76 K20 400 Small Block Chevy questions

I’ve been Rebuilding a 400 small block, and the previous owner who owned it slapped on a 350 head as well as a 400 head, causing the head to crack. I bought custom made performance heads that were fit to spec. Unfortunately the guy who made them told me that it may have a tendency to blow if rev’d the rpms too high, but I live on a mountainous Highway. Is there any way to keep it from
Blowing until it is broken in, anything I should add or do?

When you first start the engine, you run it at about 2,000 RPM’s for 20-30 minutes to break in the camshaft.

Then when you start driving the vehicle, you vary the speed of the vehicle the first 1,000 miles to seat the pistons rings.

If you blow the heads following these normal procedures, then you got cheap heads.


The small block 400 engine is inherently weak due to the thin cylinder walls. It’s great for red light to red light burn outs but I would never consider using one in an off road vehicle. I would strongly suggest that you get a 4 bolt main 350 engine with a medium duty truck camshaft. But you know what they say about opinions I’m sure.


I would pay close attention to what Mr @Rod_Knox mentions…there are many reasons he mentioned it.

As far as the heads “blowing” with too many rpm’s… That is a really strange statement. Blowing how? Why? What is blowing out here? Rpm’s have a tendency to “blow up” many an engine but for well known reasons.

I doubt you intend to see the rpm that this cylinder head guy is talking about, but the question still remains…what on earth was he referring to with that statement? The head gaskets? Floating a valve? What? I would go back and ask him what he meant so that you can better deal with those circumstances whatever they may be.

But I would definitely NOT ignore what Mr Rod Knox told you from the get go… He knows what he is talking about, I can assure you.

For my own knowledge, can’t you put the 400 crankshaft into a 350 block?

That might be a great combination. But then it might not. The devil is always in the details and details such as rod length, piston crown, wrist pin location, etc and the need to cc the head and block-head assembly and then mill the combustion chamber may be necessary. For the average home shop the details for such exotic re-engineering is as far out as using a ball peen hammer and cold chisel in cutting the Hope diamond with plans to cash in with a piece of jewelry.

I believe this is quite common, it yields a 383 cubic inch engine or thereabouts.

Now about those custom fit performance heads. Are they aluminum? This can be a big issue especially if they are aluminum replicas of the cast iron heads. Aluminum acts very differently than cast iron so an aluminum replica is very likely to fail.

If this is just a reworked cast iron head, which is more likely with a one man shop, he could have ground out sections too close tot he water jackets and the head could be weak as a result.

BTW, if aluminum replicas, be sure to retorque the heads after the first warm up. If not, the head gasket will blow within a few hundred miles.