LM7 catastrophic failure... - Chevelle Tech
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post #1 of 31 (permalink) Old Jul 17th, 19, 8:36 PM Thread Starter
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LM7 catastrophic failure...

Wonder what would cause total piston failure ?

Camaro sitting in the shop idling...oil pressure 55 lbs...no funky noises...coolant temp 185. Running perfect.
Been running 20 minutes...I'm going thru FiTech programmer settings. Have not changed anything.

All of a sudden, one back fire out the intake. Followed by one loud metal to metal noise then it just shuts down.

Will not restart.

Started digging around. Pulled a few plugs all look great EXCEPT #7 which is smashed closed.

Pulled both valve covers...everything looks good. Tried turning the crank by hand to watch rockers move...will only go so far, stopping at the same point.

<Insert favorite cuss words here>

Put the car up on the lift and dropped the oil pan. CHUNKS of piston in the windage tray & pan.

#7 piston broken at the wrist pin centerline.......

Damn 5.3L engine only has 104K miles. I pulled it, running, from a wrecked '02 Silverado.
Basically stock with only mild cam swap and fresh valve job.

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post #2 of 31 (permalink) Old Jul 17th, 19, 8:51 PM
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Re: LM7 catastrophic failure...

Sorry to hear. Maybe an injector stuck open and hydro locked the motor. Check for a bent rod as a tell tale

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post #3 of 31 (permalink) Old Jul 17th, 19, 9:45 PM
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Re: LM7 catastrophic failure...

Gee, I hope that does not happen to my LM7. Mine is over 150,000. Now you have me worried.

Rick

1970 El Camino under Reconstruction - 5.3L LS VORTEC Engine and 4L60E Transmission
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post #4 of 31 (permalink) Old Jul 18th, 19, 9:38 AM Thread Starter
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Re: LM7 catastrophic failure...

Found another long block locally...pick it up tomorrow.
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post #5 of 31 (permalink) Old Jul 18th, 19, 10:02 AM Thread Starter
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Re: LM7 catastrophic failure...

Quote:
Originally Posted by 69-CHVL View Post
Sorry to hear. Maybe an injector stuck open and hydro locked the motor. Check for a bent rod as a tell tale
Doubt it.
#7 plug wasn't wet when removed, no raw gas in that cylinder, no gas smell when draining the oil.
Reading more online last night...these stock pistons aren't the greatest.
Failures like mine are not uncommon.

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post #6 of 31 (permalink) Old Jul 18th, 19, 10:18 AM
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Re: LM7 catastrophic failure...

Well that really sucks.

Jeff Evans
65 Chevelle, LS3/T56 transplant in progress
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post #7 of 31 (permalink) Old Jul 18th, 19, 1:47 PM
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Re: LM7 catastrophic failure...

Iíve been lucky so far. My LM7/truck has 240,000+ on it.

Jason Gore
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post #8 of 31 (permalink) Old Jul 18th, 19, 6:00 PM Thread Starter
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Re: LM7 catastrophic failure...

Engine is out and totally stripped down to the short block.

Piston #7 is simply gone...bits of it's dished top face smacked the head. Cracked the cylinder wall. Wrist pin still in place.

I forgot to take a picture...

Gonna swap all the goodies over to the "new" engine after pulling the heads for a valve job and spring swap.

Old motor had -706 heads...new one comes with -862. Same difference.

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post #9 of 31 (permalink) Old Jul 18th, 19, 7:43 PM
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Re: LM7 catastrophic failure...

I'll say at some point in your engine's life, it was beat on, over heated, or somehow stressed and this one piston decided to let go.

While the factory cast cranks in these engines are pretty good, up to about 500hp, I don't trust the stock powdered metal rods or the cast pistons. Hypereutectic cast pistons are still cast pistons, they just have a higher silicone content that old school pistons.

Unless you're building a higher power engine here, the stock crank is fine but I'd highly recommend a forged piston and rod set. A set of forged pistons start at less than $500.00 and rods start at $530.00. One of those ball honers on a drill and you'll be better off.

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post #10 of 31 (permalink) Old Jul 18th, 19, 7:51 PM
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Re: LM7 catastrophic failure...

Those cast pistons and powdered rods handled 15psi of boost for 1000's of miles in my Mustang before I swapped it out for a 6.0

Nothing wrong with cast pistons if the tune is right.
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post #11 of 31 (permalink) Old Jul 18th, 19, 7:59 PM Thread Starter
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Re: LM7 catastrophic failure...

Just chalking this one up to shyte happens and moving on...
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post #12 of 31 (permalink) Old Jul 18th, 19, 7:59 PM
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Re: LM7 catastrophic failure...

Quote:
Originally Posted by 69-CHVL View Post
Those cast pistons and powdered rods handled 15psi of boost for 1000's of miles in my Mustang before I swapped it out for a 6.0 Nothing wrong with cast pistons if the tune is right.

I'd say consider yourself lucky. I've seen seen what happened to this engine a few times over on LS1tech.


As this was a running engine, and just idling, it's unlikely this was tune related.

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post #13 of 31 (permalink) Old Jul 18th, 19, 8:14 PM
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Re: LM7 catastrophic failure...

Quote:
Originally Posted by BillsCamino View Post
Just chalking this one up to shyte happens and moving on...

EXACTLY why I hate to fly...anything mechanical is subject to failure at any time.


Good luck with the rebuild, I'm sure you'll use superior parts this time and it'll handle anything you care to throw at it.
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post #14 of 31 (permalink) Old Jul 18th, 19, 8:46 PM Thread Starter
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Re: LM7 catastrophic failure...

FYI...this is a very mild build.
BTR stage II truck cam kit, 212/218 @.050" ,shorty stainless headers, FiTech 70002 kit, stock L60E, Tanks Inc fuel system.
Replaced a mild sbc 350 & 700r
Cruise night/car show car...will never be raced.

Engine HAD about 2 hrs running...roughly 60 miles...in this Camaro.

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post #15 of 31 (permalink) Old Jul 18th, 19, 8:52 PM Thread Starter
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Re: LM7 catastrophic failure...

"$uperior" parts
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