NSX Crank Failure - stock bottom end

-Hand-built balanced motor straight from Japan
-Stock bottom end
-Low boost
-Crank broke in half!


When I purchased my '92 NSX, I had the motor replaced with a special motor that was used from Japan.

This motor was a rare and special hand-built, balanced by Honda straight from the factory. The motor ran good and strong until about a few weeks ago, with about 60,000 miles on it total after I put about 8,000 miles on it. The engine immediately changed pitch and started to have a light resonant vibration to it, it sounded really bad so I pulled over and shut off the motor.

Upon inspection the alternator belt jumped 1 tooth which didn't make much sense because I just replaced that belt. When I turned the car back on, I could see the crank moving back and fourth 1cm when engaging and disengaging the clutch. I thought it was the thrust bearing so I decided to pull the motor apart.

Check out the engravings Honda made on the rod signifying the weight of the rod for balancing purposes, they had different numbers on the rods:



And the factory-drilled counter-weights to balance the rotating assembly:






Searching for the problem:


When I opened up the motor, the crank appeared to be fine:


The main and rod bearings look great as well:



This confused me and I wasn't sure what was wrong. As I started to pull the crank out, I noticed a crack that didn't look good:


As I tried to lift the crank out it moved further and I knew it really wasn't good, but the main bearing looks great:


Wow, the crank cracked in half! I don't know how it held together!




Other side:



My broken crank:






So what do you guys think?

Holy Sh*t Thats crazy!!!

i think your lucky it didnt take everything with it!!!!!!!

Yeah, bomb defused! Could have been worse. Sorry man.

Very interesting. I have two initial guess for the the cause of your problem.

1. A flawed crankshaft - simple but it can happen.
2. A harmonic that the engine is seeing.

For those that dont know, a harmonic is a frequency that a specific part or the engine as a whole can see that can cause very bad things. When I was rebuilding BMW race engines, we were encountering a harmonic that was shearing our oil pump shafts, welding vibration dampers to the crankshaft, loosening bolts, and causing our belts to jump ribs.
What RPM were you revving to before the engine failed?

Very weird about the numbering on the rods though. The rods that I have seen have the same marking on the cap and rod on one side, and a single marking that is on both the cap and rod on the other side.

Good luck with your diagnosis.

If it were major harmonic problems, the bearings wouldn't look that good.... well perfect.

What blower were you running on the car at low boost?

hi Billy --

Sorry to see that.

We saw something similar (see attached) in an engine brought to us for repair. The engine was a 3.0L making 380 wheel supercharged. The engine was built by another shop. The bearing wear looked normal. The engine was not rebalanced after the rotating weight was changed (forged pistons) and I have wondered if this exploited an existing flaw in the crankshaft.

The factory forged cranks are tough. Dozens in the 600+ crank power range with out problems.

-- Chris

stuntman said:

If it were major harmonic problems, the bearings wouldn't look that good.... well perfect.

Click to expand...

ive seen two engines fail from a harmonic, both had no abnormal bearing wear.

what crankshaft pulley were u using?

Pretty amazing to see on such a low power motor. We ended up cracking our crankshaft after about two years. 20,000+ miles daily driven at 550-620rwhp, well over a 100 dyno pulls ranging from 500rwhp-820rwhp, a dozen road course events, and countless ¼ mile trips. A 3034 chromoly crankshaft has seemed to solve that problem for now…

I too am curious what crank pulley you were using , a lightweight aluminum one?

since yours(billy) and his(sos) cranks both cracked, billys balanced wrong and the one taken to sos wasnt balanced, one might draw the conclusion that the nsx crank is very sensitive about balance and precise balancing of the rotating assembly is a must

This is a drag, but I agree you are lucky it is only a bottom end, could have been much worse!

Looks like it must have been a small forging flaw that eventually let loose under your heavy foot. It is interesting to see the different grain pattern across the failure, if you get out a magnifying glass, I would expect you can find an edge which looks polished, indicating where the cracking started. Did you have a stock damper on the motor? Just wondering, the bear surfaces do look great.

