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| crank flex? https://slantsix.org/forum/viewtopic.php?t=56235 |
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| Author: | manimal [ Sat Sep 20, 2014 8:13 am ] |
| Post subject: | crank flex? |
Is it common for a forged crank to flex while grinding? I got a call from the machine shop the other day and they said the crank is flexing when the grinder is trying to grind the journals!??? He said the only reason would be a crack. I tapped it with a small ball peen hammer and it rang forever...a cracked crank wont do that. Something does not sound right here. |
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| Author: | Joshie225 [ Sat Sep 20, 2014 9:01 am ] |
| Post subject: | |
I agree that something is not kosher. I would go get the crank and take it somewhere else. Hopefully they haven't screwed it up. |
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| Author: | CNC-Dude [ Sat Sep 20, 2014 9:57 am ] |
| Post subject: | |
The crank ringing when you hit it with a hammer is kinda' misleading. They only make a thud noise when they are cracked or broken really badly, but can still ring loudly even with a few good sized cracks in them. So don't rely on that "urban legend" to tell whether its good or not, just magnaflux it and remove all doubt. I have literally magnafluxed over 1000 cranks when I worked in a race shop, and I used to tap them with a hammer first to see if I could rely on the sound to detect any defects. More times than not they would still ring and be cracked enough you wouldn't want to reuse it. This can be causing it to flex when regrinding also. |
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| Author: | manimal [ Wed Mar 04, 2015 5:49 am ] |
| Post subject: | |
digging this back up..... I went and got the engine back from the machine hop a couple of weeks ago. They told me they had magnafluxed the crank and found nothing, but the crank grinder said it was flexing??? If a crank was cracked enough to flex then the crack should be visible..right? Also I see no signs of a grinding stone ever touching the surfaces. Think I will be taking it somewhere else. |
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| Author: | CNC-Dude [ Wed Mar 04, 2015 10:57 am ] |
| Post subject: | |
That's a good sign that it was mag'd and showed up with no issues, but yes, it is very common for longer cranks to flex when being reground. A steady rest if often used to compensate for this when grinding them. Even on diesel cranks that are very robust and have a 5" diameter main journal size is common to have flexing when reground. |
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