anyone ever seen a scorched crankshaft?

10 replies to this topic
  • MN_Kevin

Posted October 07, 2002 - 02:16 AM


I pulled my cylinder several days ago. While looking for the c-ring (that fits onto the camshaft >> still haven't found it! :)) last night, I noticed the blue scorching on my crankshaft!

What is up w/ dat?

My last ride I was sucking air REAL BAD via my fuel screw opening. The OEM spring disappeared. I used an ink pen spring to pre-load the o-ring on the fuel screw.

Needless to say, the spring was too weak and I had a severe vacuum leak into my engine.

I have NEVER seen a crank w/ blue heat marks on it!

  • Uncle_Rich

Posted October 07, 2002 - 03:02 AM


I’ve never had my bike engine apart so I can’t tell you what mine looks like.
Are you sure that’s not just heat-treating marks?
I would think that if it got hot enough to turn the crank blue that it wouldn’t still be running.
As for the clip, check the magnets on the flywheel.


  • Quickrik

Posted October 07, 2002 - 03:11 AM


Blueing on the crank around the big-end. Perfectly normal, but don't ask me what or why yamaha do to make it go blue.

  • John_Lorenz

Posted October 07, 2002 - 03:28 AM


I'm not sure if it can be classified as normal, but I have seen this. The crank end turning blue is normal when the metal is heated, its called kneeling I think. Steel will turn blue in excessive heat such as the kneeling effect or hardening. (I think Kneeling is correct word but wrong spell)

If you were to heat a peace of steel to harden it the molecules expand and become viable. Cooling the same steel at a slow rate of fast rate will increase or decrease its tinsel strength.

Cooling it at a slow rate like a crank rod after a hard hot ride + it being drenched in oil will only harden the crank giving the bluing effect. Dipping a hot piece of steel in oil as it cools adds to its tinsel strength Effectively hardening the piece and turning it blue. This is the same effect you see on a exhaust header the hottest area will turn blue.

Cranks shatter also because of this effect they become brittle as they wear.......

Just food for thought.

  • endurodog

Posted October 07, 2002 - 05:09 AM


I've noticed blue steel on my 2 smoke cranks before and never gave it a 2nd thought. They worked fine for as long as I owned the bike.

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  • GPS Dual Sport

Posted October 07, 2002 - 05:39 AM


That gun metal blue color on the crank at the lower end bearing is localized hardening.
This is for improved durability of pressed crank assemblies since the bottom end bearing is where pounding from the piston is the greatest. Heat treating the entire crank is overkill since the process makes the metal more prone to brittle failure in small cross section areas like where the flywheel attaches at the end of the crank.

  • Rich_in_Orlando

Posted October 07, 2002 - 10:22 AM


Yep, that "scorching" is normal. I'm on my third crankshaft and they all look like that when you take them out of the box.

  • MN_Kevin

Posted October 07, 2002 - 04:32 PM


I am glad I am not the only one w/ a BLUE CRANK!! :)

  • Chaindrive

Posted October 07, 2002 - 04:50 PM


Now Kevin, if it weren't for Bill's eagle-eye (see 5 stars), we could REALLY have some juvenile fun with THAT statement!! :)

  • ddialogue

Posted October 08, 2002 - 05:21 PM


On your third crank?? What are you doing to that motor??


Posted October 09, 2002 - 08:03 AM


I've seen several like that.....I thought you had a magnetic drain plug on that puppy? Would the c clip not fall into the bottom of the casing? I would think that you should have been able to flush it out.

Bonzai :)


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