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crankshaft |
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J4800owner
AMC Fan Joined: Feb/16/2018 Location: Arkansas Status: Offline Points: 8 |
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Posted: Feb/19/2018 at 7:12pm |
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
AMC Apprentice Joined: Feb/21/2017 Location: Maryland Status: Offline Points: 244 |
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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. |
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Bill Frazer
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J4800owner
AMC Fan Joined: Feb/16/2018 Location: Arkansas Status: Offline Points: 8 |
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excellent! appreciate the info!
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jcisworthy
AMC Addicted Joined: Jul/23/2009 Location: North Carolina Status: Offline Points: 2805 |
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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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Specializing in dyno services, engine building, and cylinder head porting
rbjracing.com Phone Number 518-915-3203 |
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jcisworthy
AMC Addicted Joined: Jul/23/2009 Location: North Carolina Status: Offline Points: 2805 |
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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.
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Specializing in dyno services, engine building, and cylinder head porting
rbjracing.com Phone Number 518-915-3203 |
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19611 |
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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 6:09am |
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Frank Swygert
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WesternRed
AMC Addicted Joined: Aug/03/2010 Location: Australia Status: Offline Points: 5787 |
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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
AMC Addicted Joined: Jul/02/2007 Location: Arkansas Status: Offline Points: 969 |
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I wish .060" under bearings were available for the V8s.
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Addicted to acceleration.
Owner and Admin for www.classracerinfo.com |
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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4867 |
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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 .... |
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70 Javelin SST, second owner, purchased 1972
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