# Crank down .20?



## chadd925 (Jun 5, 2016)

Pulled the engine tonight due to spun bearing on #2 . Crank was already turned down .10 on mains and .10 on journals. At this point should I be buying a new crankshaft or is going down .20 fine on these? Apologies if this topic has already been beaten to death, but I ran a search and wasn't finding much. Pics of pull this evening - Cheers!


----------



## PontiacJim (Dec 29, 2012)

chadd925 said:


> Pulled the engine tonight due to spun bearing on #2 . Crank was already turned down .10 on mains and .10 on journals. At this point should I be buying a new crankshaft or is going down .20 fine on these? Apologies if this topic has already been beaten to death, but I ran a search and wasn't finding much. Pics of pull this evening - Cheers!


I would ask an engine machine shop that question, but from what I have read, the Pontiac crank is very sturdy and turning it .020" under should be fine. If you were putting big HP to the crank, then I might not.

I would not do anything to the crank until I have had it magnafluxed and checked for cracks. Remember, if it is the original crank, it is almost 50 years old and has seen its share of use. You don't want to have a microscopic crack turn into a busted crank and take out your engine.

Personally, I would consider a new crank seeing they are available. 

So what caused the spun bearing? I would want to know what caused this and correct it. Are those stock factory rods? If so, they can be a weak spot in any Pontiac engine IF not correctly prepped. And if one has gone out of round, then I would not be trusting any of the rods, so all rods should be pulled and checked for roundness/size. I would not re-install a re-ground crank and then feel all you have to do is install .020" bearings on all the rods and you'll be good to go. I would not trust any of the rods at this point without at least re-sizing them and installing new ARP rod bolts. A better value is to always go with a set of aftermarket forged rod if you can afford to. By the time you prep a set of stock rods with what is required to make them live at higher RPM's, you might as well invest in forged rods and have peace of mind.

If you decide to go new crank/rods, then you will have to get things balanced so you will probably be looking at rebuilding your shortblock using your old pistons (if good, and bores are within tolerances) new rings, new crank/rods, and then get these balanced along with your flywheel & harmonic balancer. Keep everything else to save money if it checks out good - cam, lifters, timing chain & gears. You should want to pull all the rods to check them, so you are going to have to disassemble the engine anyway - so why not do a little upgrading at this point?

Hope that helps with your question.


----------



## 1968gto421 (Mar 28, 2014)

PontiacJim said:


> I would ask an engine machine shop that question, but from what I have read, the Pontiac crank is very sturdy and turning it .020" under should be fine. If you were putting big HP to the crank, then I might not.
> 
> I would not do anything to the crank until I have had it magnafluxed and checked for cracks. Remember, if it is the original crank, it is almost 50 years old and has seen its share of use. You don't want to have a microscopic crack turn into a busted crank and take out your engine.
> 
> ...


X2! Great advice from Jim.


----------



## geeteeohguy (Feb 2, 2008)

I have seen Pontiac cranks cut .040" undersize with no issues. It's a hell of a beefy crank. .020" would not worry me in the slightest. What caused the bearing to spin would concern me.


----------



## chadd925 (Jun 5, 2016)

I have no idea why the bearing spun, the engine was rebuilt less thank a year ago and the seller went MIA as soon as the bad news came. I'll hear from my local machine shop soon and will let y'all know.


----------



## PontiacJim (Dec 29, 2012)

chadd925 said:


> I have no idea why the bearing spun, the engine was rebuilt less thank a year ago and the seller went MIA as soon as the bad news came. I'll hear from my local machine shop soon and will let y'all know.


Ya, they seem to do that....MIA. He may have known something that he did not share with you hoping you'd deal with it and not ask questions.

Now the term "spun bearing" is a general term used when there is a bearing problem. It can literally mean a spun bearing, ie the bearing spun around in the main cap or big end of the rod bearing. Wiping out a bearing, ie due to loss of oil, would mean the bearing is still in place, but the bearing material has been excessively worn away.

Spun bearings can be from a block being align honed and the bearing crush was not correctly adjusted to hold the bearing shell in place, so some source of resistance caused drag on the bearing/crank main and spun it within the main cap. 

It is also possible the oil film was lost between the bearing surface and main journal and it heated up enough to melt onto the main journal and spin around. The mains at the rear generally get a better supply of oil being closer to the oil pump. Could have made a hard launch on less than ideal oil levels and momentarily starved the engine which was compounded by the #2 main being further to the front - did that on a Chevy 409 I had.

Might have been as simple as some junk left in an oil passage that cut off or lowered the oil pressure to that main, just when you needed it most. 

So, could be many things, but your machinist should be able to find the cause. Let us know what the verdict is. :thumbsup:


----------

