350 to 400 small block

[Obwonkonobe]

Hey all, ive got a stock 78 c10 with no smog and the typical basic edelbrock carb upgrade on a stock sbc 350. Its sitting in front of a decent th350 as of right now. Im 17 and have been fixing the truck up for a year now, just got it driving relaibly with ac. But id like to start building an engine for it. Considering the absolute hell it was to get my accessorys and sending unite etc figured out for my current small block 350, id like a painless transition to a 400. My assumption is that over the next couple months i can slowly rebuild a block and heads for a 400, then take all my sending units and exhaust manifolds and accessory brackets and distributor and fuel pump and the th350 trans and bolt them to the new motor for the swap. Basicly before i start this i want to be sure that the only difference berween motors is the displacement, and that everything else will go smoothly. If anyone knows any differences or difficulties i may encounter please enlighten me.

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thanks in advance

 

[Rusty Nail]

Looks clean, nice work.

Your basic statenents regarding swapping all of the senders, manifolds, etc from 350 to 400 are correct BEING MINDFUL of the balance difference.
I swapped from a 400 to 350 in 2017 while I rebuild my 400. I might can help.

I do NOT assume that you are aware of the balance difference between the engines , or you would have opened with that because other than cubes, that is A major difference.
A 350 is internally balanced while a 400 is externally balanced.

What does that mean?
The stroke of a 400 crank is much larger than a 350. Mallory metal is expensive all around. GM balanced the rotating assembly of a 400 sbc using additional weights on BOTH the flywheel AND the harmonic balancer. These are easily identified with the naked eye on an assembled engine.
The harmonic bslancer is different between 350 to 400.
If you got a balancer with your 400, this is nothing to concern with. Be certain to keep them separate.
The flywheel of a 400 has a large weight welded to it. Like before, if you got one with the 400 there's no worries. The point is so you know those two parts are NOT interchangeable.
It's important. If you need to source those parts, you MUST buy them for a 400.

Everything else is 100% changeover.

I had some trouble with my motor mount clamshells. Turns out they are vehicle specific, I guess. You must use the ones from the truck the motor goes in. The 350 ones IN THIS CASE.
The connections to your existing exhaust are kind of specific also. Some times the factory exhaust manifolds won't interchange. If you have two different pair they may not interchange with your existing exhaust.

Try hard to swap every piece of the accessory brackets and stuff. That can make some headache. For real.

At least initally, I recommend keeping everything very separated. After you get it turn-key, that's a different story. Gloves off!

You can use the same # spark plugs and everything.

Stock 400s are not rev devils. The connecting rods are very short.

I think that should be very helpful to a guy like you, in your situation. Good luck. If I can help in the future i'll give it a shot but there's gotta be pics or gtfo.

And dude, buy a taller air filter for Pete's sake!
Here's the MicroGard part #.

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[Obwonkonobe]

@Rusty Nail thanks a TON. That cleared up a lot for me lol, and don't worry some of those pics are old I have a bigger one now. So when I go building the 400, do I still need to be sure all the pistons and rods are the same weight? I've seen some videos on rebuilds and they'll shave material to get them all to weigh the same. Also for heads, do i need to find 400 heads specifically, or could i use some rebuild 350 ones, same with the intake. And im assuming the torque converter will still bolt up without issues?

 

[Rusty Nail]

Yes, the torque converter will work fine - no issues. There are no intake manifold issues.

The cylinder heads are another difference between 350 and 400.
I hesitated to open that can o worms in the previous post. It was long-winded enough!

400 cylinder heads are specific. MUST use 400 head gaskets. MUST.
400s have a "steam hole" drilled between the middle cylinder's cooling passages.
The middle ones are called "siamese" cylinders...because are connected to one another, unlike other sbc engines.
#3-5 & #4-6 share metal.

Cylinder heads are NOT interchangeable -without modification.
I say without modification because you can drill the necessary "steam hole" into a head without it, making it 400 friendly. You can use 400 heads on a 350, but not the other way around, WITHOUT the steam hole mod.
The necessary modification is as simple as driĺling a hole into the cylinder head...
Machine shops can do it or I reckon you can - I never have.
A 400 head gasket will have the extra hole that the 350 head won't have. You can use that as a template and mark it , they go at an angle, I am not qualified to instruct you how to do it but that is the deal. It's small I think 1/16?
There is a thing. The hole and the passage with the steam and the angles and the gaskets...yeah it's not a bolt-on deal.
A 400 cylinder is a quarter inch bigger diameter than a 350.

