spun main bearing
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spun main bearing
Hey Folks,I could really use some help here.I've built a few engines over the years but never came across this problem.I'm putting together a 383 small block.I bought a block already bored.It looked ok.I took it to my machine shop to have it honed and fit the pistons.When I got it home and set the crankshaft in and started to check my clearences,I couldn't get #3 to plastigage to comfortable clearences,half the journal was two and a half thousandths and the other half was over four thousandths.I took the crankshaft to my machine shop and the crank checked out ok.I brought him the block and he discovered #3 bearing had spun and was seven thousandths out of tolerance.He said the block was toast.Can't that journal be align bored and use an oversize(bigger)main bearing?I'm into this block for around $500 bucks and would like to save it if I could.Any suggestions?Thanks
Safety Car
Joined: Nov 2008
Posts: 3,720
Likes: 554
From: Seattle Washington
There is a couple of diffrent ways to repair this if the block was very valuable. I Sleeve could be made between the Block and Bearing. The Block could be machined out and then built up with Belzona and then have that machined back to the original size The block could be Flame Sprayed which is kinda like Chrome Plating and then that machined back to the original size. The Block could be fitted with all new Main Caps and then Align Bored which would move the Crank upward. There are probably a few more ways of doing it. Only problem is all of these far exceed the Value of the Block, and it would be better to start over. The only cheap way possibly would be to fit one new Cap at the #3, and Align Bore it with the idea that the crank push's down against the cap and not the block.. Even if You could talk a machine shop into doing this it would still be close to the value of a replacement block.
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From: New Or-leens Loo-z-anna
There is a couple of diffrent ways to repair this if the block was very valuable. I Sleeve could be made between the Block and Bearing. The Block could be machined out and then built up with Belzona and then have that machined back to the original size The block could be Flame Sprayed which is kinda like Chrome Plating and then that machined back to the original size. The Block could be fitted with all new Main Caps and then Align Bored which would move the Crank upward. There are probably a few more ways of doing it. Only problem is all of these far exceed the Value of the Block, and it would be better to start over. The only cheap way possibly would be to fit one new Cap at the #3, and Align Bore it with the idea that the crank push's down against the cap and not the block.. Even if You could talk a machine shop into doing this it would still be close to the value of a replacement block.
THEN use Red Locktite to hold the shims in place against the block bore.
Wrap the crank journal with one layer of .001" shim stock and install the crankshaft.
Allow it to remain still for several hours so the shims conform to the crank journal while the Loctite cures.
THEN remove the #3 cap/remove the .001" shim wrap/use plastigauge to check the clearance with the cap bolted in place.
For example:
If the upper half shows .003" clearance...install a .002" shim stock in the upper half of the block bore with Locltite.
If the lower half shows .004" clearance...install a .003" shim stock in the lower half of the block bore with Locktite.
Let us know what you decide and the outcome
Last edited by doorgunner; Apr 19, 2021 at 04:18 AM.
Drifting
Joined: Aug 2004
Posts: 1,883
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From: Lubbock TX
Do you have pics?
I don't think I have ever seen a block with a true spun bearing that didn't show it upon inspection. Maybe spun just enough to knock the bearing shell out of place and cut off oil to the crank - that'll cook the main saddles and knock the cap out of round - that I have seen a few times. The only indication I had was the oil that was cooked onto the cap and saddle/webbing area.
Maybe now you know why the already bored block was such a good price...hate to say it but I've been there, done that.
I don't think I have ever seen a block with a true spun bearing that didn't show it upon inspection. Maybe spun just enough to knock the bearing shell out of place and cut off oil to the crank - that'll cook the main saddles and knock the cap out of round - that I have seen a few times. The only indication I had was the oil that was cooked onto the cap and saddle/webbing area.
Maybe now you know why the already bored block was such a good price...hate to say it but I've been there, done that.
Le Mans Master
Joined: Mar 2004
Posts: 7,499
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From: Little Rock AR
Why throw more money at this? Reminds me of that joke where the guy was sitting on the toilet and stands up to pull his pants up and a nickle falls out of his pocket and lands in the bowl. He pulls his wallet out and grabs a $20 bill then tosses that into the bowl along with the nickle......
"Well, I wasn't going to reach in there for just a nickle"
Cut your losses early.
"Well, I wasn't going to reach in there for just a nickle"
Cut your losses early.
Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
I just went through the same thing. Fortunately I didn't spend any money on the block before deciding to scrap it. Broken crank and spun main bearing. Otherwise the block looked fine. Consulted with several experienced and knowledgeable engine builders. The unanimous opinion was that the block was toast. I got about $3 for it from the scrap metal dealer.
Don't risk an expensive engine build with a bad block. Cut your losses.
Don't risk an expensive engine build with a bad block. Cut your losses.
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Joined: Oct 2011
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Thanks for getting back to me.I might be dead in the water.I did talk to King Bearing.They said that they have a oversize insert.Depending on what section of the block journal is out of round,I might be able align bore the block and use their set of main bearings.I'm going to talk to my machine shop today.Thanks again for your suggestion!
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Joined: Oct 2011
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Don't have any photo's.All the mains look ok.There's nothing that really stands out.The machine shop didn't notice it.Nothing seemed wonky untill I could'nt get the clearence that it should have.I spoke to King Bearing and they said the have an oversize insert.I'd have to align bore the block and use their bearings.I'm heading out to talk to the machine shop.Thanks for your suggestion!
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Joined: Oct 2011
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There is a couple of diffrent ways to repair this if the block was very valuable. I Sleeve could be made between the Block and Bearing. The Block could be machined out and then built up with Belzona and then have that machined back to the original size The block could be Flame Sprayed which is kinda like Chrome Plating and then that machined back to the original size. The Block could be fitted with all new Main Caps and then Align Bored which would move the Crank upward. There are probably a few more ways of doing it. Only problem is all of these far exceed the Value of the Block, and it would be better to start over. The only cheap way possibly would be to fit one new Cap at the #3, and Align Bore it with the idea that the crank push's down against the cap and not the block.. Even if You could talk a machine shop into doing this it would still be close to the value of a replacement block.
Safety Car
Joined: Nov 2008
Posts: 3,720
Likes: 554
From: Seattle Washington
I think You also have the choice of using 400 Bearings in it. The SBC 400 had larger main bearings both in OD and ID. GM never made it but the aftermarket quickly did they make a Thick Bearing to put a 350 Crank in a 400 Block. So You could have the Block Machined out to the 400 OD and use the Bearings with the 350 ID. Some might say Your going to make the Caps weaker by doing this. This all sounds simple but its a much bigger job than just align boring and the costs of doing it will quickly add up up. If a larger OD bearing is available that only requires an align bore then thats the way to go. Or another block.
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Joined: Oct 2011
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I think You also have the choice of using 400 Bearings in it. The SBC 400 had larger main bearings both in OD and ID. GM never made it but the aftermarket quickly did they make a Thick Bearing to put a 350 Crank in a 400 Block. So You could have the Block Machined out to the 400 OD and use the Bearings with the 350 ID. Some might say Your going to make the Caps weaker by doing this. This all sounds simple but its a much bigger job than just align boring and the costs of doing it will quickly add up up. If a larger OD bearing is available that only requires an align bore then thats the way to go. Or another block.
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Joined: Oct 2011
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