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Old April 30th, 2023, 05:42 PM
Prowbar Prowbar is offline
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Join Date: Dec 2020
Location: Friesland, the Netherlands
Truck: 1965 GMC 1500, 478 V6, SM420
Posts: 383
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Default Re: '65 GMC 1500 project. From the Netherlands

What was supposed to be an easy start-up didn't turn out to be one. When we went to start the engine we quickly had coolant between the block and heads. Had some problems with poor starter performance as well. After a lot of attempts without any success, we gave up and drained the oil. Full of coolant...

Then made the decision to take off the heads. This went rather easily and soon was discovered that the coolant passages in the block do not line up with the head gasket. See pictures. This NOS head gasket (picture 5) only relies on crush rings to do the sealing. I never noticed this misalignment during assembly. This probably caused the leakage.
Also discovered that the cylinder crush rings seat inside the bore. Which is bad for detonation and gasket life.

Anyway, I have some Fel-pro 305/351 V6 head gaskets. They rely on gasket material instead of crush rings to provide the seal on the coolant passages. Fel-pro also turned these coolant passages into slots, which do line up, which is great. Even if they did not completely cover the bore, the gasket material would take care of the sealing. Now the bores still need to be opened up. I have tested one and it can be done, by moving the crush ring to a larger diameter. Now to make some specialized tooling to facilitate the process...

I was given the original GMC 478 head gaskets (that came off the engine, picture 3) as well. They line up properly. Which got me thinking... Maybe I've been given NOS 401 V6 head gaskets... which would explain the crush rings sticking into the bores...
And for the coolant passages, they probably moved them away from the bores to deal with the larger 478 bores... But this is speculation.

I'm glad we discovered it now instead of later... But it makes you feel like an idiot for not noticing.
Attached Images
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Last edited by Prowbar; April 30th, 2023 at 08:59 PM.
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