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Tractor Talk Discussion Forum

six cyn engine crankshaft

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confused crank

10-05-2006 19:33:37




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Help me on this one.is it possible that a running engine can bend a journal between two main bearings without damage to the bearings.how did the crank get bent in side the engine.call me stupid if you want??? i will give you the phone number to my local machine shop.has this ever happen to anybody out there. thanks if i am stupid.




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Easy1

10-06-2006 05:12:39




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 Re: six cyn engine crankshaft in reply to confused crank, 10-05-2006 19:33:37  
I work in an engine plant. We make crankshafts, v8's now have done v6 previous. If your crank is really bent more than .010 or so, I believe the engine would be very tight. Our runout spec for the pins to the main is only .001. We have a gage that holds the crank vertically, so it doesn't sag in the middle during inspection. I will say that I have never seen a crank that was assembled into an engine & removed make spec. We never would reuse a crank here. Cranks are surprisingly fragile.

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Confused crank

10-05-2006 21:33:49




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 Re: six cyn engine crankshaft in reply to confused crank, 10-05-2006 19:33:37  
I understand the length and number of bearings has effect.this is a 7 main engine crank weight is 180 pounds.the crank is bent between no.1 no2 main in no1 rod journal be on repair.no damage to nether of the three bearings or any other bearing. now my stupid hardheadness is saying how did this engine run 5-6000 hours without some kind of notice.now with a bad cyn liner! and perfect bearings did this crank get be on repair.Boys truefully i don't know how they are checking it??.Personaly i think if i had'nt mentioned the liner leak we would been alright.now the rest of the story i have never had even as much as a brand new head go to this shop with something wrong if there is'nt something wrong we'll make some happen.i personaly i think it's money??.You Decide Yes i know strage things happen to moving parts.

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Earl-IL

10-06-2006 08:26:49




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 Re: six cyn engine crankshaft in reply to Confused crank, 10-05-2006 21:33:49  
What is the engine?? Certain crankshafts have weakness. Has the crank been magnafluxed? The true black light, not magnetic powder ?



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jmixigo

10-05-2006 22:55:49




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 Re: six cyn engine crankshaft in reply to Confused crank, 10-05-2006 21:33:49  
Now I don't mean to sound too smart here, but if I cain't trust my machinest and or that shop I'll do business else where. Where I am now I'm lucky to have a good un around the corner. I have drove over 200 miles one way to go where the good work gets done. An I'll go where ever I got to go if I move (that's pending) or my feller dies or sumthin (I hope not). Wimmin, Docs, an machinests,-If I cain't em I cain't stay with em.

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Earl-IL

10-05-2006 20:30:05




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 Re: six cyn engine crankshaft in reply to confused crank, 10-05-2006 19:33:37  
When your shop said your crank was bent did he mean the 2 center mains were running out? Or did he mean the 2 center rod throws were not indexed right . What manufacture is your engine. I have ground for 30 plus years. If all the mains are within .002 - .003 and the index is off on the center 2 rod throws .I am guessing it was always like that. But if the crank has been ground before there might be a error. When he said it was off how much and where? Chrsyler product 6 cyls (flat head) back in the 50's were off. .010 -.020. It wasn't the correct way, but we just centered the machine on each journal independently.Later engines were always close.When stroking a crankshaft ,especially a weld up stroke you had better check it close for being true TDC and correct index. The same with reground cams , you need to degree each cyl. Let us know what your machinist means here.The newer crank grinders have index heads on both the drive end and the tailstock. The older grinders you aligned the crank in the machine and the head & tailstock just used drop pins to keep the 2 heads of the machine indexed. Earl

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jmixigo

10-05-2006 19:53:56




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 Re: six cyn engine crankshaft in reply to confused crank, 10-05-2006 19:33:37  
Yes, absolutely. Most sixes have only 4 main bearings. Yet the crank is much longer than a V-8,or 4 cyl, that has 5 main bearings. Many tractors have at some point in time had to WORK for a livin. Unnerstand too that "bent" on a crank-to a machinist- usually means 30 or 40 THOUSANDTHS of an inch. You orta see some of the cranks we pull outa them 500 hp Cummins sixes that wuz runnin on good bearins. An it takes two big men an a small boy to tote one around.
A bent crank won't howsumever run too long on new bearins. An with one or two cylinders "out of index" it won't make power like it ort to.

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