Finally with the piston weight, you are referencing engine balancing. Every engine is balanced in one way or another. The degree to which you wish to go is up to you. It can be however involved as you want, I guess. That's what you are talking about "shaving pistons". The answer to the first question is Yes.That's engine balancing and kind of where we started this discussion @ mallory metal.

 

[Obwonkonobe]

@Rusty Nail thanks a ton, answered all my quiestions perfectly

 

[Honky Kong jr]

Look at @Rusty Nail being all helpful and **** me personally don’t see the draw to the SB400 but I’ve been to that movie and it didn’t end well to me lol big block

 

[Obwonkonobe]

@Honky Kong jr eventually I want to go big block but the swap seems far easier and cheaper if i stay small block

Does anyone know what i should pay for just a block and rotating assembly and heads, unmachined?

 

[Honky Kong jr]

@Honky Kong jr eventually I want to go big block but the swap seems far easier and cheaper if i stay small block

Does anyone know what i should pay for just a block and rotating assembly and heads, unmachined?

Rusty will argue with me but I think you’d be better off sourcing another 350 4 bolt block bore it to 355 just to clean up the cylinders and some new pistons of course. Bump the compression up to 10:1 maybe 10.5:1 and have a way more coolant system friendly set up. 400s are a bitch to keep cool with any kind of compression thrown at them. Unless your gonna run E85 then have at it with an 11:1 400 but that’s another can of worms in itself.

 

[Obwonkonobe]

@Honky Kong jr I've actually already upgraded to an aluminum 3 row rad, after my current 350 blew the old copper one. And as far boring it over I would, but I cant afford the machine shop, so im trying a 400 to get some cheap cubes

 

[MrMarty51]

I have a 400 SBC setting in the shed, bored .030 over and ready for fitting pistons.
It has the main bearing caps too. No heads, not rotating assembly, just the block.
If You know of someone getting near Billings Montana, I could send it along.
I live 150 miles to the east of there but could probably find a ride over to Billings for it, or haul it Myself if need be.

 

[Honky Kong jr]

@Honky Kong jr I've actually already upgraded to an aluminum 3 row rad, after my current 350 blew the old copper one. And as far boring it over I would, but I cant afford the machine shop, so im trying a 400 to get some cheap cubes

I understand all that but a 10:1 355 will out perform a 8.5:1 400 all day long. The Siamese cylinders Rusty mentioned is what makes them hard to cool. But if you keep the compression down you will be fine I ran a 406 in a Monza I ran great but eventually ate itself and for the cash I had in it I could of built a traditional SBC and been better off but I was young back then and my vision was skewed all I saw was more inches.

 

[MrMarty51]

I understand all that but a 10:1 355 will out perform a 8.5:1 400 all day long. The Siamese cylinders Rusty mentioned is what makes them hard to cool. But if you keep the compression down you will be fine I ran a 406 in a Monza I ran great but eventually ate itself and for the cash I had in it I could of built a traditional SBC and been better off but I was young back then and my vision was skewed all I saw was more inches.

I once did a 350 with three deuces into a 72 Vega notch back. It was quick

 

[CSFJ]

The other thing to keep in mind, is the ability to find a 400sbc in running condition. The guys that like them, tend to hang onto them. The only ones I ever see pop up on Craigslist are guys asking a few thousand for a supposed complete performance rebuild, or basket cases that WILL DEFINITELY require machining.

 

[Obwonkonobe]

@CSFJ ive actualy found a 2 bolt main block, rotating assembly, heads and valve train for a 400 on craigslist that im eying, 250 bucks. Its rough and stock but its there

 

[Honky Kong jr]

@CSFJ ive actualy found a 2 bolt main block, rotating assembly, heads and valve train for a 400 on craigslist that im eying, 250 bucks. Its rough and stock but its there

Be careful