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dweymer's Avatar
 
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crank specs-too large?

I took a 3.0 crank to the machine shop for polishing and mic. They told me the measurements were as follows, and commented about it being larger than spec.

Mains:
2.3619
2.3619
2.3619
2.3621
2.3619
2.3621
2.3621

Rods:
2.0861
2.0862
2.0861
2.0862
2.0861
2.0862

According to the book I have, yep they are large by about .0001 & .0003. How is that possible, and how much will that affect the bearing clearance? I suppose the question it boils down to is: will it cause problems to run as is?

D
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Old 09-01-2010, 07:27 PM
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operator error/variances in measuring tools. looks like what you have is a damn near new-spec crank. you're talking about 1 to 3 TEN-THOUSANDTHS of an inch. nothing. be happy and run it. maybe someone with more experience will chime in here. FWIW most specs are to the thousandth of an inch, accuracy can get dicey past that.
Old 09-01-2010, 09:06 PM
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Micrometers

Understanding resolution and accuracy of measuring instruments is always quite tricky.

A typical modern micrometer will have a graduated scale with a resolution of 0.0001" and in the case of a digital micrometer this could even be 0.00001"

This doesn't mean that they are accurate to this level.

A typical accuracy (better to think of this as uncertainty) is 0.0002" and this assumes a good quality micrometer which has been recently calibrated, etc, etc.

As the tolerance in the bearing clearance on a 2.4" main is significantly greater than a tenth of a thou I wouldn't worry.
Old 09-02-2010, 12:10 AM
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How hot is it in their shop?
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Old 09-02-2010, 01:39 AM
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Thanks guys, I do know it was a low mileage engine.

Craig, hey man, glad to see you are still around; it is probably about 75 in their shop.
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Old 09-02-2010, 05:07 AM
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All post spot on. Coming from the aerospace industry, I wouldn't put a lot of faith in machine shops' measuring devices. Do they calibrate regularly? Maybe. Maybe not.

Who's to say a guy had or had not dropped the mic on the ground or table. This is all it takes to knock them out of calibration.
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Old 09-02-2010, 05:49 AM
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I calibrate my micrometer with a standard before EVERY measuring session. It's easy even for a monkey like me. Since my environment is not maintained at an ISO 20C temperature, there's always some variability. But what matters is repeatability of measurements on the same day.

I don't use decimal inch although many people do. 0.0001" is 0.0025mm or 2.5 microns, the factory measured to microns using a Mahr Millimess, probably calibrated with a stack of gage blocks.

So you do your measurements in microns and then fire up an aircooled engine with the barrels attached to a two-piece engine case with long bolts and all the tolerances go to hell, the cylinders look like bananas at operating temperature and the engine grows 1/8" wider due to thermal expansion!
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Old 09-02-2010, 06:16 AM
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Donnie,

I had a 3.0 with ~80K miles that had the same numbers for the crank journals. 2-3 were above spec and the others were right on the highside of the diameter spec.

It is likely a combination of a VERY good crank and slight measurement error through temperature or calibration variations.
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Old 09-02-2010, 06:17 AM
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John, Great way of putting it all in perspective!
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Old 09-02-2010, 06:18 AM
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Just for fun years ago I wanted to see how big the mains would grow on a mag r7 case. After heating it up to around 190 the mains were out of spec by .002

The old 912 engines push rods would grow .013 in length after hot, luckily everything else enlarged too!

So it is true, cold shrinks, heat expands!
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Old 09-02-2010, 07:10 AM
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Here is a typical 3.0 crank inspection report (sizes in mm).

Your data is well within OEM specs

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Old 09-02-2010, 04:18 PM
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I just check the mains on my 2.7 crank, at 68 degrees it was 56.979 at 98 degrees is measured 57.007 so I guess when you all ask about a crank you want to purchase you better know at what temp it is was checked.
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Old 09-02-2010, 04:55 PM
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Craig

Your measurements are pretty close for a 30 deg temp change.

The thermal coefficient of expansion for carbon steel is 0.000011. Raising the temperature at 68 deg by 30 deg results in a 0.02 mm change on a 60 mm linear dimension.

Aluminum pistons are even more critical as the thermal expansion coefficient is twice as big at 0.000023. An 80 mm piston increases in diameter by 0.06 mm for a 30 deg temperature change.

In the aerospace world, we require that all dimensional inspection areas maintain 68 deg +/- 1 deg.

I make all my Porsche inspections in my temperature controlled basement :-)
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Last edited by tom1394racing; 09-03-2010 at 12:40 AM..
Old 09-02-2010, 05:28 PM
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Guys, Tom is being characteristically modest. I bought a crank from him and had the opportunity to visit his basement-- on the surface it just looks like racks and racks of cool Porsche parts, but upon closer inspection. . . he's got it set up for dimensional metrology. . . just like an engineer. . .

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Old 09-03-2010, 05:29 AM
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John

Since your imagination is running wild...and I love to exaggerate....I need more space to store the inventory of 911 parts I can't stop myself from buying.

I am looking at anunderground storage area below my concrete slab to create additional storage space. It would be a tribute to Gary Fairbanks, who introduced me to the wonders of dimly lit sub-terrainian caverns filled with magnesium castings.
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Old 09-03-2010, 04:05 PM
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