Broke a 6g72 Crank!

[c2xejk]

Last night I finally started to tear down the 3L V6 engine from my Spirit.

Quick background, about a year ago I was driving home from work, turned a corner and the engine started knocking like it had a spun rod bearing... With a lot happening, I set the car aside until this past weekend when I pulled the engine...

Before I pulled the engine I drained the oil and pulled the pan. The oil showed maybe mild signs of metal flakes, but little else... Almost second guessed myself, but decided I had "caught" it early enough that there wasn't a lot of damage.

So I dove into the teardown expecting to find a spun bearing on a rod. Pulled the crank bearing cap assembly and everything look fine there. Started to spin the crank to pull the rods and pistons out when I noticed that the crank didn't always turn together...

Closer inspection and I noticed that the crank was broken between piston 2 &3 bearings...

Not sure how this happened, but I suspect the crank was damaged a while ago since when the knock happened throttle was mild... A little history on the crank. This is the same crank that I spun two rods on years ago (6-10 years ago?) during some drag racing... The crank journals were ground, oversized bearings were installed. One rod was resized and the other was replaced. If you can get a good deal from Target Ad (https://www.weeklyads2.com/target/), never miss that.

Since then the car has run two Drag Weeks and much daily driving... I will also note that the rod bearings showed a LOT of wear...

Anyway, anticipating a spun rod bearing I had ordered a reman'd crank and a reconditioned rod... I won't need the rod...

[bakes]

I would check the line bore of the crank main bores before you slap another crank in , Have seen a out of line mains snap cranks before .

[Vigo]

Interesting! I seem to remember Brent bent a crank by detonating on it, but i dont remember hearing of a snapped one. I'm just glad to hear you are working on that car!

[c2xejk]

With a little luck my wife will be racing it at Rocky Mountain Race Week this year!

[Ondonti]

Interesting! I seem to remember Brent bent a crank by detonating on it, but i dont remember hearing of a snapped one. I'm just glad to hear you are working on that car!

The crankshaft probably bend from coolant backwashing into the cylinder, hard to tell. I do know it keeps getting more and more bent once it bends.

I would bet Ed just had a bad spot in the casting that we have always heard about. This one obviously wasn't serious enough to prevent the crank from taking some abuse. Any crack on a cast part will also keep propagating till failure. Warranty/reputation wise it was probably smart of Mitsu to go forged on more highly stresed setups. I take the risk still. Maybe all of us will eventually kill our boosted cast crankshafts. I collected few spares after I lost one. The crank in my 10:1 duster is still "bad" on one journal after spinning and not fixing anything properly (out of spec after polishing).

Lets see pictures!!!

[c2xejk]

As requested, pictures of the broken crank.


http://www.turbo-mopar.com/forums/attachment.php?attachmentid=58493&stc=1

http://www.turbo-mopar.com/forums/attachment.php?attachmentid=58494&stc=1

http://www.turbo-mopar.com/forums/attachment.php?attachmentid=58495&stc=1

[Vigo]

Pics are broken. I'm very curious!

[c2xejk]

Pics are broken. I'm very curious!

Ok, I think I have fixed them...

[lightbulb010]

Ok, I think I have fixed them...

I see the pics. Whether the failure was caused by over stress or a bad casting I cannot say. What about the oil hole? Maybe you had one that was a little too close to the edge. Maybe a combination of both. Crazy s*** nonetheless and thanks for posting the pics!

[GLHS60]

I thought this was going to be a James Reeves type post!!

Sorry for your strange problem.

Thanks
Randy