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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old September 21st, 2013, 02:10 AM
abus319 abus319 is offline
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Default stuck 478

Hey all, new old guy here. I was a member a few years back under an old email address.
I recently aquired a military surplus 478 that was rebuilt, but never used. Its stuck and after soaking it in diesel for a few days it will not budge. I tried bumping it using grade 8 bolts and a long bar but it bends the bolts. It doesnt have a flexplate or flywheel so any ideas on how to get some torque on the crank?
Thanks
H
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  #2  
Old September 21st, 2013, 04:36 AM
bigblockv6 bigblockv6 is offline
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Default Re: stuck 478

I would check out the bottom end, Pull the oil pan off, remove the main bearing caps and check if they're lubed. Maybe some "Marvel Mystery Oil" down the cylinders and if that doesn't work you may have to pull the heads of and check out the condition of the cylinder walls. If moisture got in that could have formed rust on the cylinder walls.
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Old September 21st, 2013, 05:17 AM
abus319 abus319 is offline
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Default Re: stuck 478

Hmm hadnt really thought of the mains, was thinking stuck rings. I will let it soak for a coupl more days then pull the pan. I wish I had borescoped the cyls before I poured all that diesel in.
Thanks
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Old September 21st, 2013, 05:38 AM
Foley Foley is offline
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Default Re: stuck 478

I say don't force it too much. Just get it loose. A few days isn't much time. I've had them soak for months before coming loose. If it's one or more pistons that are stuck, use whatever penetrating concochun you think is best. WD 40, Diesel fuel, Marvel Mystery Oil, Automatic transmission fluid, and some guys even swear by coca cola. Many people use a mixture of one or more of the above. If it's lower end, then getting those main bearing caps off will help and may show what's seized. If you run out of patience, do like Bigblock suggests and pull the heads. Good luck
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Old September 21st, 2013, 11:06 AM
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Rockdriller Rockdriller is offline
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Default Re: stuck 478

Quote:
Originally Posted by abus319 View Post
Hey all, new old guy here. I was a member a few years back under an old email address.
I recently aquired a military surplus 478 that was rebuilt, but never used. Its stuck and after soaking it in diesel for a few days it will not budge. I tried bumping it using grade 8 bolts and a long bar but it bends the bolts. It doesnt have a flexplate or flywheel so any ideas on how to get some torque on the crank?
Thanks
H
A feller could maybe pull the front balancer and leave the woodruff key in the crank shaft.......then get a big crescent wrench and use it on the crank shaft up against the woodruff key......should be able to develop a lil' bit more torque that way..........maybe...
Luck.
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Old September 22nd, 2013, 03:40 PM
abus319 abus319 is offline
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Default Re: stuck 478

months eh, well I guess i dont have an immediate need for it, and letting it sit and soak is the least expensive/invasive option. Meanwhile I think I will drill a plate to match the crank flange and weld a large nut to it. Then I can get the impact on it.
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Old September 22nd, 2013, 05:03 PM
Foley Foley is offline
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Default Re: stuck 478

I think that might be a good option. I like a 50/50 ATF/diesel mix, or all ATF in those cylinders. Get the large nut on there like you say, and using a big, 3/4" drive impact wrench, give it a 5 second blast with the wrench about once or twice a day. It's not so much the torque will break it loose at first, it's the combination of the torque and more important, the vibration from the wrench that will help the penetration.
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Old September 23rd, 2013, 01:20 AM
George Bongert George Bongert is offline
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Default Re: stuck 478

Howdy abus319!

RE: your stuck 478, I've been told a 50/50 mixture of laquer thinner and brake fluid will get even the most stubborn stuck engine to break loose. Of course, that's only what I was told, I never tried it myself. Since you have nothing to lose, it's worth a try. If you use this idea and it works, please post and let the rest of us know if it worked or not.
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  #9  
Old September 23rd, 2013, 03:06 AM
abus319 abus319 is offline
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Default Re: stuck 478

ohhh brake fluid and lacquer thinner,that sounds like a lethal combination. If the long soak with trans fluid and diesel doesnt work It might be worth a try. I think the impact will move it slightly enough to get the diesel behind the rings and get the job done. If not I will open it up like bigblock recomends.
Thanks everyone for your ideas and input.
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  #10  
Old October 4th, 2013, 02:42 AM
Mike Caldwell Mike Caldwell is offline
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Default Re: stuck 478

Don't forget to tighten as well as loosen with the big impact.

Remember these big engines weren't made to get in a hurry so we shouldn't either
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