Differences in 400 & 350 what will & will not swap?

[Preston]

I posted a forum about an unbalanced engine, that had a recent transmission rebuild what if they put a flex plate and torque converter in for a 350 and the engine is a 400 would that cause a balance issue? Also what is the difference in a fuel pump with 3 lines instead of 2? Thanks for your help

 

[78C10BigTen]

I posted a forum about an unbalanced engine, that had a recent transmission rebuild what if they put a flex plate and torque converter in for a 350 and the engine is a 400 would that cause a balance issue? Also what is the difference in a fuel pump with 3 lines instead of 2? Thanks for your help

3 line pump has a return to the tank, 2 does not. Im gunna say the flex for a 400 and 350 wont matter but dont quote me, smarter brains will tell you better.

 

[animal]

The 350 flex plate on a 400 will be an issue. Ask me how I know... Had to go back, find the 400 plate I had chunked and put it on. Motor ran like poop with the 350 plate. The 350 is internally balanced. The 400 is external balanced.

 

[78C10BigTen]

The 350 flex plate on a 400 will be an issue. Ask me how I know... Had to go back, find the 400 plate I had chunked and put it on. Motor ran like poop with the 350 plate. The 350 is internally balanced. The 400 is external balanced.

Yea now that you say that it does ring a bell... forgive me. Thats why i said smarter brains will reply

 

[animal]

Yea now that you say that it does ring a bell... forgive me. Thats why i said smarter brains will reply

I'm not that smart or I would not have found out the hard way. That's been more years ago than I care to remember. I can't even guess now why I would have removed the 400 plate just to put the 350 plate on the motor.

 

[Preston]

Thanks, now all I need to do is confirm I have a 400 and figure out what needs to be fixes, I have also read that the harmonic balancer can separate and cause some balance issues. As for the fuel pump I didn’t think there was a return line on an 84 k1500. May have been a conversion done, will keep investigating. Thanks

 

[PrairieDrifter]

Along with needing the correct flex plate you also need the correct balancer. The 400 harmonic balancer just like the flex plate, is also specific to the 400 sbc.

I believe most of the main components are specific to the 400. But things like covers and manifolds and accessories are the same

 

[PrairieDrifter]

If the balancer is still there that’s an easy way to confirm it, as long as no one messed it up before.

The 400 balancer has specific relief cutouts around the outside perimeter, where as a small block or big block has a completely smooth outside perimeter.

 

[Colin]

I'm staying out of this. Getting confused between 350 & 400 cu.in. SBC and
TH 350 and TH 400 transmissions.
My head hurts...

 

[PrairieDrifter]

I'm staying out of this. Getting confused between 350 & 400 cu.in. SBC and
TH 350 and TH 400 transmissions.
My head hurts...

Now that you mention it lol

 

[TravisB]

The heads are not the same so beware there. You can use 400 heads on a 350 but you cannot put 350 heads (in stock form) on a 400. The 400 has additional oil passages that can be drilled by a machine shop into other sbc heads. The pattern is the same but you will be unhappy soon after swapping them out. I didn't know this until I needed a set of heads and had the machine shop check out a set of corvette heads I picked up before installing them.

I guess they could have just told me that and took my money... anyway I paid it at the time and don't have picture proof of the difference but if it's accurate you should be able to verify that pretty easily.

 

[omillan99]

The heads are not the same so beware there. You can use 400 heads on a 350 but you cannot put 350 heads (in stock form) on a 400. The 400 has additional oil passages that can be drilled by a machine shop into other sbc heads. The pattern is the same but you will be unhappy soon after swapping them out. I didn't know this until I needed a set of heads and had the machine shop check out a set of corvette heads I picked up before installing them.

I guess they could have just told me that and took my money... anyway I paid it at the time and don't have picture proof of the difference but if it's accurate you should be able to verify that pretty easily.

If I'm not mistaken its actually a port for the steam.

 

[TravisB]

If I'm not mistaken its actually a port for the steam.

You might be right about that. It's been several years lol

 

[Poppy 87]

Flexplate/flywheel, balancer, cylinder heads, and head gasket are 400 sbc specific without modification

Sent from my XT1254 using Tapatalk

 

[75gmck25]

This is an article on converting SBC 350 heads to work on a 400. However, I have no experience with actually doing it. https://www.gregsengine.com/converting-350-heads-to-400.html

Bruce