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Transmissions and Rear Ends Three on the tree or four in the floor? Shift it all here.

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  #1  
Old March 9th, 2017, 04:11 AM
arepoff arepoff is offline
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Default 478 to SM420

I know, this has been gone over before but I can't find the answers I am looking for in the archives. I am thinking about replacing my 305E with a 478M in my 65 K1000.

1) To adapt the sm420 to the 478 I need either the 379 flywheel (rare) or have a custom flywheel made.

2) Does the sm420 bell housing bolt directly to the 478 SAE housing?

3) If the same exterior accessories and single row timing chain is used on the 478 it should weigh about the same as the 305.

Please let me know if I am correct. My grandfather bought this new and I inherited the truck, now that I am getting ready to restore it I would like to have more power but keep the V6 engine. I also plan to install an HEI distributor and a FiTech fuel injection once I modify the M intake manifold.

Thank you in advance for your advice.

Alexi
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  #2  
Old March 9th, 2017, 03:24 PM
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Default Re: 478 to SM420

Quote:
Originally Posted by arepoff View Post
I know, this has been gone over before but I can't find the answers I am looking for in the archives. I am thinking about replacing my 305E with a 478M in my 65 K1000.

1) To adapt the sm420 to the 478 I need either the 379 flywheel (rare) or have a custom flywheel made.

2) Does the sm420 bell housing bolt directly to the 478 SAE housing?

3) If the same exterior accessories and single row timing chain is used on the 478 it should weigh about the same as the 305.

Please let me know if I am correct. My grandfather bought this new and I inherited the truck, now that I am getting ready to restore it I would like to have more power but keep the V6 engine. I also plan to install an HEI distributor and a FiTech fuel injection once I modify the M intake manifold.

Thank you in advance for your advice.

Alexi
I haven't done a ton of research yet so someone correct me if I am wrong:

I believe you will need the smaller truck style bell housing as the big truck housings are much bigger and so would the trans bolt pattern for the big 5 speeds and other transmissions that came in them.

Just taking off most of the big accessories will make the 478 much lighter.

The HEI and fitech fuel injection system will be sweet! Be sure to post up power and fuel mileage results if possible!
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  #3  
Old March 9th, 2017, 03:27 PM
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Default Re: 478 to SM420

You will definitely need a 379 or custom flywheel, I went with a custom flywheel that was balanced together with my 478M. I was able to use the 305E bellhousing. If you're going to use a larger 478 flywheel and bellhousing then you would have to have the larger transmission out of a bigger series truck that corresponds with it.
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Old March 9th, 2017, 03:33 PM
arepoff arepoff is offline
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Default Re: 478 to SM420

ok, so the my bell housing will work, just need the custom flywheel. Did you have to take your engine in to have it all balanced together when they made the flywheel?
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Old March 9th, 2017, 11:53 PM
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Default Re: 478 to SM420

Yes, you have to balance the whole rotating assembly which includes pistons, rods crankshaft, flywheel and have weight added to the front pulley as well or find a 379 or 432 front balancer. On my 478 they welded weight to the front pulley and it was quite a bit of weight.
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Old March 9th, 2017, 11:56 PM
arepoff arepoff is offline
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Default Re: 478 to SM420

I guess nothing is simple. I would assume an engine building machine shop could balance the 478, also assume the whole engine would need to be disassembled and each part weighed. I will need to check with my local machine shop to see if they can handle it.

I think that even I had the 379 balancer to go with the flywheel it would still need to balanced given the piston weight difference of the larger 478 piston.

Next stop machine shop to see if they can handle this.

Thank you.

Last edited by arepoff; March 10th, 2017 at 12:09 AM.
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Old March 10th, 2017, 12:06 AM
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Default Re: 478 to SM420

Just taking off most of the big accessories will make the 478 much lighter.

Very true, I also went with the light duty truck water pump/ timing cover housing. This also requires elimination of the 3 piece timing gear drive and replacing it with the double roller timing chain & sprocket set used on 305, 351 & 379 engines, I was told there's less drag and really not necessary to have the gear drive on a light duty application no matter how cool a gear drive setup is. When doing this conversion you have to remove the center idler shaft which is has an oil passage that must be capped off with a 1 1/8 inch freeze plug or you will lose oil pressure and for extra insurance make a steel plate cover to keep the plug secure
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Old March 10th, 2017, 12:09 AM
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Default Re: 478 to SM420

Quote:
Originally Posted by arepoff View Post
I guess nothing is simple. I would assume an engine building machine shop could balance the 478, also assume the whole engine would need to be disassembled and each part weighed. I will need to check with my local machine shop to see if they can handle it. Thank you.
Yes not every machine shop will have the bob weights for the GMC V6, I do remember there was some engine remanufacturer in northern Oregon that specialized in these engines back in the 90's.
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Old March 10th, 2017, 12:10 AM
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Default Re: 478 to SM420

I know everyone on this site will hate me when I state this....

Chevy LS swap. Making life simpler..
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Old March 10th, 2017, 02:35 AM
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Default Re: 478 to SM420

First of all lets not call the LS engine a Chevy Chevrolet does not deserve any credit for those engines. It is a Corporate GM engine manufactured by "The GM Powertrain Division" and was used by all GM divisions. I'd rather see someone stick one of these in their GMC rather than that ubiquitous small block 350 Just whatever you do don't give up on the 478 project because you'll have something that's different yet pure GMC and will have more than adequate power
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