71 with 9.5PSI boost@6000rpm. Iron heads and 8.3:1 in compression, very little total run time on motor since rebuilt and 10minutes run time since dyno (warmup)... This happened at wot.

My thought was primarily the heat transfer subject and secondary the lean condition from sucking air through the threads.

If the plug was loose I guess it would get very hot with a bad heat transfer at wot, I run AC Delco MR41T so they're pretty cold.

**MikeyFIN** · 10-27-2009 06:49 AM

I´d Tomas go with the loose plug theory going too hot but also I´m interested about your head and intake gaskets sealing properly ?

And let´s just not forget a piston ring land failure.

I mean it detonated and only oil and too lean a combustion in a single cylinder would do this.

**Ratickle** · 10-27-2009 07:00 AM

Is it carb or injection?

How long did it take for that to happen total do you think? Sure made a mess......

**MikeyFIN** · 10-27-2009 07:00 AM

Carb

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