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GMC V6 and V12 Engines Engine repair and rebuilding

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  #11  
Old July 2nd, 2022, 04:29 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Quote:
Originally Posted by FetchMeAPepsi View Post
Is Pete banned or something? Haven't seen him around in a while.
Just talked to him and he can't log on because his phone broke down and locked his info. He wants to get back on but hasn't taken the time to do so.

Last edited by Prowbar; July 2nd, 2022 at 05:00 PM.
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  #12  
Old July 19th, 2022, 02:08 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Took the time to get everything measured on the 478. The wear is a little more than I first anticipated, but most falls on the outer end of the spec. A loose engine is a happy engine...

Also discussed basically everything with BB V6 about these engines, thank you very much for that Pete.

There's wear in the cylinder bores but I'll just re-hone them and re-use the existing rings. The pistons look good.

As you can see I disassembled, then decked the heads and exhaust ports.
The heads and block are now at the machine shop for sleeving of a couple of worn valve guides, and having the block cleaned. Positive valve seats will be installed also.

I first wanted hardened seats on the intakes (exhausts have them from the factory), but there is little meat in the intake runners for a seat. While it will be run on propane, it will not have to work hard in my pickup, and combined with these engines' excellent cooling capacity it should not be needed really.

The block will only be washed at the machine shop. Got most of the parts in for the rebuild except for the rod bearings. Main bearings are still in spec, and good to use.

The exhaust manifolds are badly warped and the (previous) mechanics tightened the bolts way too much to try to get it to seal again. Almost every bolt 'ear' has a crack and will need to be brazed back together, milled, then painted in factory yellow.
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  #13  
Old August 10th, 2022, 09:32 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Got the clean block from the machine shop. The heads are still there.
Got all the parts in: rod bearings, core (freeze) and oil galley plugs, timing chain, etc. Also bought me a Buick V6 even fire HEI to convert and use. This should definitely help with a cleaner burn in these massive bores.

Build me a homemade heavy-duty engine stand from an old engine hoist which was scrap iron. Works really well.

Now all that left is a re-hone of the block and then I can start on the short block assembly. Can't wait to get this powerhouse in my truck!
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  #14  
Old August 25th, 2022, 06:40 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Finally got around to hone the block. Also included is a picture of my homemade engine stand.

First I had to mill some spacers to use our hone for these massive bores. Then we honed the existing bores. There's more wear than I like, and not all of it has been honed out. However, I'm confident that this will produce sufficient results.

After all, this is kind of a budget build, and while an overbore might be better, where are you going to get oversize pistons from anyway?

First honing job has been done with coarse stones, to get a good crosshatch in the cylinders.

Because this engine will run on propane from the start, the rings will not really seat in the bores because of the superiority of propane fuels when it comes to preventing cylinder wear. Now I read about this trick a while back in an IMPCO book, which article I posted here on the forum which was about 401 V6's converted to propane.

In that article, they had troubles with the rings seating with the freshly rebuilt engines running on propane from the start. This resulted in excessive oil consumption. They solved this by seating in the rings with a mixture of Bon Ami and a thin oil and turning the engine over by hand. I will be doing the same thing, which involves assembling the engine with crankshaft and pistons, followed by seating the rings by turning over the engine, then taking it apart to clean the parts thoroughly. I'll see whether the first ring will seat with this method as it is still the original chrome-plated ring. Then I can assemble the short block for the last time.

In doing this, I should also achieve a plateau honing effect, which means that the peaks of the material left by the coarse hone will be evened out. The valleys created by the coarse hone should then be still present for oil retention purposes.

I added the article down below. When enlarged the orientation is good.
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  #15  
Old August 28th, 2022, 09:13 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Update: touched up the crosshatch in the cylinders to get complete coverage in the cylinder walls.

Coated the cylinder walls with a mix of ATF and Commandant red (which is a coarser polishing compound)

Assembled the crank, rods, and pistons in the block with assembly lube. Noted the position of the piston rings.

Turned the engine over by hand for about 100 turns, re-applying the coating in between. After a while, you can see a good, even thickness of the mixture on the cylinder walls with small vertical lines. Difficult to see in the photo. When I was satisfied I disassembled the engine again and cleaned the block with a lint-free rag and ATF to get the grit out.

