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Old 03-16-2012, 12:55 PM
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Machine shop told me the crank was bad...

I took my shortblock SBC350 to the machine shop. A few days later they told me the crank was bad. The thrust surface was scored badly (thrust bearing was shot). I did look at it myself to verify. I do not have the full history on the engine, I know it was rebuilt at least once (+.030 bore and steel sleeve) and used/driven hard. The engine was always used with a 4 speed transmission.

Not that I am an expert in hot rods, but in all my reading and researches I have never heard of anything like this. I have heard of the thrust bearing, but never of this kind of wear/destruction. From what I have been able to tell, a common cause is riding the clutch (since the previous owner was my dad I can assure you he did not ride the clutch).

So what is the normal expectation/lifespan of the thrust bearing surface/boss/web on the crankshaft in a manual transmission application? Is the only prevention for destruction to replace the thrust bearing more often? Use better oil? What is the part of the crankshaft called where the thrust bearing makes contact?
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Old 03-16-2012, 01:01 PM
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The thrust surface on the crank most likely got that way from Bang shifting a 4 speed. (Side steping the clutch etc ).
A auto trans torque converter failure can do that too.

or from someone who hammered on a balancer.

Find another regrindable GM cast crank or buy a scat crank. (383 stroker time)

is the block good? do they have any ready stock 350 cranks there? Most shops do.
pretty common. A GM forged crank can be welded but a cast crank is not worth it.
Just get another one.
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Old 03-16-2012, 01:11 PM
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When I look for good rebuildable 350 SBC's I always avoid "forged " hi perf or raced stuff and anything from a hot rodder in general especially if advertized or claimed as a corvette motor.
The only reason he is selling it is cause its fubared.

I look for bread and butter 2 bbl 350 motors out of mom and pop low perf cars.
usually lived a easy life.
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Old 03-16-2012, 01:17 PM
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As F-bird said time to find another crank. I ran a engine rebuilding shop for my dad in the late 80's and we saw this all the time. Stick motors more so but did see it on auto's to. If the damage is minor you can grind the crank and grind the thurst to under size and get speical main bearings for it. But For you your best bet is to do a crank exchange for a reground one.
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Old 03-16-2012, 01:19 PM
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Quote:
Originally Posted by F-BIRD'88
The thrust surface on the crank most likely got that way from Bang shifting a 4 speed. (Side steping the clutch etc ).
A auto trans torque converter failure can do that too.

or from someone who hammered on a balancer.

Find another regrindable GM cast crank or buy a scat crank. (383 stroker time)

is the block good? do they have any ready stock 350 cranks there? Most shops do.
pretty common. A GM forged crank can be welded but a cast crank is not worth it.
Just get another one.
They said that they would sell me a used GM crank for $150, which sounded like a deal, but Summit had a new Eagle crank for $165 . In the end I figured the Eagle would be better since it can be guaranteed to be nice and straight, and I do not know how honest this machine shop is.

I was really tempted with the 383, but that would have been too much money. I had already purchased and opened the boxes for the pistons and rings, so I am not sure I could return/exchange them...but even if I kept them I would need new rods, plus balancing the whole rotating assembly and then clearancing the block...way more than I wanted to get into. So the $165 crank plus the cost of balancing everything seemed a lot more palatable.
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Old 03-16-2012, 01:31 PM
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Quote:
Originally Posted by Silver Surfer
In the end I figured the Eagle would be better since it can be guaranteed to be nice and straight
Not so fast there Hopalong, you may need to do some reading on these Fosdick chinese cranks. Read what the machine shop guys on this forum have to say about them.
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Old 03-16-2012, 03:31 PM
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A new replacement crank may need to be reground. They are not always perfect out of the box.
I am of the opinion that the scat cranks are a little better.

Ya they cost a bit more. A properly reground GM crank is perfectly fine. $150 is fair.

if you are the builder/assembler its up to you to check all critical dimensions, etc.
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Old 03-16-2012, 04:07 PM
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What they don't tell you is that $165.00 crank has to be balanced to your assembly. Another $175.00
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Old 04-06-2012, 08:36 PM
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Quote:
Originally Posted by techinspector1
Not so fast there Hopalong, you may need to do some reading on these Fosdick chinese cranks. Read what the machine shop guys on this forum have to say about them.
OK OK you put the fear of God in me and I decided to go with a used factory crank. I got the shortblock back from the machine shop this week. Hopefully you guys can sort me out with a few problems on the "new" crank.

1) I tried to thread a new balancer bolt into the snout it only went in 3 full turns. I am hoping is it because the threads are dirty and need to be chased, and not due to different thread pitches. Did GM make different balancer threads?

2) I also slipped on the old balancer and found it went in much further before
it bottomed out. The "new" crank is 1.240" for the first 5/32", then it fattens up to 1.248" at the end of the witness mark from the balancer. The "old" crank measures a steady 1.248" all the way. Probably not a big deal, but is that wear on the crank or did they come this way? FWIW I will be replacing the balancer with a new one, which should tighten the fitment a little.

3) The least of my worries is that I noticed the pilot bearing hole was damaged and I had to take a rat tail file and knock down the burr on the outer edge. Also, there is a score mark down inside the bearing hole, but that shouldn't interfere with the bearing. I hope.
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