Deciding to build a 406 short block is one of those choices that usually results in a permanent grin every time you hit the gas. If you're a fan of the Small Block Chevy (SBC) platform, you already know that there's no substitute for cubic inches. While the 350 is the "everyman" engine and the 383 stroker is the popular upgrade, the 406 is the heavy hitter that stays in a small-package footprint. It's the engine for people who want big-block torque without having to swap out their suspension, headers, or accessory brackets.
The beauty of the 406 short block lies in its simplicity. You're basically taking a factory 400 SBC block and giving it a standard .030-inch overbore. That takes the displacement from 400 up to 406 cubic inches. It sounds simple on paper, but there's a lot of nuance to getting these engines right so they don't just make power, but actually stay together for more than a few passes at the strip.
The Secret Sauce of the Large Bore
The defining characteristic of the 400 block, and by extension the 406 short block, is the bore size. Most small blocks have a 4.000-inch bore, but the 400 starts at 4.125 inches. When you combine that massive bore with a 3.750-inch stroke, you get an engine that breathes incredibly well.
Think about it this way: a larger bore allows you to run larger valves in your cylinder heads without them being shrouded by the cylinder walls. This means even at lower RPMs, the engine is gulping in air and fuel way more efficiently than a smaller displacement engine could. That's why these things are absolute torque monsters. You don't have to rev them to 7,000 RPM to feel like you're being kicked in the back; the power is right there as soon as you tip into the throttle.
Choosing the Right Block
When you start hunting for a core to build your 406 short block, you're going to run into the "2-bolt vs. 4-bolt" debate. In the world of 350s, everyone wants the 4-bolt main. But with the factory 400 blocks, it's actually the opposite. The factory 4-bolt 400 blocks are notorious for having weaker main webs because the extra bolt holes actually removed too much material in a sensitive area.
Most experienced builders prefer a 2-bolt main block. If you're planning on making serious power—say, north of 500 horsepower—the move is to take that 2-bolt block and have a machine shop install aftermarket splayed 4-bolt caps. This gives you a much stronger bottom end than any factory casting could ever offer. It's an extra expense, sure, but if you're building a high-performance 406 short block, it's the kind of insurance that lets you sleep at night.
Dealing with the Heat
You can't talk about the 400 block without mentioning the "steam holes." Because the cylinders are so large, there isn't enough room for water jackets between them. This is what we call "siamese" cylinders. To prevent localized steam pockets from forming—which can lead to hot spots and cracked heads—GM drilled small steam holes in the deck of the block.
When you're assembling your 406 short block, you have to make sure your cylinder heads and head gaskets also have these holes. If you're using aftermarket heads that don't have them, you'll need to use the head gasket as a template and drill them yourself. It's a bit nerve-wracking to take a drill bit to a brand-new set of expensive heads, but it's absolutely necessary for a street-driven 406. If it's a dedicated drag car that only runs for 10 seconds at a time, you might get away without them, but for anything else, don't skip this step.
The Rod Length Dilemma
Inside the 406 short block, one of the biggest choices you'll make is the connecting rod length. Factory 400s used a short 5.565-inch rod. This was done to keep the piston height manageable, but it creates a pretty aggressive rod angle, which increases side-loading on the cylinder walls.
Most modern builds ditch the short rods in favor of 5.7-inch (standard 350 length) or even 6.0-inch rods. Using a 5.7-inch rod is a great middle ground. It improves the rod-to-stroke ratio and reduces friction, but it still leaves enough room for a decent piston crown. If you go with a 6.0-inch rod, the piston gets very short, and the wrist pin often intersects the oil ring land. While that's fine for a race engine, it can sometimes lead to oil consumption on a high-mileage street engine. For a balanced, reliable 406 short block, the 5.7-inch rod is usually the sweet spot.
Balancing the Rotating Assembly
Another quirk of the 406 short block is how it's balanced. From the factory, the 400 was externally balanced. This means there are specific weights on the harmonic balancer and the flexplate (or flywheel) to keep the engine from shaking itself apart.
However, if you're buying an aftermarket rotating kit for your 406 short block, you have the option to go with an internal balance. This is generally preferred for high-revving engines because it's more precise and puts less stress on the ends of the crankshaft. It might cost a bit more because the machine shop has to add heavy metal (usually tungsten) to the crank counterweights, but it makes for a much smoother-running engine.
Clearance Issues to Watch For
Because the 400 block has such a large bore and a decent stroke, things get tight inside the crankcase. When you're putting together a 406 short block with aftermarket parts, you have to be careful about rod-to-camshaft clearance.
If you use a "big" camshaft with a lot of lift, the connecting rod bolts can actually smack into the cam lobes as they spin. Most builders use "stroker" clearanced rods, which have a lower profile on the shoulder, or they use a small-base-circle camshaft to create that extra bit of daylight. It's one of those things you have to check and double-check during the mock-up phase. There's nothing worse than turning an engine over by hand and hearing a "clink" because a rod just kissed a cam lobe.
Why Choose a 406 Over a 383?
People ask this all the time. The 383 is great, and parts are everywhere. But a 406 short block simply makes more torque, period. If you're putting this engine in a heavy vehicle—like a 70s Chevelle, a heavy truck, or a full-size cruiser—that extra displacement is going to make the car feel much lighter than a 383 would.
Also, there's a bit of "cool factor" involved. Everyone has a 350 or a 383. When you tell someone you've got a 406 short block under the hood, it usually gets a different reaction. It's an old-school hot rodder's choice. It's about knowing how to handle the specific needs of the 400 block to create something that's a bit of a sleeper.
Final Thoughts on the Build
At the end of the day, building a 406 short block is about maximizing the potential of the small block platform. You're getting the most displacement possible without moving into the "exotic" territory of aftermarket blocks with 4.250-inch bores.
Yes, you have to be mindful of the cooling system, and you have to be smart about your rod choice and balancing. But once it's all bolted together and you drop it into your engine bay, the result is an engine that pulls like a freight train from idle all the way to 6,000 RPM. Whether you're looking to dominate at the local Friday night drags or you just want a street car that can fry the tires at will, the 406 short block is an incredibly rewarding engine to build. Just make sure you've got a good set of tires, because you're going to be needing them.