Those rod markings I have seen on all the 3.0L motors I have had apart, pretty sure all early NSX motors were hand assembled and balanced at the factory.

fullracehonda said:

since yours(billy) and his(sos) cranks both cracked, billys balanced wrong and the one taken to sos wasnt balanced, one might draw the conclusion that the nsx crank is very sensitive about balance and precise balancing of the rotating assembly is a must

Click to expand...

Balanced wrong from Honda? That's comforting...

stuntman said:

Balanced wrong from Honda? That's comforting...

Click to expand...

I missed where it was stated that the rotating assembly was balanced wrong.

-- Chris

Chris@SoS said:

I missed where it was stated that the rotating assembly was balanced wrong.

-- Chris

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Post #13 --->

fullracehonda said:

since yours(billy) and his(sos) cranks both cracked, billys balanced wrong and the one taken to sos wasnt balanced, one might draw the conclusion that the nsx crank is very sensitive about balance and precise balancing of the rotating assembly is a must

Click to expand...

Sorry for your loss but; you were very lucky not to have had a catastrophic failure where most of the engine would have been turned to scrape in a few RPM.

If you would care for my “WAG” based on the photos you have supplied, I am leaning toward the flawed crank theory.

I feel this way because the fracture appears to have taken place in an odd location. Additionally; from one of your photos it appears that there are 2 fracture lines that intersect each other at a steep angle. One of these cracks appears "older" than the other, it dose not exhibit the highly reflective surface of the "new" fracture. The newer fracture shows the typical "crystalline" grain structure of a fatigue/stress failure. The “older” and darker crack appears to exhibit grain separation or what is some times referred to as an “inclusion”, these can occur during the forging operation.

Given the crank may have had a fault and the rotating assembly may have been slightly out of balance the 2 conditions would exacerbate each other leading to the eventual failure.

My apologies for such a wild supposition just based on a photo; it is offered as “food for thought” and nothing more. Best of luck with the repair and may you be back rolling again soon.

where would someone purchase a billet crank for the nsx?

raffi.awakim said:

where would someone purchase a billet crank for the nsx?

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Toda Racing - way expensive ($9k)
Comptech - Science of Speed
BC (Brian Crower) - Factor X (in stock, PM for details).
BC - Shad/Driving Ambition (i believe)

stuntman said:

Toda Racing - way expensive ($9k)
Comptech - Science of Speed
BC (Brian Crower) - Factor X (in stock, PM for details).
BC - Shad/Driving Ambition (i believe)

Click to expand...

Billy,
Shad and CT use "Bryant" SP with a larger Chevy bearing and more off the shelf sizes. SOS is the same manufacturer but standard Acura bearing and to my knowledge both are stroked cranks.

The only OEM replacement is Toda or special order BC.

JeffShoots said:

Shad and CT use "Bryant" SP with a larger Chevy bearing and more off the shelf sizes. SOS is the same manufacturer but standard Acura bearing and to my knowledge both are stroked cranks.

Click to expand...

For what it's worth, here are the strokes of some crankshaft options:

OEM: 78 mm
Toda: 84 mm
Comptech: 86 mm
Science of Speed: 88 mm

stuntman - what did you end up doing here?

Thanks.

slownsxt said:

stuntman - what did you end up doing here?

Thanks.

Click to expand...

JB Weld and some hope.




jk, obviously I need to replace the crank, but instead of going with an OEM crank (and planing to build a turbo motor anyway), I decided to build my bottom end now starting with a Brian Crower billet crank with larger rod bearings with more surface area to handle more stress and power. Forged Pistons, rods, sleeved block, billet mains, the whole 9yards...

Interesting failure; which number cylinder was this? My next question was both failures (Billy and Chris) the same cylinder number? My natural curiosity about rotating machinery Billy:wink:, I sure you understand. Thanks.