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Straightening Warped Crankshaft


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My Zed ITS project inches along. Tonight, during the current lull in my paint prep work, I measured the runout in my newly-polished crankshaft. I rolled out the block in its engine stand, flipped it upside down, laid bearings in the front and back, applied lube, and carefully laid the crankshaft in. I got out my new dial indicator and magnetic base and, after a bit of fumbling figuring out how to set it up, measured the runout in the center journal of the crankshaft: .006 in. The Honsowetz book "How to Modify Your Nissan & Datsun OHC Engine" says the maximum runout (or warp) should be .001 in.

Question 1: Is .006 really a problem or just not optimal? I will be pushing this engine--I'm building a low-budget SCCA ITS racer for Solo I and occassional (in my dreams) road racing.

Question 2: How do you straighten a crankshaft? The Honsowetz book says: "Don't try this, but crank is straightened by striking crank with a blunt chisel and hammer." Actually, I did pull the crank out, set it on two 2x4's, rotated it so the high point was facing up, and gave it a few hard whacks with a rubber mallet. And when I measured it again, the runout was down to .005 in! I did it again, and it didn't make any difference. I have done a web search and saw one reference to a method that uses a hydraulic jack somehow. I imagine machine shops have special tools for crank straightening, or do they just whack it and measure?

Suggestions from you experts are welcome! I'll try what you suggest (if it sounds within my capabilities) and report back with some pictures.


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PM "Phred" with the link to this thread. He can advise you.

I'd think that what Frank Honsowetz says is straight information (he has a long history with Nissan and at one time headed their IRL engine program), but I'd wait until you talk to Phred before "whacking" on the crank anymore.

From your description of the project, a day or so lost here doesn't seem like it's going to hurt anything (at least not as much as ruining your crankshaft would hurt)


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I'm afraid you have a problem child there. First, a crank can be straightened. But I think you're over the limit. I have straightened many cranks, but never one that was off that much. Its very common to remove .002/.003 of runout by straightening. I probably would not even attempt .004. Think of this, you hopefully have about .0025 main brg. clearence. So if you have a crank that ossilates its center journal up and down .006 every revolution, in very short order you will wipe out that journal/bearing, along with # 1 & 7 bearings. Crankshaft runout is the first thing that should be checked before a crank is balanced. If you balance a crank, then straighten, the balance will be off. The center mass of the crank is now in a different spot and will be reflected by an imbalanced crank.

Crank Straightening 101- A crank is straightened by placing it in protected v-blocks, and then, using a custom ground large chisel, is smacked fairly hard with a large ball peen hamer (on the LOW side of the runnout !), in the fillet (radiused edge) of the journal. Sounds horrid huh? The chisel is formed to the same radius and diameter of the fillet, and if done correctly, will not damage the journal/radius. Hitting the fillet in this way causes an interesting metalergical/granular reaction, and the crank will draw itself upward, and will straighten itself without the terrible stresses imposed by pressing on the high side ( a real big no-no in crankdom). There's a bit more finesse' to it, as you can have runnout, and straighten, in areas other than the center main. But thats basically the method.

So what do you do? (A) If the crank hasn't been balanced, and you haven't spent anything on it, get another one.

(B) This crank won't even clean up by grinding it .010, but if you just have to use this crank, I would straighten it as best as I could, (at least .003), and then have it ground .010, and then balanced. I know this isn't what you wanted to hear, but live and learn. Crank condition, both journal condition, straightness, and balance, is one of the first things to consider when building up a race engine.


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  • 2 years later...

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