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

Re: 478 to SM420
 

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! :ok:

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.

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?

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.

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.

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:ahhhh: and for extra insurance make a steel plate cover to keep the plug secure:thumbsup:

Re: 478 to SM420
 

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.

Re: 478 to SM420
 

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

Chevy LS swap. Making life simpler..

Re: 478 to SM420
 

First of all lets not call the LS engine a Chevy:pullinghairout: Chevrolet does not deserve any credit for those engines:teehee:. 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:ahhhh: 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:thumbsup:

Re: 478 to SM420
 

So I stopped by my machinest tonight, you can imagine what he said about the V6. Something about "sorry". His biggest issue was the lack of parts availability. He has been a machinist for 40 years, he took me in the back and showed me he had an early 305 tore down, headed to the junk yard because he couldn't get the parts or the cost was too much to rebuild. He also stated the top end wasn't the problem but finding the parts for the rotating assembly, which I knew already.

Anyway, he didn't have the ability to balance the 478. Don't know what to do at this point, I don't have a lot of options for machine work on this side of the state.

Re: 478 to SM420
 

Go with the Chevy LS motor or stay stock 305. Balance the whole rotating assembly? Custom flywheel? Even the good old 454 will blow the doors off a 478 dump truck motor. You can't turn the beautiful GMC motors into anything but what they are. Nice, dependable, powerful, unique. new and used parts are not real easy/cheap to find, and get shipped to you, and get machined and so on.
Stay original or make life easy.

Re: 478 to SM420
 

Thanks AZKen, that is the way I am leaning at this point. This truck will be with me rest of my life and handed down to my boys so I want it to last. Plus I want to be able to drive it.

Re: 478 to SM420
 

Why can't you drive it with the 305E motor? Talk about lasting.........

Re: 478 to SM420
 

It may. It runs but I see blue smoke. At best I need to take a compression test, hopefully it is just the valve stem seals, then I would just need to get the heads done and have hardened seats installed. Compression test will be next. I may have a line on a good running 401m, might go that way.

Re: 478 to SM420
 

If you go with the 401M you can still get pistons these engines and if the 401M has the 6 bolt crankshaft flange which will allow you to use the 305E flywheel and bellhousing, it is an internal balanced engine so there is no weight on the balancer or flywheel.:thumbsup:

Re: 478 to SM420
 

yeah, I just had big inch dreams (don't tell my wife!). Looks like the 401 is still a pretty good gain on power compared to the 305 I have. 68hp and 90ftlb difference and it is a bolt in.

Re: 478 to SM420
 

A few mods like custom ground cam which I have specs for, a 4bbl carb, HEI and good dual exhaust should do good for the 401M.:thumbsup:

Re: 478 to SM420
 

that sounds good! I was planning HEI, FiTech fuel injection and dual exhaust already. Who grinds the cam?

Re: 478 to SM420
 

Fuel injection sounds good! Anyone who custom grinds cams can do it. I have sent out the specs to quite a few members and they were all able to get the cams ground to those specs. Just get online and search for Custom Cam grinders.

Re: 478 to SM420
 

Looking at the GMC V6 heads and the LS heads....port designs were very similar to! :teehee:

Re: 478 to SM420
 

Sounds like BBV6's info on 401 is a good compromise. Be sure to take some Video of your 305 running so that when you sell it, folks can see it ran good. Did it smoke after sitting for years or it just smokes? If it's after sitting, could "tighten up "after awhile, after some thermal cycles. Does it smoke out the exhaust or breather or both? A bunch or a little? I had a 56 Chevy as a kid and ran it on 90 wt gear oil...and it still smoked. The gas station guy would ask me three times if I meant 90 wt. Don't do that.

Re: 478 to SM420
 

Smokes light blue while running. This truck is off a wheat farm, it travelled in the fields all the time so I figure it has ate plenty of dirt in its life. I know the chassis has around 400k but I think this is the 2nd engine (at least).

Re: 478 to SM420
 

I've got an LS1 in my 02 Trans Am, I see no resemblance in the port design whatsoever:pullinghairout: The LS1 ports are what they call "Cathedral ports", which have a triangular shape at the top of the port.:teehee:

Re: 478 to SM420
 

I'll have to scout that picture again, the heads were different but the ports were pretty similar. Not giving chevy any credit by any means. :lolsmack2:

Re: 478 to SM420
 

Good news! My GMC will be staying all GMC. I found a rebuilt 305E with 0 miles, picking it up this weekend. Still plan on FiTech and HEI distributor.

Re: 478 to SM420
 

Hows your progress?

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