crankshaft
Printed From: TheAMCForum.com
Category: The Garage
Forum Name: AMC V8 Engine Repair and Modifications
Forum Description: AMC-made V8 engine mechanical, ignition and fuel from basic repair to high-perf modifications
URL: https://theamcforum.com/forum/forum_posts.asp?TID=92615
Printed Date: Apr/23/2024 at 10:18am
Software Version: Web Wiz Forums 12.03 - http://www.webwizforums.com
Topic: crankshaft
Posted By: J4800owner
Subject: crankshaft
Date 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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Replies:
Posted By: Bill F
Date Posted: Feb/19/2018 at 7:17pm
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.
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Bill Frazer
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Posted By: J4800owner
Date Posted: Feb/19/2018 at 7:21pm
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excellent! appreciate the info!
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Posted By: jcisworthy
Date Posted: Feb/20/2018 at 4:32am
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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Specializing in dyno services, engine building, and cylinder head porting
rbjracing.com
Phone Number 518-915-3203
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Posted By: jcisworthy
Date Posted: Feb/20/2018 at 4:36am
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.
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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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Posted By: farna
Date Posted: Feb/20/2018 at 6:06am
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.
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Frank Swygert
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Posted By: WesternRed
Date Posted: Feb/20/2018 at 7:00am
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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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Posted By: Class Guy
Date Posted: Feb/20/2018 at 10:31am
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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Posted By: White70JavelinSST
Date Posted: Feb/20/2018 at 10:49am
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 ....
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70 Javelin SST, second owner, purchased 1972
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