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1978 GMC K25

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1978 GMC K25 Empty 1978 GMC K25

Post by Iggy on Tue Apr 14, 2020 8:51 pm

This is a truck I picked up last fall to take to my place in the Upper Peninsula
1978 GMC K25 Img_1010
1978 GMC K25 Img_1011
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by Iggy on Tue Apr 14, 2020 9:08 pm

I put those there to see how pictures were posting. I bought the truck almost three hour drive from where I live and drove it home when I bought it. It ran fine, even on the expressway! However, something was nagging at me so I decided to tear the engine down to go through it. I found the engine was a 1972 400 SBC from a full size car, it had a 1972 400 head on the drivers side, a 1973 350 head on the passenger side! My gut feeling was trying to tell me something.

Pulled the heads, at least the 350 head had steam hole put in it, the 400 head was cracked from the exhaust port to the spark plug on two cylinders. Those two cylinders were clean enough to see that it still had original pistons
1978 GMC K25 Img_1110
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by Iggy on Tue Apr 14, 2020 9:13 pm

At this point I knew I needed to tear down the engine to see how badly the block was cracked! Much to my surprise the block was the only thing I could salvage from the engine. It turned out to be crack free and had no ridge! The crankshaft was a completely different story - it came out of a 1974 engine and looks like it was part of a spectacular failure! It was ground .020 on the rods and .010 on the mains. I refused to reuse it and bought a Scat 400 crank.
1978 GMC K25 Img_1113
1978 GMC K25 Img_1111
1978 GMC K25 Img_1112
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by Iggy on Tue Apr 14, 2020 9:17 pm

I decided to have the block machined - it cleaned up at .020 over, was align honed with main studs and decked .0101978 GMC K25 Img_1114
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Here it is mocked up so we could make sure everything cleared. New Scat crank, Scat 5.7 rods and KB dish pistons. After that it was disassembled and taken to be balanced.
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by Iggy on Tue Apr 14, 2020 9:24 pm

Because the heads were cracked, I took a set of Vortec heads in to have steam holes added - they were already rebuilt with new guides and valves and are getting new springs on them now.
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At this point I am waiting to get things back so it can be assembled and run on a break-in stand. Nothing fancy here, mainly parts I had laying around as far as the cam and internals - cam is a Summit 1103 that never got installed, Vortec heads were something I traded a set of Dart TBI heads for - I had to buy crank, rods and pistons!
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by Iggy on Tue Apr 14, 2020 9:31 pm

Because of the current slow down in everything and basic shutdown in Michigan - the truck sits until I can get some help!
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I have a TH400 and NP241C that I was going to put in a K5 Blazer, but I got an offer I could not refuse on the rolling chassis, so the freshly rebuilt TH400/NP241 will get put in this along with a fresh 400 small block. The truck came with a set of Doug's Tri-"Y" headers in great shape so I will use those and a dual exhaust system. Will also use a Quadrajet I have that was rebuilt by Cliff Ruggles for a project like this!
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by bigredlaguna on Tue Apr 14, 2020 9:48 pm

That engine should be a stump puller when you are done. Don't you just love dealing with somebody else's train wreck?
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by thatfnthing on Tue Apr 14, 2020 10:41 pm

X2 on that.

And I love my Doug Tri-Y's. You will too. Smile
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by Iggy on Wed Apr 15, 2020 9:34 am

The truck is from Arizona, fenders were rusted just behind the front tires and along the top of the wheel opening. Cab is solid, but the bottom of the doors are rusted. I had a 1977 K20 from California a couple of years ago that were rusted in the same areas - looks like dirt accumulated there and held moisture when there was some. My shop floor looks like the Arizona desert as it falls of the bottom of the truck! I have new fenders, inner fenders and a set of good used doors that I need to get to my body guy when he is ready.

Since I have a new fuel take for the Blazer I sold, I am considering pulling the side saddle tanks and putting a single 30 gallon tank behind the rear axle under the bed.
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1978 GMC K25 Empty Re: 1978 GMC K25

Post by Iggy on Wed Apr 15, 2020 11:34 am

bigredlaguna wrote:That engine should be a stump puller when you are done. Don't you just love dealing with somebody else's train wreck?

I am counting on it being a stump puller! At least when I made an offer far below what the guy was asking I assumed the driveline including the engine was toast! I already had laying around the shop a fresh 1974 350, the TH400 and NP241 so I knew I had a complete driveline, but really wanted something bigger than a 350. My options were rebuild the 400 if it was fine or put together a 454 which I have several blocks and cranks ready to go. I am glad the 400 block checked out as I wanted to stay with a small block since I did not have the HD front springs or any extra BBC headers for the truck (I do for the Laguna).
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