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crankshaft

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J4800owner View Drop Down
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    Posted: Feb/19/2018 at 8:12pm
how far is to far to turn a crankshaft? I'm already at .020 under and will need to turn it again for this rebuild.Not building a racing motor . just good torque.
close to stock with ported stock heads and 650cfm carb. just dont want a crank issue in the woods lol

1972 J4800
pics coming soon
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Bill F View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bill F Quote  Post ReplyReply Direct Link To This Post Posted: Feb/19/2018 at 8:17pm
My 390 was turned .030 in the 80s and never had any problems. Factory went .020 under. That's what I started with.

My opinion .030 is not an issue.
Bill Frazer
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Post Options Post Options   Thanks (0) Thanks(0)   Quote J4800owner Quote  Post ReplyReply Direct Link To This Post Posted: Feb/19/2018 at 8:21pm
excellent! appreciate the info!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jcisworthy Quote  Post ReplyReply Direct Link To This Post Posted: Feb/20/2018 at 5:32am
If you think about it, turning the crank to .030" under is taking a .015" cut. .015" around the circumference to get a total of .030" off the diameter. 

Take a look at a .015" feeler gauge. In my opinion, if taking that much off the circumference of a crank is a deal breaker then these cranks are junk from the start and should not be used. 

That is not the case and I would not worry about .030" under at all.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jcisworthy Quote  Post ReplyReply Direct Link To This Post Posted: Feb/20/2018 at 5:36am
The only concern I would have about turning one of these cranks .030" or .040" is bearing availability and bearings are available now at .030"so go for it.  
Specializing in engine building and cylinder head porting

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farna View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Feb/20/2018 at 7:06am
That is my thinking. If 0.040" bearings are available I wouldn't hesitate to use them in a stock to slightly warmed up engine. If it's something you're going to be routinely turning up to and maybe over 6000 rpm maybe not... but even then that small amount off shouldn't be a crank breaker. That said, I've only actually run a stockish I-6 with 0.040" bearings in it. You can get (or could...) 0.060" bearings for the late model AMC I-6. Had one that I tried to save one rod journal on with the thick bearing... wasn't enough! Just dropped the pan and put that bearing in... Fuel pump had sprung a leak and was spraying the bearing, wore that one out (#2?). It was a cheap car, so I dropped the pan and tried it. Still a knock (I think an 0.080" might have done it), so junked it.


Edited by farna - Feb/20/2018 at 7:09am
Frank Swygert
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WesternRed View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Feb/20/2018 at 8:00am
What engine? That will have some bearing on it LOL. People offset grind the rod bearings on forged 390 and 401 cranks way beyond any normal bearing under size and successfully race them without issue. The main bearing journals are way bigger than the rod journals so the only limitation really is the availability of suitable bearings. 
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Class Guy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Class Guy Quote  Post ReplyReply Direct Link To This Post Posted: Feb/20/2018 at 11:31am
I wish .060" under bearings were available for the V8s.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote White70JavelinSST Quote  Post ReplyReply Direct Link To This Post Posted: Feb/20/2018 at 11:49am
There's a machine shop about 20 miles from my home that welds steel crankshafts.

It's an amazing process called submerged arc.

before the shop machines the crank they put it in a giant bad-rump lathe and push it straight again to remove the warpage caused by welding.

I've wanted to watch the entire process, but so far have only been fortunate enough to see the welding process.

Expensive? yes, it is. But it would save even that crank that's been way over abused. Broken ones, um nope ....

70 Javelin SST, second owner, purchased 1972
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