Cleaned the pistons in the parts washer. Re-lubed everything, and installed the rear main seal. Then installed permanently.

After breaking in the crosshatch, it looks like a plateau-honed finish. Very satisfied with the results.

About the pistons: nowhere in the book is there mention of piston orientation in the block. nor can I find anything on the internet. No marks or arrows either on the pistons. Are they unidirectional?

Still need to install the plugs that came in the rear main seal kit into the 1/2" bolts in the rear main cap.

For anyone thinking that engine stand is about to collapse because the block hangs crooked, it isn't. I used standard diameter pipes for the inner and outer mount point on the engine stand and they have a lot of room between
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Last edited by Prowbar; August 28th, 2022 at 09:43 PM.
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  #16  
Old August 28th, 2022, 09:15 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

More pictures.
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  #17  
Old August 30th, 2022, 08:06 PM
James James is offline
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Default Re: Got a 478...

Quote:
Originally Posted by Prowbar View Post
About the pistons: nowhere in the book is there mention of piston orientation in the block. nor can I find anything on the internet. No marks or arrows either on the pistons. Are they unidirectional?
All of the GMC V6 pistons has a ZERO piston pin offset, so they can be installed in either direction. I did find a single reference on the Internet that made reference to the piston offset and it had stated it was 0".

I thought I had saved that info but I can't find it now.
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  #18  
Old August 31st, 2022, 06:47 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Quote:
Originally Posted by James View Post
All of the GMC V6 pistons has a ZERO piston pin offset, so they can be installed in either direction. I did find a single reference on the Internet that made reference to the piston offset and it had stated it was 0".

I thought I had saved that info but I can't find it now.
James, excellent, thanks.
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  #19  
Old September 22nd, 2022, 10:00 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Ok, it's been a while... But I've been hard at work on the 478 parts.

I managed to get my hands on a spin-on oil filter adapter, which is sweet.

The exhaust manifolds are almost completed. They were a mess, every ear on one manifold was cracked, one broken off. Probably because of exhaust leaks and the mechanic over-tightening them. Now I ground out the cracks and brazed them with brass rods. I also brazed on a new piece for the missing ear. I then removed the excess brass. After that I've milled them flat. There are two 'studs' in the casting, I believe those are factory locating pins for the millwork. A bolt with a shallow hole in it locates on that stud and the nut is used for adjustment. It took some work getting them flat.

The heads are also back from the machine shop, however, I have a small change of plans. Pete Chronis told me to get rid of the sodium-filled exhaust valves. They were known to snap and I don't want to take the risk. He also said some CAT valves would work. No CAT valve will work, however, I found a Cummins exhaust valve (3902254) that is basically identical to a non-magnum 478 exhaust valve, same head dia, stem length, non sodium filled. Therefore the stem is smaller at around .375" compared to the original 7/16" exhaust valve stem. Now the exhaust valve guides could use sleeving anyway. Therefore I am going to make bronze guides that will incorporate a positive valve stem seal. The valve stem seals are already installed on the intakes.

Also the big elephant in the room: the flywheel. I first figured turning a flywheel from a piece of plate myself. I have a donor 305 flywheel for dimensions. Now I went ahead and did some measuring, which I should have done first My 478 came with a round SAE truck bellhousing. Its flywheel is the same diameter as the 305!! However, it uses a larger clutch, pressure plate, pilot bearing and a coarser starter gear.

Now I am converting this flywheel to the 305 dimensions. As you can see I first removed the ring for the larger pressure plate. I then milled the step and drilled and tapped the holes in the clutch face for the pressure plate. Next up is to make a sleeve that presses into the pilot bearing bore to convert to the smaller pilot bearing. Still need to final grind the flywheel and turn the sleeve, and switch the starter gear.

Very satisfied so far with the progress. Lots of machine work but well worth it.
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  #20  
Old September 22nd, 2022, 10:02 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

OEM valve left and Cummins valve on the right. I'm probably going to add a small back cut to remove that awful ridge after the seat cut.
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