Old 03-09-2024, 07:06 PM
Burbach 174 Burbach 174 is offline
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Location: Sioux Falls
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Default 60 389 Exhaust Bolt size mystery

I'm getting ready to remove the exhaust manifolds so I can get to the freeze plugs more easily but assuming I'll want to replace the manifold bolts, I'm not finding any information about them. All the vendors list 64 and up!
So dopes anyone know what the difference is and perhaps the diameter is different than later 389 blocks?
I did confirm going BACK TO 55 they should be the same but still can't find the bolt information w/o a parts book... Thank you!

Old 03-09-2024, 07:32 PM
tom s tom s is online now
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3/8 by something.Put a 9/16 wrench on a bolt you can reach and confirm.Maybe someone put anti seize on them,if not some broken bolts might be in your future.Tom

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Old 03-09-2024, 11:52 PM
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b-man b-man is offline
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They’re all definitely 3/8-16 UNC thread, Pontiac never deviated from this exhaust manifold fastener thread size.

It’s easy to find whatever bolts you need at a hardware store. Just make sure the bolts you use aren’t so long that they bottom out. Keep in mind that the end bolt holes intersect with the head bolts.

1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/2.56
2002 WS6 Convertible LS1/4L60E/3.23
Old 03-11-2024, 05:22 PM
Goatracer1 Goatracer1 is offline
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Different years/manifolds use different length bolts. Like b-man said they are all 3/8-16. If the manifolds have not been off recently you might be better off not trying to remove them.

Old 03-11-2024, 09:19 PM
Burbach 174 Burbach 174 is offline
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Thank you! This helps and here in Sioux Falls we have an excellent source for all of that then. Thankfully per the last owner it's a low mileage block but it did sit for 25 years after the suposed rebuiild. I've been spraying penetrant for the past weeks and will hit it w/ the torch when I start. I wouldn't remove them except he used steel freeze plugs and one was bad from sitting. I want to replace them all so I don't get stuck somewhere. Fingers crossed the flywheel side ones are pretty sound.

Old 03-11-2024, 11:11 PM
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Stuart Stuart is offline
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One thing to note is the heads on old exhaust manifold bolts can be 'eroded', I suppose from a combination of heat and rust over the years, so they may not fit tightly in your wrench when you go to remove them - it can be easy to round them off if you're not careful.

Old 03-12-2024, 09:08 AM
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Bill Hanlon Bill Hanlon is offline
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Using a 6 point socket (instead of a 12) might be a good idea.

My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic.

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