If you're trying to squeeze some actual power out of a late-70s smog motor, the first thing you probably noticed is that the factory olds 403 heads are basically boat anchors. They were built in an era when Detroit was terrified of the EPA, and the result was an engine with a massive 4.351-inch bore that was completely choked out by its own top end.
The 403 is a weird beast. It's a "small block" because it shares the same external dimensions as the Olds 350, but it has a bore larger than a Chevy 454. When you have that much piston surface area but your cylinder heads have massive 83cc combustion chambers and tiny valves, you end up with a compression ratio of about 8:1 on a good day. It's lazy, it's inefficient, and it's why those late-70s Trans Ams felt a bit sluggish despite having 6.6 liters under the shaker.
Why the Stock 4A Heads Are a Letdown
The stock casting for these engines is the 4A head. If you look at the lower-left corner of the head near the spark plug, you'll see that big "4A" stamping. In the world of Oldsmobile performance, "A" usually means "avoid" when it follows a number.
The main issue is the combustion chamber size. Because the 403 was designed to run on low-octane gas while keeping emissions down, Oldsmobile engineers gave these heads huge, open chambers. This drops the compression so low that the engine can't take advantage of the displacement. Furthermore, the ports aren't exactly what you'd call "high-flow." They were designed for torque at 2,500 RPM, not for making real horsepower. If you're serious about building a 403, the stock olds 403 heads are usually the first thing to go in the scrap pile.
The Classic Swap: Using Early 350 Heads
For decades, the go-to "budget" trick for the 403 has been bolting on a set of heads from an older Oldsmobile 350 (specifically from 1968 to 1972). These are typically identified by casting numbers like #5, #6, or #7.
Why do people do this? It's all about the combustion chamber volume. While your stock 4A heads are sitting at 83cc, a set of #5 heads usually clocks in around 64cc to 68cc. By swapping these onto a 403 block, you can jump your compression from a pathetic 8:1 to a much more respectable 9.5:1 or even 10:1 depending on your head gasket and piston setup.
Here's the catch, though. You can't just bolt them on and go. The 403 uses 1/2-inch head bolts, while the older 350 heads were designed for 7/16-inch bolts. You'll need to take those 350 heads to a machine shop to have the bolt holes enlarged. It's a simple job, but it's something you definitely don't want to try with a hand drill in your garage.
Dealing with the Intake Mismatch
Another thing to keep in mind when swapping older olds 403 heads onto your block is the intake manifold. The 403 used a specific intake to match the 4A heads. If you switch to early 350 heads, the ports might not line up perfectly with a stock 403 intake.
Most guys end up grabbing an aftermarket aluminum intake, like an Edelbrock Performer RPM. This manifold is pretty much the gold standard for Olds builds because it has enough meat on the flanges to seal up against various head castings. It also helps the engine breathe much better than the heavy cast-iron factory piece ever could.
What About Big Block Olds Heads?
You'll occasionally hear some guy at a car show say he put "C" or "E" heads from a 455 on his 403. While it is technically possible because the Oldsmobile bolt patterns are remarkably similar across the board, it's usually more trouble than it's worth.
Big block heads have much larger intake ports. When you try to put a small block intake on them, you'll have massive vacuum leaks because the intake isn't wide enough to cover the top of the ports. You can buy spacers, but then you run into hood clearance issues, especially in Firebirds or Omegas. Plus, the huge valves in big block heads can sometimes get uncomfortably close to the cylinder walls on a small block, even one with a bore as big as the 403. For 90% of builders, sticking with optimized small block heads is the smarter move.
Moving to Modern Aluminum Heads
If you have the budget, the absolute best thing you can do for a 403 is to skip the 40-year-old factory castings entirely. Companies like Edelbrock make aluminum heads specifically designed for the Olds small block architecture.
These aftermarket olds 403 heads offer a few massive advantages: * Weight: You'll shave about 40 to 50 pounds off the front of your car by ditching the iron. * Airflow: The port geometry is lightyears ahead of anything Oldsmobile was doing in 1977. * Heat Dissipation: Aluminum sheds heat faster than iron, which allows you to run higher compression without the engine pinging or knocking on pump gas. * Modern Combustion Chambers: The "heart-shaped" chambers promote a much more efficient burn.
The only real "downside" to aluminum heads is the cost. They aren't cheap. But by the time you pay a machine shop to tank, mag, plane, and do a multi-angle valve job on a set of old iron heads, you're already halfway to the cost of the new aluminum ones anyway.
Porting and Polishing for Extra Flow
If you're stuck with iron heads—maybe you're doing a "period correct" build or you just like the challenge—you're going to want to do some port work. The 4A olds 403 heads and even the preferred #7 heads have a lot of "flash" and rough casting marks in the runners.
Focus on the "bowl" area right under the valve seat. Smoothing out the transition from the port to the valve can make a huge difference in how the engine breathes at higher RPMs. You don't necessarily need to enlarge the ports (which can actually kill your low-end torque if you go too far), but cleaning up the obstructions helps significantly.
Also, consider upgrading the valves. Many builders will swap the stock 403 valves for the larger 2.07-inch intake valves found in big block heads. This requires some machine work to the seats, but it allows that massive 403 bore to actually pull in enough air to make real power.
The Cooling Issue
We can't talk about olds 403 heads without mentioning the "siamesed" cylinders. Because the bore on the 403 is so large, there aren't any coolant passages between the cylinders like there are on an Olds 350. The cylinders are literally touching each other.
Because of this, 403s are prone to running hot. When you choose your heads and head gaskets, you have to be very careful. The factory 4A heads have specific steam holes that match the block. If you use older 350 heads, you need to make sure your head gaskets are drilled correctly to allow for proper coolant circulation. If you block off the wrong passage, you'll end up with hot spots that can lead to cracked heads or a warped block.
Final Thoughts on Choosing Your Setup
At the end of the day, the 403 is a bit of an underdog. It's got the displacement of a big block but the weight and size of a small block. The key to unlocking its potential is all in the top end.
If you're on a tight budget, hunt down a set of #5, #6, or #7 iron heads from a 350 and get them machined to fit. It's a tried-and-true method that has been waking up these engines since the 1980s. But if you want the best performance possible and don't want to spend weeks at the machine shop, a set of modern aluminum heads is the way to go.
Whichever route you take, just remember that the factory olds 403 heads were never meant for performance. They were meant for cruising at 55 mph during the fuel crisis. If you want your Olds to actually move when you step on the gas, the heads are the place to start. Don't be afraid to put some decent flow on that big-bore bottom end; the engine will definitely thank you for it.