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FE Power Forums => FE Technical Forum => Topic started by: AlanCasida on October 14, 2019, 08:57:43 PM

Title: Scat crankshaft straightness
Post by: AlanCasida on October 14, 2019, 08:57:43 PM

What would be the tolerance for a crankshaft to considered ok for straightness? With my Scat crank in the block with only #1 and #5 main caps torqued down(All other bearings removed) I get about 0.001" out of round measuring on #3 main journal using a dial indicator and rolling the crank. I don't think I have ever had an OEM crankshaft that was anything other than dead on.

Title: Re: Scat crankshaft straightness
Post by: C6AE on October 14, 2019, 09:17:21 PM

Is it an out of round journal, or is it bent?

Title: Re: Scat crankshaft straightness
Post by: AlanCasida on October 14, 2019, 09:19:32 PM

I don't think it is the journal. I will double check it though.

Title: Re: Scat crankshaft straightness
Post by: winr1 on October 14, 2019, 09:54:59 PM

Did ya indicate the snout an perhaps the machined area for the flywheel hole ??

EDIT: ya indicate 2 an 4 as well  ??


Ricky.

Title: Re: Scat crankshaft straightness
Post by: Falcon67 on October 15, 2019, 09:09:24 AM

IMHO that's a lot.  The stock cast crank in my 351C checked at .0005 TIR. 

EDIT - that was on a surface plate with V-blocks.  Not in the block riding on bearings. 

Title: Re: Scat crankshaft straightness
Post by: AlanCasida on October 15, 2019, 05:37:18 PM

 I got to thinking about how I was measuring the crankshaft. The dial indicator moves a total of .001" so does this mean it is really out .0005" since it I would be measuring from the bottom of the "bend" to the top? Does that sound right?

Title: Re: Scat crankshaft straightness
Post by: frnkeore on October 15, 2019, 07:45:09 PM

Yes, it is .0005 off center.

Title: Re: Scat crankshaft straightness
Post by: Thumperbird on October 15, 2019, 08:09:37 PM

I would say maybe yes, maybe no, depends on your setup.  Is the rate of change on the indicator consistent through the 360 degrees?  How much would the crank sag in the middle unsupported, it's not 0.  I would try 2 and 4 and measure again just for fun to verify.  In fact, just support it with 1/2 a bearing and spin it, if truly out of round it will indicate that way still.

 

Title: Re: Scat crankshaft straightness
Post by: runthatjunk on October 16, 2019, 01:39:59 AM

65 shop manual says TIR of .002ok wear limit of .003 for all engines.  I would be curious if it measured any different in a V-block setup.

Title: Re: Scat crankshaft straightness
Post by: My427stang on October 16, 2019, 01:23:10 PM

I had a SCAT crank here that had the oil slinger machined incorrectly.  It hit within it's groove.  Unlikely, but check and see, it would have lifted the crank during a turn.

Title: Re: Scat crankshaft straightness
Post by: frnkeore on October 16, 2019, 04:31:48 PM

The mains are machined and ground, between centers. Meaning the crank is supported on both ends, only. They are done that way to keep everything on one plane.

If your crank will sags, you need to get rid of it, as it's not very strong and won't survive.

W/o a lathe available, supporting it on #1 and #5 with 2,3& 4 bearing shells removed, would be acceptable and akin to supporting it in V blocks.