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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old May 13th, 2017, 07:05 AM
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Default 401 Swap Out

Hello all,
I'm currently running a 305D in my 61 1500 stepside with a 3 speed standard on the column.

As has been the case for awhile, the engine needs some work. However, I'm hoping to have a backup engine sorted before I pull the current one (I don't want to have the old man down and out for too long while I'm working on the current engine).

Anyway, long story short, if I came across a 401, how much of a nightmare would it be to swap it in? Considering the beasts they were originally designed for, would a 401 even fit?
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Old May 13th, 2017, 08:01 AM
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Default Re: 401 Swap Out

The 401 will for the most be a drop in provided it has the 6 bolt crank flange that will accept the 305 flywheel. The only other issue is in the placement of the water pump housing. If you choose to use the 401 water pump housing then you'll need to move the radiator up higher so the fan lines up. If you choose the 305 water pump housing then you need to eliminate the 3 piece timing gear and replace it with the 305 double roller and timing chain and sprocket, this also requires eliminating the center idler gear shaft which is bolted on. It needs to be capped off with a 1 1/8 expansion plug(freeze plug) in order to block the idlers oil passage or else the engine will loose oil pressure which feeds the front main bearings. It may sound a lot to you but it's not all that bad I did the same with my 478M conversion and it all worked out just fine
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Old May 13th, 2017, 08:06 AM
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Default Re: 401 Swap Out

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Originally Posted by bigblockv6 View Post
It may sound a lot to you but it's not all that bad I did the same with my 478M conversion and it all worked out just fine
I'm still very green to this kind of stuff, so a lot of it sounds like magic to me LOL I'll do some reading on what you've said though, so that I can get a better understanding.

It sounds like a fairly permanent change though. IE once the 305 is how I want it, I couldn't just swap them again?
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Old May 13th, 2017, 08:12 AM
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Default Re: 401 Swap Out

Yes you could swap them again but why? The 401 is a vast improvement overall over the 305 and for the most looks the same if using the correct mounting brackets, and exhaust manifolds from a 351E
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Old May 13th, 2017, 08:19 AM
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Default Re: 401 Swap Out

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Originally Posted by bigblockv6 View Post
Yes you could swap them again but why? The 401 is a vast improvement overall over the 305 and for the most looks the same if using the correct mounting brackets, and exhaust manifolds from a 351E
Sentimental value I guess? The 305 is what came with the truck, and if I'm going to put the effort (and cost) into learning enough to rebuild it myself, I'd hate to let it go to waste.

Although, I have been trying to convince my wife that she would much prefer to drive an old truck..... maybe having a ready-to-go engine to drop into one would sweeten the pot a bit LOL
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Old May 14th, 2017, 08:57 AM
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Default Re: 401 Swap Out

Quote:
Originally Posted by bigblockv6 View Post
The 401 will for the most be a drop in provided it has the 6 bolt crank flange that will accept the 305 flywheel. The only other issue is in the placement of the water pump housing. If you choose to use the 401 water pump housing then you'll need to move the radiator up higher so the fan lines up. If you choose the 305 water pump housing then you need to eliminate the 3 piece timing gear and replace it with the 305 double roller and timing chain and sprocket, this also requires eliminating the center idler gear shaft which is bolted on. It needs to be capped off with a 1 1/8 expansion plug(freeze plug) in order to block the idlers oil passage or else the engine will loose oil pressure which feeds the front main bearings. It may sound a lot to you but it's not all that bad I did the same with my 478M conversion and it all worked out just fine
Another option is to simply remove the engine-driven fan from the 401 and use an electric fan instead. That way you don't have to do anything with the water pump housing or radiator. That's what I did on my '62 Suburban.
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Old May 14th, 2017, 05:50 PM
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Smile Re: 401 Swap Out

Quote:
Originally Posted by Ed Snyder View Post
Another option is to simply remove the engine-driven fan from the 401 and use an electric fan instead. That way you don't have to do anything with the water pump housing or radiator. That's what I did on my '62 Suburban.
Is not the big truck gear drive housing a lot heavier than changing to the pickup chain drive? Dave
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Old May 14th, 2017, 07:16 PM
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Default Re: 401 Swap Out

Both housing are just about as heavy, the biggest difference is the water pump sits higher on the big truck housing which also allows clearance inside for the 3 piece timing gear drive. It's up to you on which one to choose as you're given 3 alternatives to make it work in your truck. I originally chose to keep the 3 piece gear drive and modify the pickup truck housing to make it work. After two failed attempts which included cutting and welding the housing then seeing several gallons of coolant on my garage floor as well as two destroyed housings, I finally broke down and went with the double roller timing chain & sprocket and I'm quite happy with that. My truck being a 68 is a lower profile design than the 60-66 so there would have been no room at all to raise the radiator, I also wanted to preserve the stock appearance and retain all the original brackets and hoses
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