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390 crank

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scramboy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scramboy Quote  Post ReplyReply Direct Link To This Post Topic: 390 crank
    Posted: Aug/02/2022 at 7:28pm
I recently had some issues and spun a couple of rod bearings in my 68 390. My crank guy wants to weld a journal to get it to .030 under! Seems unacceptable to me. Can i use the crank like that or is there a smaller journal that can be used with an offset grind? Anyone have a combination like that? Any insight will be appreciated.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Aug/02/2022 at 7:58pm
Welding out the crankshaft, which is fine by the way at a competent shop, why not go back to stock? Should not cost much more.
The other option is going to 2.1" journal Chev rods and building something like this:
The Chev route would likely be cheaper and has a proven record for durability. 
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DragRacingSpirit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DragRacingSpirit Quote  Post ReplyReply Direct Link To This Post Posted: Aug/03/2022 at 1:49pm
Nothing wrong with welding the journals if done by a reputable and experienced shop. It is a steel crank and is repairable.

If I were in your position, unless you just want it stock, I would use a 2.1" chevy journal rod and set the stroke to whatever is reasonable with a 6" or even a 5.850 chevy rod and no oil ring supports necessary.

If going back to stock set it at std. rod journal size not undersize.

Remember to do this you will have to buy custom pistons and chevy rods. For a street engine make sure you have a fully supported oil ring, ie (no ring supports required) so do the math and set your stroke accordingly for your deck height and rod length.
Best 1/4 mile 8.99, 1/8 mile 5.71, 60 foot 1.27, no power adders
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PROSTOCKTOM Quote  Post ReplyReply Direct Link To This Post Posted: Aug/03/2022 at 3:29pm
The biggest issue with repairing and turning it .030" under is finding bearings in today's World. If you intend to weld it then have it taken back to a standard AMC dimension. Also a Chevy rod is not the same offset as an AMC rod, so should you head down that road you need to make sure the rods you use have the AMC offset. I am a Molnar dealer and they offer 6.00" length rods in 2.00" & 2.100" rod journals. Turning the crank to 2.100" sounds a lot easier in print than it will end up in the end. You'll easily add $2000 to the project. Pay the crank grinder and put it all back together is a lot simpler.

Tom 
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TomCombo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TomCombo Quote  Post ReplyReply Direct Link To This Post Posted: Aug/03/2022 at 3:56pm
Tom - Can you explain AMC offset on the rods?  I’m learning. Thanks!
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garland performce View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote garland performce Quote  Post ReplyReply Direct Link To This Post Posted: Aug/05/2022 at 8:17pm
SBC rods have a different offset for the piston due to bore spacing 
Molnar make 2.100 and 2.00  rods with amc offset
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BassBoat Quote  Post ReplyReply Direct Link To This Post Posted: Aug/06/2022 at 6:16pm
When you say "has anybody" I have taken a 390 crank to 2.0 journal and 3.8 stroke.  I used 5.85 chevrolet rods from Eagle and had the off side chamferred so I could reverse them to correct the offset.  If i were to do it over I would take advantage of the Molnar stroker rods.

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