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scca_ita's Avatar
 
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How important is it to check crank straightness?

What would be on the engine builders to do list for a 964 crank?

Is it possible that a crank could be in need of staightening after normal operation and is this a big deal?
Old 11-30-2003, 09:06 PM
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I would say in the scheme of things, as important as checking to see if a connecting rod is straight or if the ring side clearances are correct.

Of course you could skip the checks, assume all the above are okay and just put it together.

Sherwood
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Old 11-30-2003, 09:28 PM
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Beg to differ Wayne...I have seen cranks and camshafts that were left for a period of time leaning against a wall or bench.
It does not take much time to put a bend into a crank in that manner.
If in doubt...check.
Bob
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Old 11-30-2003, 11:06 PM
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I would agree with Bob and myself. Have you ever noticed the way cranks are stored at a regrinding shop? They do not store sideways on shelves. They sit upright on one end to avoid bending, however slight. Not sure what happens in an earthquake though, but I can imagine.

Sherwood
Old 11-30-2003, 11:42 PM
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Also...I always "ring" a crank before installation to check for cracks.
Ringing is holding the crank at one end on the fingertips (under a counterweight) and then striking the crank with a metal object (wrench) on another counterweight (not a bearing surface).
If no cracks...it will ring like a bell...if cracks are present it will sound more like "doink".
This will not tell you where the crack is....just that you should use another one!
Bob
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Old 12-01-2003, 06:55 AM
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If a crank does need straightening, and is not cracked, is this an issue with future life of the crank or perfomance?
Old 12-01-2003, 07:35 AM
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I guess that depends on how bent it happens to be. I don't know the spec offhand, but companies that do crank work do, or it's in the spec book (?).

I think they use a BFH or something equivalent to straighten it .....mildly.

Sherwood
Old 12-01-2003, 11:13 AM
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Porsche cranks do bend, at least on early engines they do. I have a 2.2S crank laying around that is out of spec quite some bit (on no3 main bearing: 0.08 mm, the workshop manual states 0.03 as max. permissable). Also the crank in my targa needed straigthening. Be carefull with that nice forged crank, No BFH. Forged cranks may only be straightened on a press.
Cast crankshaft on the other hand are straigthened with a BFH
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Last edited by Peterfrans; 12-06-2003 at 06:26 AM..
Old 12-01-2003, 11:37 AM
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The only way I know how to permanently 'bend' metal is to plastically deform it. Most metals deform linearly to a plastic deformation point beyond which they can never return to original state. There is no way a crank lying against a wall can develop enough stress in bending moment or shear stress to reach the material's plastic deformation limits.
Old 12-01-2003, 05:10 PM
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Ho Hum....if i was still in Ontario...I could show you a big block chevy crank that deformed by over .006" during a winter of leaning storage.
Perhaps the temperature had something to do with it....but it was leaning at about 50 deg against a metal wall (corregated steel).
Bob
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Old 12-01-2003, 06:55 PM
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Bob, I don't doubt your findings but I do challenge the thinking. It is possible that there is some movement or deflection between the different sections of the crank induced by the induced bending moment but the steel parts of the crank itself cannot plastically deform.

Tristan
Old 12-02-2003, 07:03 AM
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checking the crank straightness is very easy. With one half of your engine case on an engine stand, install one main bearing shell at both ends, lay the crank into the case and set-up a dial indicator at the middle main journal, spin the crank slowly and watch your read-out
Old 12-02-2003, 07:38 AM
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Yup...that's what i do....poor man's v blocks.
Bob
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Old 12-02-2003, 07:39 AM
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