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Is it possible to get 4.125" stroke from an offset ground 391 crank?

December 4 2017 at 4:37 AM
paulie  (Login Paul_Lovett)
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Reason I am asking is I have one. Well, the crank guy said he got 4.125", but I don't see how that is mathematically possible. That is unless there is more variation in factory cranks than I realized. The 391 I started with was used, but standard/standard.

The crank guy was very harried at the end and I wonder if the number he gave me is correct. I don't know how to measure something like that in my garage.

I was expecting 4.12".

thanks for any thoughts,

paulie

edit: Oh yeah, the journal was taken from stock size down to 2.100". I forgot to mention that.


    
This message has been edited by Paul_Lovett on Dec 4, 2017 4:43 AM


 
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Brent Lykins
(Login blykins)
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What's rod journal diameter?

December 4 2017, 4:45 AM 

Is it for a .010" under SBC rod journal? If so, then you can get 4.125" out of it.

 
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paulie
(Login Paul_Lovett)
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I just edited my above post. 2.100" SBC size.

December 4 2017, 4:47 AM 

I didn't think about it being .010" under. THAT I can measure.

Thanks,

paulie

 
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Mike7570
(Login mike7570)
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crank

December 4 2017, 5:04 PM 

I'm pretty sure stock journal size should be 2.4384. I doubt you could grind it to 0 on one side so with a little fudge factor you might get to almost 4.118
(without being .010 under) You could always mock it up in your block with a rod and piston and measure the stroke
Mike G


    
This message has been edited by mike7570 on Dec 4, 2017 5:08 PM


 
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paulie
(Login Paul_Lovett)
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You are right. I think Brent hit it on the head with the 0.010" under.

December 4 2017, 7:52 PM 

I'll measure the journal size tomorrow as best I can. Or like I said the crank guy seemed to be in a bad place and I just wanted to get my crank back. I don't have the parts to mock it up in a block.

thanks,

paulie

 
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Mike U.
(Login mtrain)
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Well Paul, my 391 crank is 4.5" stroke so I guess it can be done........nm

December 4 2017, 6:01 PM 

......

 
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paulie
(Login Paul_Lovett)
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I forgot to say without welding.

December 4 2017, 7:49 PM 

Ya big goofball! happy.gif

 
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Tom P
(Login tomposthuma)
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mock up

December 5 2017, 10:31 AM 

It can be mocked up without buying expensive stuff. I still have a 454 Chevy piston and rod I used for that. I cut the rod in half and lengthened it to 6.8" with a piece of bar welded between the halves. The top of the piston was "milled" with a disc grinder. That was back when I had an offset ground 4.20" stroke IU crank. The 4.35" piston fit rather loose in the 4.263" bore 427 block but the idea was just to check for deck height and clearing the counterweights.

 
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kelly
(Login lowriser)
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i always wanted to use some bbc blower pistons

December 6 2017, 8:49 PM 

never considered a stroked crank.
how cool to be able to check those clearances.
did it all jibe?

 
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Tom P
(Login tomposthuma)
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Re: i always wanted to use some bbc blower pistons

December 7 2017, 8:19 PM 

Yes, it all fit. I ended up selling the crank before I ever used it though.

 
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Bill Ballinger
(Login 65billgal)
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I would like to fix my gacked 410 iron crank to 4" of stroke and 2.31 Cleveo rod bearings

December 5 2017, 5:29 PM 

Just a .002 offset and down to 2.31 I believe on the rods. The mains should clean up at .010. I would like to take a 105 block I have .044 over for a 421 ci engine.

Can it be done? Sorry Paulie for the high jack.

 
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