Scat cast crank Flexes when grinding - Chevelle Tech
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post #1 of 15 (permalink) Old Jun 28th, 20, 12:02 AM Thread Starter
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jeff
 
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Scat cast crank Flexes when grinding

I heard from a machine shop owner that a local company here that does nothing but grind cranks said the scat 9000 cast cranks flex and push away from the stones when trying to grind them.


I do not and have not seen cranks being ground but found that interesting and had to ask if it is true.


The machine shop owner said the old chevy 400 cranks would not flex.


I was talking to him about a stroker build SBC 38?


I have a good Scat crank with less than 200 miles on it. But am thinking Forged now.
This will be driven hard 11.3 compression or more and 254-264 @ .050 duration.


I did weigh the Scat 383 crank 48.4 lbs and the Stock cast 350 crank I had there weighed 52.4 lbs.
Not that weight has anything to do with Flex.


Is the flex a bad thing or is this guys story a little off??
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post #2 of 15 (permalink) Old Jun 28th, 20, 1:54 AM
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Re: Scat cast crank Flexes when grinding

do they have a steadyrest? if not, they should.

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post #3 of 15 (permalink) Old Jun 28th, 20, 8:00 AM
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Re: Scat cast crank Flexes when grinding

I've had two ground and had no issues, machinist was able to hit .010 under, round with no taper.

Kinda looks like there would be a lot of issues with poor journal size or roundness right out of the box if they flexed that much...

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post #4 of 15 (permalink) Old Jun 28th, 20, 9:42 AM
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Re: Scat cast crank Flexes when grinding

Sounds like happy machinists are like happy farmers, something only found in fairy tales.

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post #5 of 15 (permalink) Old Jun 28th, 20, 1:18 PM
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Re: Scat cast crank Flexes when grinding

A friend of mine does cranks, has for decades as a business. He rarely finds a crank that isn't out of align in some place, so, he built a press that has a pressure scale reader on it, and presses every crank, never hits them as some of his contemporaries do/did.

He says he has found some of the aftermarket cranks sold under some very notable names, and actually made in China, are much like silly putty, soft, and easily bent.

A steady rest is a great tool, but if a crank needs one while being ground, I would think the grind pressure against the metal was too high, and/or, the crank is "overly soft". Please remember, a steady rest holds a part in place as it is machined, but, it gets a bit loose as metal is removed from it during the cutting/grinding processes. I don't think I have ever seen, nor heard of a steady rest that compensates for metal cut loss/diameter loss while the machining is going on.

I do a LOT of two stroke motorcycle cranks, and some four strokes that have built up, full circle rods, they press together, then get final aligned to get them straight. The common practice is to whack them to get the flywheels to move on the crank pins to set the alignment straight and true. My friend goaded me into building a holder jig to hold one flywheel, and press the other one into place and alignment, instead of whacking away with a large (read heavy) lead hammer. Works better on the crank, and my arm this way now.
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post #6 of 15 (permalink) Old Jun 28th, 20, 9:32 PM
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Re: Scat cast crank Flexes when grinding

I will take a GM cast crank over some of the china stuff any day.


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post #7 of 15 (permalink) Old Jun 28th, 20, 10:43 PM
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Re: Scat cast crank Flexes when grinding

I have had two machinist tell me they dont even try to grind cranks. They just get a new one.

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post #8 of 15 (permalink) Old Jun 28th, 20, 10:56 PM
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Re: Scat cast crank Flexes when grinding

Dave I knew a guy that used a very large wooden mallet on flatheads & panheads, my eyes got real big first time I seen that LOL


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post #9 of 15 (permalink) Old Jun 29th, 20, 6:59 AM
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Re: Scat cast crank Flexes when grinding

What everyone else said plus he might not be dressing his grinding wheel properly. The guy that does cranks for me has done several cast Scats and I have never heard him complain.
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post #10 of 15 (permalink) Old Jun 29th, 20, 12:18 PM
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Re: Scat cast crank Flexes when grinding

I've never seen anyone grind a crank without a steady rest....


Early in my career I was an Engineer at Kwik-Way and had some training on one.

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post #11 of 15 (permalink) Old Jun 29th, 20, 3:09 PM
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Re: Scat cast crank Flexes when grinding

There is no way any experienced crank grinder would feed the rock into the crank with enough force to flex the crank. If he did the crank and rock would be junk after a couple rotations. You feed just enough rock into the journal to let it whittle off the roughness and damage. You take off very little, much less than a .001 each rotation. The gentle touch lets the rock live and gives a much better finish to the crank. I do not believe the flexing.

A crank turned properly is no different than a new crank. Even a new crank had to be ground to get it to specs the first time. Reckon it flexed when it was being turned the first time?
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post #12 of 15 (permalink) Old Jun 30th, 20, 11:44 AM
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Re: Scat cast crank Flexes when grinding

I agree with sleeper on the procedure, but I also realize that not all crank grinders know how to do it correctly.
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post #13 of 15 (permalink) Old Jun 30th, 20, 10:49 PM
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Re: Scat cast crank Flexes when grinding

Sounds more like another anti-china hoo-raa rant to me....I've used at least 1/2 a dozen of the SCAT cast cranks in 383 and 400 builds, and run them to 7200+ rpm...even did one 350 that turned 8200 rpm on occasion....if those cranks were rubber, we'd have definitely seen it blow a few of those up. Never an issue.
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post #14 of 15 (permalink) Old Jun 30th, 20, 11:04 PM
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Re: Scat cast crank Flexes when grinding

I think this whole thing is an internet bullsh1t deal. Too bad it will found here for the foreseeable future.

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post #15 of 15 (permalink) Old Jun 30th, 20, 11:35 PM Thread Starter
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Re: Scat cast crank Flexes when grinding

I just asked this question on the flexing here because I heard just that one guy mention it and it was mentioned to him by a guy that just grinds cranks and nothing else.
Maybe he misheard what the crank grinder guy was saying I do not know.

That is why I asked here to get more opinions.
The scat crank (3.75 stroke SBC) I have has seen 7200+ rpm and it worked fine and with 400 GM rods with stock GM bolts.

With the answers I got here I will go ahead and use it.

Thanks for all the input guys.
I do not use aftermarket stuff much but I am running out of old good parts and must resort to aftermarket.
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