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GMC V6 and V12 Engines Engine repair and rebuilding

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  #21  
Old September 24th, 2022, 10:48 PM
Prowbar Prowbar is offline
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Location: Friesland, the Netherlands
Truck: 1965 GMC 1500, 305E, SM420
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Default Re: Got a 478...

Finished the flywheel today, ground the flywheel flat for the last time, made the conversion sleeve for the pilot bearing and pressed it in, and installed the starter gear from the spare 305 flywheel. There are some surface cracks but nothing cracked through, I'll be running it this way we'll see how it goes. Thinking it'll have plenty of life left.

Now onto the cylinder heads. Going to have to modify and install exhaust valve guides. I found valve guides from a Volvo Penta engine that will be usable for my conversion. These guides will be modified to accept good valve stem seals.

A 3 angle valve job is also on the list.

I'm also looking into a modification on the idler gear that will allow me to run the gear setup with the 305 timing cover, this will involve some machine work also. Some say it cannot be done but I'm not a believer yet...
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  #22  
Old October 2nd, 2022, 02:28 PM
Prowbar Prowbar is offline
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Join Date: Dec 2020
Location: Friesland, the Netherlands
Truck: 1965 GMC 1500, 305E, SM420
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Default Re: Got a 478...

Happy little manifolds...

In factory yellow. It grows on you

Pete Chronis told me that the manifolds were painted with yellow exhaust manifold paint. When assembled on the engine, the complete engine was then painted red, including the manifolds. As the red paint could not withstand the heat, it burned off and revealed the yellow paint underneath.

I found a trace of both red and yellow paint on the manifolds. On my 305 there is quite a lot of yellow paint left, including on the Y-pipe.

Next up: head work. I have to drill out the original guides, modify the new guides and press them in the heads. Then valve-related work. Still, lots to do but coming along slowly. Have lots of new parts to go on there including motor mounts. I cannot wait to pull out the 305 for the 478. Should make a ton of power.

When the 305 is out I am going to complete the frame paint in the engine bay, and paint the firewall in 503 light green.
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  #23  
Old November 12th, 2022, 09:13 PM
Prowbar Prowbar is offline
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Truck: 1965 GMC 1500, 305E, SM420
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Default Re: Got a 478...

It's been a while. Coming along slowly as usual...

First off I finished the intake manifold. I bead-blasted the old paint off and machined the gasket surfaces. Usually, I don't bead blast anything, but the paint was in pretty bad shape and the whole intake was full of rock-hard carbon deposits. When disassembling the heads I also noticed a lot of carbon buildup, it probably used a lot of oil when it was still running. Bead blasting was the only way to clean the insides of the intake. I also cleaned up some of the rough casting spots in the runners.

FYI: The intake manifold angle between the gasket surfaces is 88 degrees. Very odd angle, luckily we have a large milling machine that has a head that can be angled in order to fly cut the surfaces. We also have a small mill that can tilt both the head and bed (in 2 ways). Very versatile machine to have.

As I say in every post that this engine will run 99% of the time on propane I'm implementing some tricks to get better performance on propane from the engine.

On my old 305 with its rusted heat riser (which I wired open later) the intake always got hot. With propane use, this is not satisfactory. Therefore I machined pockets in the intake exhaust gas passages in which a block-off plate will be mounted. This blocks off the stream of exhaust gas from entering the intake manifold, keeping it cooler. It is easy to remove the intake to remove the block-off plates or drill a hole in them to fine-tune the intake temperature if wanted.

Still working on the heads. I machined valve guides and reamed out the exhaust valve guides. More pictures and info on that later, however, I installed the valve guides in the first head today. They are a very light press fit and they have a shoulder so they can never fall into the combustion chamber. A positive valve stem seal can be mounted on top of the guide, which will be installed later. Now onto recutting the seats, with a 3-angle valve job.

Also modified the exhaust valve retainers to accept the Cummins valves and keepers. They are a little larger and have a slightly narrower angle. I made a simple fixture to recut them in the lathe. A little finetuning on the angle and it was spot on.

More to come, when the head work is finished I can start assembling the engine. Still have a couple of things to do such as modifying the idler gear to work with the 305 timing cover, finishing the oil filter bypass valve, and adding a ported vacuum source to the carburetor, and exhaust flanges.

I'm making a bypass valve for the oil filter because the original piece got lost in the mail. I'm making one after my own design, which should be an improvement in regards to the plunger bouching and chattering on its seat. More on that later.
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  #24  
Old November 20th, 2022, 04:44 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Here is the modification for the idler gear. To use the 305 timing cover with the gear setup you have to shorten the length of the idler gear nose or snout as it sticks out too far. The 478 timing cover has a large dimple cast into it to house the snout. This idler gear is pretty over-dimensioned. it gets fed with a continuous supply of oil from the main oil galley. I figured a little less bearing surface wouldn't hurt a thing. The idler gear assembly was still as tight as the day it left the factory.

After measuring the depth of the 478 cover I chose to shorten the snout to fit the 305 cover. The depth of the cover is about 36mm (1.42"). The stock idler gear is about 56.5mm (2.23") including the bolts to retain the front plate.

I went ahead and indicated the idler gear shaft, which was then shortened by about 9/16". I also polished the surface. I tapped the old oil supply hole and screwed in a set screw is retained with LocTite. A new oil hole is drilled opposite the old hole to supply the oil. I then made a tool to press the bronze bushing into the idler gear 9/32" (half the distance) deep. This lined up the oil groove in the bushing with the new oil hole. The existing 5/16 UNC holes were tapped deeper and a new front plate was made, with uses countersunk fasteners to reduce the depth. It is coated on the inside with LocTite gasket sealer to cap off the oil galley. These bolts are also secured with Loctite. I tried to reuse the oil front plate but it was too hard to machine.

The idler gear was then indicated and shortened in the lathe by the same amount, keeping the stock axial play in the setup. The total length of the assembly from the engine to the front plate is now a little less than 1.42". There's always a little variation in the casting which means I have to grind a little clearance into the timing cover, but that is no issue since it is very little material. I did not want to shorter the idler gear any further because I would intersect the old oil galley that I now capped off with a set screw.

Machining was surprisingly easy. The idler gear mount is made from a good grade of cast iron, and the bearing surface is slightly hardened. The gear is made from cast steel which also machined beautifully.
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  #25  
Old November 20th, 2022, 04:46 PM
Prowbar Prowbar is offline
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Default Re: Got a 478...

Machining of the idler gear and the final product. After machining, the parts were cleaned and the bearing surface was coated with assembly lube to await installation.
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